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* [PATCH v15 7/8] Row pattern recognition patch (tests).
@ 2024-03-28 10:30 Tatsuo Ishii <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Tatsuo Ishii @ 2024-03-28 10:30 UTC (permalink / raw)
---
src/test/regress/expected/rpr.out | 821 +++++++++++++++++++++++++++++
src/test/regress/parallel_schedule | 2 +-
src/test/regress/sql/rpr.sql | 392 ++++++++++++++
3 files changed, 1214 insertions(+), 1 deletion(-)
create mode 100644 src/test/regress/expected/rpr.out
create mode 100644 src/test/regress/sql/rpr.sql
diff --git a/src/test/regress/expected/rpr.out b/src/test/regress/expected/rpr.out
new file mode 100644
index 0000000000..e8998ebf45
--- /dev/null
+++ b/src/test/regress/expected/rpr.out
@@ -0,0 +1,821 @@
+--
+-- Test for row pattern definition clause
+--
+CREATE TEMP TABLE stock (
+ company TEXT,
+ tdate DATE,
+ price INTEGER
+);
+INSERT INTO stock VALUES ('company1', '2023-07-01', 100);
+INSERT INTO stock VALUES ('company1', '2023-07-02', 200);
+INSERT INTO stock VALUES ('company1', '2023-07-03', 150);
+INSERT INTO stock VALUES ('company1', '2023-07-04', 140);
+INSERT INTO stock VALUES ('company1', '2023-07-05', 150);
+INSERT INTO stock VALUES ('company1', '2023-07-06', 90);
+INSERT INTO stock VALUES ('company1', '2023-07-07', 110);
+INSERT INTO stock VALUES ('company1', '2023-07-08', 130);
+INSERT INTO stock VALUES ('company1', '2023-07-09', 120);
+INSERT INTO stock VALUES ('company1', '2023-07-10', 130);
+INSERT INTO stock VALUES ('company2', '2023-07-01', 50);
+INSERT INTO stock VALUES ('company2', '2023-07-02', 2000);
+INSERT INTO stock VALUES ('company2', '2023-07-03', 1500);
+INSERT INTO stock VALUES ('company2', '2023-07-04', 1400);
+INSERT INTO stock VALUES ('company2', '2023-07-05', 1500);
+INSERT INTO stock VALUES ('company2', '2023-07-06', 60);
+INSERT INTO stock VALUES ('company2', '2023-07-07', 1100);
+INSERT INTO stock VALUES ('company2', '2023-07-08', 1300);
+INSERT INTO stock VALUES ('company2', '2023-07-09', 1200);
+INSERT INTO stock VALUES ('company2', '2023-07-10', 1300);
+SELECT * FROM stock;
+ company | tdate | price
+----------+------------+-------
+ company1 | 07-01-2023 | 100
+ company1 | 07-02-2023 | 200
+ company1 | 07-03-2023 | 150
+ company1 | 07-04-2023 | 140
+ company1 | 07-05-2023 | 150
+ company1 | 07-06-2023 | 90
+ company1 | 07-07-2023 | 110
+ company1 | 07-08-2023 | 130
+ company1 | 07-09-2023 | 120
+ company1 | 07-10-2023 | 130
+ company2 | 07-01-2023 | 50
+ company2 | 07-02-2023 | 2000
+ company2 | 07-03-2023 | 1500
+ company2 | 07-04-2023 | 1400
+ company2 | 07-05-2023 | 1500
+ company2 | 07-06-2023 | 60
+ company2 | 07-07-2023 | 1100
+ company2 | 07-08-2023 | 1300
+ company2 | 07-09-2023 | 1200
+ company2 | 07-10-2023 | 1300
+(20 rows)
+
+-- basic test using PREV
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w,
+ nth_value(tdate, 2) OVER w AS nth_second
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (START UP+ DOWN+)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+ company | tdate | price | first_value | last_value | nth_second
+----------+------------+-------+-------------+------------+------------
+ company1 | 07-01-2023 | 100 | 100 | 140 | 07-02-2023
+ company1 | 07-02-2023 | 200 | | |
+ company1 | 07-03-2023 | 150 | | |
+ company1 | 07-04-2023 | 140 | | |
+ company1 | 07-05-2023 | 150 | | |
+ company1 | 07-06-2023 | 90 | 90 | 120 | 07-07-2023
+ company1 | 07-07-2023 | 110 | | |
+ company1 | 07-08-2023 | 130 | | |
+ company1 | 07-09-2023 | 120 | | |
+ company1 | 07-10-2023 | 130 | | |
+ company2 | 07-01-2023 | 50 | 50 | 1400 | 07-02-2023
+ company2 | 07-02-2023 | 2000 | | |
+ company2 | 07-03-2023 | 1500 | | |
+ company2 | 07-04-2023 | 1400 | | |
+ company2 | 07-05-2023 | 1500 | | |
+ company2 | 07-06-2023 | 60 | 60 | 1200 | 07-07-2023
+ company2 | 07-07-2023 | 1100 | | |
+ company2 | 07-08-2023 | 1300 | | |
+ company2 | 07-09-2023 | 1200 | | |
+ company2 | 07-10-2023 | 1300 | | |
+(20 rows)
+
+-- basic test using PREV. UP appears twice
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w,
+ nth_value(tdate, 2) OVER w AS nth_second
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (START UP+ DOWN+ UP+)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+ company | tdate | price | first_value | last_value | nth_second
+----------+------------+-------+-------------+------------+------------
+ company1 | 07-01-2023 | 100 | 100 | 150 | 07-02-2023
+ company1 | 07-02-2023 | 200 | | |
+ company1 | 07-03-2023 | 150 | | |
+ company1 | 07-04-2023 | 140 | | |
+ company1 | 07-05-2023 | 150 | | |
+ company1 | 07-06-2023 | 90 | 90 | 130 | 07-07-2023
+ company1 | 07-07-2023 | 110 | | |
+ company1 | 07-08-2023 | 130 | | |
+ company1 | 07-09-2023 | 120 | | |
+ company1 | 07-10-2023 | 130 | | |
+ company2 | 07-01-2023 | 50 | 50 | 1500 | 07-02-2023
+ company2 | 07-02-2023 | 2000 | | |
+ company2 | 07-03-2023 | 1500 | | |
+ company2 | 07-04-2023 | 1400 | | |
+ company2 | 07-05-2023 | 1500 | | |
+ company2 | 07-06-2023 | 60 | 60 | 1300 | 07-07-2023
+ company2 | 07-07-2023 | 1100 | | |
+ company2 | 07-08-2023 | 1300 | | |
+ company2 | 07-09-2023 | 1200 | | |
+ company2 | 07-10-2023 | 1300 | | |
+(20 rows)
+
+-- basic test using PREV. Use '*'
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w,
+ nth_value(tdate, 2) OVER w AS nth_second
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (START UP* DOWN+)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+ company | tdate | price | first_value | last_value | nth_second
+----------+------------+-------+-------------+------------+------------
+ company1 | 07-01-2023 | 100 | 100 | 140 | 07-02-2023
+ company1 | 07-02-2023 | 200 | | |
+ company1 | 07-03-2023 | 150 | | |
+ company1 | 07-04-2023 | 140 | | |
+ company1 | 07-05-2023 | 150 | 150 | 90 | 07-06-2023
+ company1 | 07-06-2023 | 90 | | |
+ company1 | 07-07-2023 | 110 | 110 | 120 | 07-08-2023
+ company1 | 07-08-2023 | 130 | | |
+ company1 | 07-09-2023 | 120 | | |
+ company1 | 07-10-2023 | 130 | | |
+ company2 | 07-01-2023 | 50 | 50 | 1400 | 07-02-2023
+ company2 | 07-02-2023 | 2000 | | |
+ company2 | 07-03-2023 | 1500 | | |
+ company2 | 07-04-2023 | 1400 | | |
+ company2 | 07-05-2023 | 1500 | 1500 | 60 | 07-06-2023
+ company2 | 07-06-2023 | 60 | | |
+ company2 | 07-07-2023 | 1100 | 1100 | 1200 | 07-08-2023
+ company2 | 07-08-2023 | 1300 | | |
+ company2 | 07-09-2023 | 1200 | | |
+ company2 | 07-10-2023 | 1300 | | |
+(20 rows)
+
+-- basic test with none greedy pattern
+SELECT company, tdate, price, count(*) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (A A A)
+ DEFINE
+ A AS price >= 140 AND price <= 150
+);
+ company | tdate | price | count
+----------+------------+-------+-------
+ company1 | 07-01-2023 | 100 | 0
+ company1 | 07-02-2023 | 200 | 0
+ company1 | 07-03-2023 | 150 | 3
+ company1 | 07-04-2023 | 140 |
+ company1 | 07-05-2023 | 150 |
+ company1 | 07-06-2023 | 90 | 0
+ company1 | 07-07-2023 | 110 | 0
+ company1 | 07-08-2023 | 130 | 0
+ company1 | 07-09-2023 | 120 | 0
+ company1 | 07-10-2023 | 130 | 0
+ company2 | 07-01-2023 | 50 | 0
+ company2 | 07-02-2023 | 2000 | 0
+ company2 | 07-03-2023 | 1500 | 0
+ company2 | 07-04-2023 | 1400 | 0
+ company2 | 07-05-2023 | 1500 | 0
+ company2 | 07-06-2023 | 60 | 0
+ company2 | 07-07-2023 | 1100 | 0
+ company2 | 07-08-2023 | 1300 | 0
+ company2 | 07-09-2023 | 1200 | 0
+ company2 | 07-10-2023 | 1300 | 0
+(20 rows)
+
+-- last_value() should remain consistent
+SELECT company, tdate, price, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ORDER BY tdate
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (START UP+ DOWN+)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+ company | tdate | price | last_value
+----------+------------+-------+------------
+ company1 | 07-01-2023 | 100 | 140
+ company1 | 07-02-2023 | 200 |
+ company1 | 07-03-2023 | 150 |
+ company1 | 07-04-2023 | 140 |
+ company1 | 07-05-2023 | 150 |
+ company1 | 07-06-2023 | 90 | 120
+ company1 | 07-07-2023 | 110 |
+ company1 | 07-08-2023 | 130 |
+ company1 | 07-09-2023 | 120 |
+ company1 | 07-10-2023 | 130 |
+ company2 | 07-01-2023 | 50 | 1400
+ company2 | 07-02-2023 | 2000 |
+ company2 | 07-03-2023 | 1500 |
+ company2 | 07-04-2023 | 1400 |
+ company2 | 07-05-2023 | 1500 |
+ company2 | 07-06-2023 | 60 | 1200
+ company2 | 07-07-2023 | 1100 |
+ company2 | 07-08-2023 | 1300 |
+ company2 | 07-09-2023 | 1200 |
+ company2 | 07-10-2023 | 1300 |
+(20 rows)
+
+-- omit "START" in DEFINE but it is ok because "START AS TRUE" is
+-- implicitly defined. per spec.
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w,
+ nth_value(tdate, 2) OVER w AS nth_second
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (START UP+ DOWN+)
+ DEFINE
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+ company | tdate | price | first_value | last_value | nth_second
+----------+------------+-------+-------------+------------+------------
+ company1 | 07-01-2023 | 100 | 100 | 140 | 07-02-2023
+ company1 | 07-02-2023 | 200 | | |
+ company1 | 07-03-2023 | 150 | | |
+ company1 | 07-04-2023 | 140 | | |
+ company1 | 07-05-2023 | 150 | | |
+ company1 | 07-06-2023 | 90 | 90 | 120 | 07-07-2023
+ company1 | 07-07-2023 | 110 | | |
+ company1 | 07-08-2023 | 130 | | |
+ company1 | 07-09-2023 | 120 | | |
+ company1 | 07-10-2023 | 130 | | |
+ company2 | 07-01-2023 | 50 | 50 | 1400 | 07-02-2023
+ company2 | 07-02-2023 | 2000 | | |
+ company2 | 07-03-2023 | 1500 | | |
+ company2 | 07-04-2023 | 1400 | | |
+ company2 | 07-05-2023 | 1500 | | |
+ company2 | 07-06-2023 | 60 | 60 | 1200 | 07-07-2023
+ company2 | 07-07-2023 | 1100 | | |
+ company2 | 07-08-2023 | 1300 | | |
+ company2 | 07-09-2023 | 1200 | | |
+ company2 | 07-10-2023 | 1300 | | |
+(20 rows)
+
+-- the first row start with less than or equal to 100
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (LOWPRICE UP+ DOWN+)
+ DEFINE
+ LOWPRICE AS price <= 100,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+ company | tdate | price | first_value | last_value
+----------+------------+-------+-------------+------------
+ company1 | 07-01-2023 | 100 | 100 | 140
+ company1 | 07-02-2023 | 200 | |
+ company1 | 07-03-2023 | 150 | |
+ company1 | 07-04-2023 | 140 | |
+ company1 | 07-05-2023 | 150 | |
+ company1 | 07-06-2023 | 90 | 90 | 120
+ company1 | 07-07-2023 | 110 | |
+ company1 | 07-08-2023 | 130 | |
+ company1 | 07-09-2023 | 120 | |
+ company1 | 07-10-2023 | 130 | |
+ company2 | 07-01-2023 | 50 | 50 | 1400
+ company2 | 07-02-2023 | 2000 | |
+ company2 | 07-03-2023 | 1500 | |
+ company2 | 07-04-2023 | 1400 | |
+ company2 | 07-05-2023 | 1500 | |
+ company2 | 07-06-2023 | 60 | 60 | 1200
+ company2 | 07-07-2023 | 1100 | |
+ company2 | 07-08-2023 | 1300 | |
+ company2 | 07-09-2023 | 1200 | |
+ company2 | 07-10-2023 | 1300 | |
+(20 rows)
+
+-- second row raises 120%
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (LOWPRICE UP+ DOWN+)
+ DEFINE
+ LOWPRICE AS price <= 100,
+ UP AS price > PREV(price) * 1.2,
+ DOWN AS price < PREV(price)
+);
+ company | tdate | price | first_value | last_value
+----------+------------+-------+-------------+------------
+ company1 | 07-01-2023 | 100 | 100 | 140
+ company1 | 07-02-2023 | 200 | |
+ company1 | 07-03-2023 | 150 | |
+ company1 | 07-04-2023 | 140 | |
+ company1 | 07-05-2023 | 150 | |
+ company1 | 07-06-2023 | 90 | |
+ company1 | 07-07-2023 | 110 | |
+ company1 | 07-08-2023 | 130 | |
+ company1 | 07-09-2023 | 120 | |
+ company1 | 07-10-2023 | 130 | |
+ company2 | 07-01-2023 | 50 | 50 | 1400
+ company2 | 07-02-2023 | 2000 | |
+ company2 | 07-03-2023 | 1500 | |
+ company2 | 07-04-2023 | 1400 | |
+ company2 | 07-05-2023 | 1500 | |
+ company2 | 07-06-2023 | 60 | |
+ company2 | 07-07-2023 | 1100 | |
+ company2 | 07-08-2023 | 1300 | |
+ company2 | 07-09-2023 | 1200 | |
+ company2 | 07-10-2023 | 1300 | |
+(20 rows)
+
+-- using NEXT
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (START UPDOWN)
+ DEFINE
+ START AS TRUE,
+ UPDOWN AS price > PREV(price) AND price > NEXT(price)
+);
+ company | tdate | price | first_value | last_value
+----------+------------+-------+-------------+------------
+ company1 | 07-01-2023 | 100 | 100 | 200
+ company1 | 07-02-2023 | 200 | |
+ company1 | 07-03-2023 | 150 | |
+ company1 | 07-04-2023 | 140 | 140 | 150
+ company1 | 07-05-2023 | 150 | |
+ company1 | 07-06-2023 | 90 | |
+ company1 | 07-07-2023 | 110 | 110 | 130
+ company1 | 07-08-2023 | 130 | |
+ company1 | 07-09-2023 | 120 | |
+ company1 | 07-10-2023 | 130 | |
+ company2 | 07-01-2023 | 50 | 50 | 2000
+ company2 | 07-02-2023 | 2000 | |
+ company2 | 07-03-2023 | 1500 | |
+ company2 | 07-04-2023 | 1400 | 1400 | 1500
+ company2 | 07-05-2023 | 1500 | |
+ company2 | 07-06-2023 | 60 | |
+ company2 | 07-07-2023 | 1100 | 1100 | 1300
+ company2 | 07-08-2023 | 1300 | |
+ company2 | 07-09-2023 | 1200 | |
+ company2 | 07-10-2023 | 1300 | |
+(20 rows)
+
+-- using AFTER MATCH SKIP TO NEXT ROW
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ AFTER MATCH SKIP TO NEXT ROW
+ INITIAL
+ PATTERN (START UPDOWN)
+ DEFINE
+ START AS TRUE,
+ UPDOWN AS price > PREV(price) AND price > NEXT(price)
+);
+ company | tdate | price | first_value | last_value
+----------+------------+-------+-------------+------------
+ company1 | 07-01-2023 | 100 | 100 | 200
+ company1 | 07-02-2023 | 200 | |
+ company1 | 07-03-2023 | 150 | |
+ company1 | 07-04-2023 | 140 | 140 | 150
+ company1 | 07-05-2023 | 150 | |
+ company1 | 07-06-2023 | 90 | |
+ company1 | 07-07-2023 | 110 | 110 | 130
+ company1 | 07-08-2023 | 130 | |
+ company1 | 07-09-2023 | 120 | |
+ company1 | 07-10-2023 | 130 | |
+ company2 | 07-01-2023 | 50 | 50 | 2000
+ company2 | 07-02-2023 | 2000 | |
+ company2 | 07-03-2023 | 1500 | |
+ company2 | 07-04-2023 | 1400 | 1400 | 1500
+ company2 | 07-05-2023 | 1500 | |
+ company2 | 07-06-2023 | 60 | |
+ company2 | 07-07-2023 | 1100 | 1100 | 1300
+ company2 | 07-08-2023 | 1300 | |
+ company2 | 07-09-2023 | 1200 | |
+ company2 | 07-10-2023 | 1300 | |
+(20 rows)
+
+-- match everything
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ORDER BY tdate
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ AFTER MATCH SKIP PAST LAST ROW
+ INITIAL
+ PATTERN (A+)
+ DEFINE
+ A AS TRUE
+);
+ company | tdate | price | first_value | last_value
+----------+------------+-------+-------------+------------
+ company1 | 07-01-2023 | 100 | 100 | 130
+ company1 | 07-02-2023 | 200 | |
+ company1 | 07-03-2023 | 150 | |
+ company1 | 07-04-2023 | 140 | |
+ company1 | 07-05-2023 | 150 | |
+ company1 | 07-06-2023 | 90 | |
+ company1 | 07-07-2023 | 110 | |
+ company1 | 07-08-2023 | 130 | |
+ company1 | 07-09-2023 | 120 | |
+ company1 | 07-10-2023 | 130 | |
+ company2 | 07-01-2023 | 50 | 50 | 1300
+ company2 | 07-02-2023 | 2000 | |
+ company2 | 07-03-2023 | 1500 | |
+ company2 | 07-04-2023 | 1400 | |
+ company2 | 07-05-2023 | 1500 | |
+ company2 | 07-06-2023 | 60 | |
+ company2 | 07-07-2023 | 1100 | |
+ company2 | 07-08-2023 | 1300 | |
+ company2 | 07-09-2023 | 1200 | |
+ company2 | 07-10-2023 | 1300 | |
+(20 rows)
+
+-- backtracking with reclassification of rows
+-- using AFTER MATCH SKIP PAST LAST ROW
+SELECT company, tdate, price, first_value(tdate) OVER w, last_value(tdate) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ORDER BY tdate
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ AFTER MATCH SKIP PAST LAST ROW
+ INITIAL
+ PATTERN (A+ B+)
+ DEFINE
+ A AS price > 100,
+ B AS price > 100
+);
+ company | tdate | price | first_value | last_value
+----------+------------+-------+-------------+------------
+ company1 | 07-01-2023 | 100 | |
+ company1 | 07-02-2023 | 200 | 07-02-2023 | 07-05-2023
+ company1 | 07-03-2023 | 150 | |
+ company1 | 07-04-2023 | 140 | |
+ company1 | 07-05-2023 | 150 | |
+ company1 | 07-06-2023 | 90 | |
+ company1 | 07-07-2023 | 110 | 07-07-2023 | 07-10-2023
+ company1 | 07-08-2023 | 130 | |
+ company1 | 07-09-2023 | 120 | |
+ company1 | 07-10-2023 | 130 | |
+ company2 | 07-01-2023 | 50 | |
+ company2 | 07-02-2023 | 2000 | 07-02-2023 | 07-05-2023
+ company2 | 07-03-2023 | 1500 | |
+ company2 | 07-04-2023 | 1400 | |
+ company2 | 07-05-2023 | 1500 | |
+ company2 | 07-06-2023 | 60 | |
+ company2 | 07-07-2023 | 1100 | 07-07-2023 | 07-10-2023
+ company2 | 07-08-2023 | 1300 | |
+ company2 | 07-09-2023 | 1200 | |
+ company2 | 07-10-2023 | 1300 | |
+(20 rows)
+
+-- backtracking with reclassification of rows
+-- using AFTER MATCH SKIP TO NEXT ROW
+SELECT company, tdate, price, first_value(tdate) OVER w, last_value(tdate) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ORDER BY tdate
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ AFTER MATCH SKIP TO NEXT ROW
+ INITIAL
+ PATTERN (A+ B+)
+ DEFINE
+ A AS price > 100,
+ B AS price > 100
+);
+ company | tdate | price | first_value | last_value
+----------+------------+-------+-------------+------------
+ company1 | 07-01-2023 | 100 | |
+ company1 | 07-02-2023 | 200 | 07-02-2023 | 07-05-2023
+ company1 | 07-03-2023 | 150 | 07-03-2023 | 07-05-2023
+ company1 | 07-04-2023 | 140 | 07-04-2023 | 07-05-2023
+ company1 | 07-05-2023 | 150 | |
+ company1 | 07-06-2023 | 90 | |
+ company1 | 07-07-2023 | 110 | 07-07-2023 | 07-10-2023
+ company1 | 07-08-2023 | 130 | 07-08-2023 | 07-10-2023
+ company1 | 07-09-2023 | 120 | 07-09-2023 | 07-10-2023
+ company1 | 07-10-2023 | 130 | |
+ company2 | 07-01-2023 | 50 | |
+ company2 | 07-02-2023 | 2000 | 07-02-2023 | 07-05-2023
+ company2 | 07-03-2023 | 1500 | 07-03-2023 | 07-05-2023
+ company2 | 07-04-2023 | 1400 | 07-04-2023 | 07-05-2023
+ company2 | 07-05-2023 | 1500 | |
+ company2 | 07-06-2023 | 60 | |
+ company2 | 07-07-2023 | 1100 | 07-07-2023 | 07-10-2023
+ company2 | 07-08-2023 | 1300 | 07-08-2023 | 07-10-2023
+ company2 | 07-09-2023 | 1200 | 07-09-2023 | 07-10-2023
+ company2 | 07-10-2023 | 1300 | |
+(20 rows)
+
+-- ROWS BETWEEN CURRENT ROW AND offset FOLLOWING
+SELECT company, tdate, price, first_value(tdate) OVER w, last_value(tdate) OVER w,
+ count(*) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ORDER BY tdate
+ ROWS BETWEEN CURRENT ROW AND 2 FOLLOWING
+ AFTER MATCH SKIP PAST LAST ROW
+ PATTERN (START UP+ DOWN+)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+ company | tdate | price | first_value | last_value | count
+----------+------------+-------+-------------+------------+-------
+ company1 | 07-01-2023 | 100 | 07-01-2023 | 07-03-2023 | 3
+ company1 | 07-02-2023 | 200 | | |
+ company1 | 07-03-2023 | 150 | | |
+ company1 | 07-04-2023 | 140 | 07-04-2023 | 07-06-2023 | 3
+ company1 | 07-05-2023 | 150 | | |
+ company1 | 07-06-2023 | 90 | | |
+ company1 | 07-07-2023 | 110 | 07-07-2023 | 07-09-2023 | 3
+ company1 | 07-08-2023 | 130 | | |
+ company1 | 07-09-2023 | 120 | | |
+ company1 | 07-10-2023 | 130 | | | 0
+ company2 | 07-01-2023 | 50 | 07-01-2023 | 07-03-2023 | 3
+ company2 | 07-02-2023 | 2000 | | |
+ company2 | 07-03-2023 | 1500 | | |
+ company2 | 07-04-2023 | 1400 | 07-04-2023 | 07-06-2023 | 3
+ company2 | 07-05-2023 | 1500 | | |
+ company2 | 07-06-2023 | 60 | | |
+ company2 | 07-07-2023 | 1100 | 07-07-2023 | 07-09-2023 | 3
+ company2 | 07-08-2023 | 1300 | | |
+ company2 | 07-09-2023 | 1200 | | |
+ company2 | 07-10-2023 | 1300 | | | 0
+(20 rows)
+
+--
+-- Aggregates
+--
+-- using AFTER MATCH SKIP PAST LAST ROW
+SELECT company, tdate, price,
+ first_value(price) OVER w,
+ last_value(price) OVER w,
+ max(price) OVER w,
+ min(price) OVER w,
+ sum(price) OVER w,
+ avg(price) OVER w,
+ count(price) OVER w
+FROM stock
+WINDOW w AS (
+PARTITION BY company
+ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+AFTER MATCH SKIP PAST LAST ROW
+INITIAL
+PATTERN (START UP+ DOWN+)
+DEFINE
+START AS TRUE,
+UP AS price > PREV(price),
+DOWN AS price < PREV(price)
+);
+ company | tdate | price | first_value | last_value | max | min | sum | avg | count
+----------+------------+-------+-------------+------------+------+-----+------+-----------------------+-------
+ company1 | 07-01-2023 | 100 | 100 | 140 | 200 | 100 | 590 | 147.5000000000000000 | 4
+ company1 | 07-02-2023 | 200 | | | | | | |
+ company1 | 07-03-2023 | 150 | | | | | | |
+ company1 | 07-04-2023 | 140 | | | | | | |
+ company1 | 07-05-2023 | 150 | | | | | | | 0
+ company1 | 07-06-2023 | 90 | 90 | 120 | 130 | 90 | 450 | 112.5000000000000000 | 4
+ company1 | 07-07-2023 | 110 | | | | | | |
+ company1 | 07-08-2023 | 130 | | | | | | |
+ company1 | 07-09-2023 | 120 | | | | | | |
+ company1 | 07-10-2023 | 130 | | | | | | | 0
+ company2 | 07-01-2023 | 50 | 50 | 1400 | 2000 | 50 | 4950 | 1237.5000000000000000 | 4
+ company2 | 07-02-2023 | 2000 | | | | | | |
+ company2 | 07-03-2023 | 1500 | | | | | | |
+ company2 | 07-04-2023 | 1400 | | | | | | |
+ company2 | 07-05-2023 | 1500 | | | | | | | 0
+ company2 | 07-06-2023 | 60 | 60 | 1200 | 1300 | 60 | 3660 | 915.0000000000000000 | 4
+ company2 | 07-07-2023 | 1100 | | | | | | |
+ company2 | 07-08-2023 | 1300 | | | | | | |
+ company2 | 07-09-2023 | 1200 | | | | | | |
+ company2 | 07-10-2023 | 1300 | | | | | | | 0
+(20 rows)
+
+-- using AFTER MATCH SKIP TO NEXT ROW
+SELECT company, tdate, price,
+ first_value(price) OVER w,
+ last_value(price) OVER w,
+ max(price) OVER w,
+ min(price) OVER w,
+ sum(price) OVER w,
+ avg(price) OVER w,
+ count(price) OVER w
+FROM stock
+WINDOW w AS (
+PARTITION BY company
+ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+AFTER MATCH SKIP TO NEXT ROW
+INITIAL
+PATTERN (START UP+ DOWN+)
+DEFINE
+START AS TRUE,
+UP AS price > PREV(price),
+DOWN AS price < PREV(price)
+);
+ company | tdate | price | first_value | last_value | max | min | sum | avg | count
+----------+------------+-------+-------------+------------+------+------+------+-----------------------+-------
+ company1 | 07-01-2023 | 100 | 100 | 140 | 200 | 100 | 590 | 147.5000000000000000 | 4
+ company1 | 07-02-2023 | 200 | | | | | | | 0
+ company1 | 07-03-2023 | 150 | | | | | | | 0
+ company1 | 07-04-2023 | 140 | 140 | 90 | 150 | 90 | 380 | 126.6666666666666667 | 3
+ company1 | 07-05-2023 | 150 | | | | | | | 0
+ company1 | 07-06-2023 | 90 | 90 | 120 | 130 | 90 | 450 | 112.5000000000000000 | 4
+ company1 | 07-07-2023 | 110 | 110 | 120 | 130 | 110 | 360 | 120.0000000000000000 | 3
+ company1 | 07-08-2023 | 130 | | | | | | | 0
+ company1 | 07-09-2023 | 120 | | | | | | | 0
+ company1 | 07-10-2023 | 130 | | | | | | | 0
+ company2 | 07-01-2023 | 50 | 50 | 1400 | 2000 | 50 | 4950 | 1237.5000000000000000 | 4
+ company2 | 07-02-2023 | 2000 | | | | | | | 0
+ company2 | 07-03-2023 | 1500 | | | | | | | 0
+ company2 | 07-04-2023 | 1400 | 1400 | 60 | 1500 | 60 | 2960 | 986.6666666666666667 | 3
+ company2 | 07-05-2023 | 1500 | | | | | | | 0
+ company2 | 07-06-2023 | 60 | 60 | 1200 | 1300 | 60 | 3660 | 915.0000000000000000 | 4
+ company2 | 07-07-2023 | 1100 | 1100 | 1200 | 1300 | 1100 | 3600 | 1200.0000000000000000 | 3
+ company2 | 07-08-2023 | 1300 | | | | | | | 0
+ company2 | 07-09-2023 | 1200 | | | | | | | 0
+ company2 | 07-10-2023 | 1300 | | | | | | | 0
+(20 rows)
+
+-- JOIN case
+CREATE TEMP TABLE t1 (i int, v1 int);
+CREATE TEMP TABLE t2 (j int, v2 int);
+INSERT INTO t1 VALUES(1,10);
+INSERT INTO t1 VALUES(1,11);
+INSERT INTO t1 VALUES(1,12);
+INSERT INTO t2 VALUES(2,10);
+INSERT INTO t2 VALUES(2,11);
+INSERT INTO t2 VALUES(2,12);
+SELECT * FROM t1, t2 WHERE t1.v1 <= 11 AND t2.v2 <= 11;
+ i | v1 | j | v2
+---+----+---+----
+ 1 | 10 | 2 | 10
+ 1 | 10 | 2 | 11
+ 1 | 11 | 2 | 10
+ 1 | 11 | 2 | 11
+(4 rows)
+
+SELECT *, count(*) OVER w FROM t1, t2
+WINDOW w AS (
+ PARTITION BY t1.i
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (A)
+ DEFINE
+ A AS v1 <= 11 AND v2 <= 11
+);
+ i | v1 | j | v2 | count
+---+----+---+----+-------
+ 1 | 10 | 2 | 10 | 1
+ 1 | 10 | 2 | 11 | 1
+ 1 | 10 | 2 | 12 | 0
+ 1 | 11 | 2 | 10 | 1
+ 1 | 11 | 2 | 11 | 1
+ 1 | 11 | 2 | 12 | 0
+ 1 | 12 | 2 | 10 | 0
+ 1 | 12 | 2 | 11 | 0
+ 1 | 12 | 2 | 12 | 0
+(9 rows)
+
+-- WITH case
+WITH wstock AS (
+ SELECT * FROM stock WHERE tdate < '2023-07-08'
+)
+SELECT tdate, price,
+first_value(tdate) OVER w,
+count(*) OVER w
+ FROM wstock
+ WINDOW w AS (
+ PARTITION BY company
+ ORDER BY tdate
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (START UP+ DOWN+)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+ tdate | price | first_value | count
+------------+-------+-------------+-------
+ 07-01-2023 | 100 | 07-01-2023 | 4
+ 07-02-2023 | 200 | |
+ 07-03-2023 | 150 | |
+ 07-04-2023 | 140 | |
+ 07-05-2023 | 150 | | 0
+ 07-06-2023 | 90 | | 0
+ 07-07-2023 | 110 | | 0
+ 07-01-2023 | 50 | 07-01-2023 | 4
+ 07-02-2023 | 2000 | |
+ 07-03-2023 | 1500 | |
+ 07-04-2023 | 1400 | |
+ 07-05-2023 | 1500 | | 0
+ 07-06-2023 | 60 | | 0
+ 07-07-2023 | 1100 | | 0
+(14 rows)
+
+--
+-- Error cases
+--
+-- row pattern definition variable name must not appear more than once
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ ORDER BY tdate
+ INITIAL
+ PATTERN (START UP+ DOWN+)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price),
+ UP AS price > PREV(price)
+);
+ERROR: syntax error at or near "ORDER"
+LINE 6: ORDER BY tdate
+ ^
+-- pattern variable name must appear in DEFINE
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ORDER BY tdate
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (START UP+ DOWN+ END)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+ERROR: syntax error at or near "END"
+LINE 8: PATTERN (START UP+ DOWN+ END)
+ ^
+-- FRAME must start at current row when row patttern recognition is used
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ORDER BY tdate
+ ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (START UP+ DOWN+)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+ERROR: FRAME must start at current row when row patttern recognition is used
+-- SEEK is not supported
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ORDER BY tdate
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ AFTER MATCH SKIP TO NEXT ROW
+ SEEK
+ PATTERN (START UP+ DOWN+)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+ERROR: SEEK is not supported
+LINE 8: SEEK
+ ^
+HINT: Use INITIAL.
diff --git a/src/test/regress/parallel_schedule b/src/test/regress/parallel_schedule
index 5ac6e871f5..db6978fb5c 100644
--- a/src/test/regress/parallel_schedule
+++ b/src/test/regress/parallel_schedule
@@ -98,7 +98,7 @@ test: publication subscription
# Another group of parallel tests
# select_views depends on create_view
# ----------
-test: select_views portals_p2 foreign_key cluster dependency guc bitmapops combocid tsearch tsdicts foreign_data window xmlmap functional_deps advisory_lock indirect_toast equivclass
+test: select_views portals_p2 foreign_key cluster dependency guc bitmapops combocid tsearch tsdicts foreign_data window xmlmap functional_deps advisory_lock indirect_toast equivclass rpr
# ----------
# Another group of parallel tests (JSON related)
diff --git a/src/test/regress/sql/rpr.sql b/src/test/regress/sql/rpr.sql
new file mode 100644
index 0000000000..0a69cc0e11
--- /dev/null
+++ b/src/test/regress/sql/rpr.sql
@@ -0,0 +1,392 @@
+--
+-- Test for row pattern definition clause
+--
+
+CREATE TEMP TABLE stock (
+ company TEXT,
+ tdate DATE,
+ price INTEGER
+);
+INSERT INTO stock VALUES ('company1', '2023-07-01', 100);
+INSERT INTO stock VALUES ('company1', '2023-07-02', 200);
+INSERT INTO stock VALUES ('company1', '2023-07-03', 150);
+INSERT INTO stock VALUES ('company1', '2023-07-04', 140);
+INSERT INTO stock VALUES ('company1', '2023-07-05', 150);
+INSERT INTO stock VALUES ('company1', '2023-07-06', 90);
+INSERT INTO stock VALUES ('company1', '2023-07-07', 110);
+INSERT INTO stock VALUES ('company1', '2023-07-08', 130);
+INSERT INTO stock VALUES ('company1', '2023-07-09', 120);
+INSERT INTO stock VALUES ('company1', '2023-07-10', 130);
+INSERT INTO stock VALUES ('company2', '2023-07-01', 50);
+INSERT INTO stock VALUES ('company2', '2023-07-02', 2000);
+INSERT INTO stock VALUES ('company2', '2023-07-03', 1500);
+INSERT INTO stock VALUES ('company2', '2023-07-04', 1400);
+INSERT INTO stock VALUES ('company2', '2023-07-05', 1500);
+INSERT INTO stock VALUES ('company2', '2023-07-06', 60);
+INSERT INTO stock VALUES ('company2', '2023-07-07', 1100);
+INSERT INTO stock VALUES ('company2', '2023-07-08', 1300);
+INSERT INTO stock VALUES ('company2', '2023-07-09', 1200);
+INSERT INTO stock VALUES ('company2', '2023-07-10', 1300);
+
+SELECT * FROM stock;
+
+-- basic test using PREV
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w,
+ nth_value(tdate, 2) OVER w AS nth_second
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (START UP+ DOWN+)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+
+-- basic test using PREV. UP appears twice
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w,
+ nth_value(tdate, 2) OVER w AS nth_second
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (START UP+ DOWN+ UP+)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+
+-- basic test using PREV. Use '*'
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w,
+ nth_value(tdate, 2) OVER w AS nth_second
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (START UP* DOWN+)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+
+-- basic test with none greedy pattern
+SELECT company, tdate, price, count(*) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (A A A)
+ DEFINE
+ A AS price >= 140 AND price <= 150
+);
+
+-- last_value() should remain consistent
+SELECT company, tdate, price, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ORDER BY tdate
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (START UP+ DOWN+)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+
+-- omit "START" in DEFINE but it is ok because "START AS TRUE" is
+-- implicitly defined. per spec.
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w,
+ nth_value(tdate, 2) OVER w AS nth_second
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (START UP+ DOWN+)
+ DEFINE
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+
+-- the first row start with less than or equal to 100
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (LOWPRICE UP+ DOWN+)
+ DEFINE
+ LOWPRICE AS price <= 100,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+
+-- second row raises 120%
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (LOWPRICE UP+ DOWN+)
+ DEFINE
+ LOWPRICE AS price <= 100,
+ UP AS price > PREV(price) * 1.2,
+ DOWN AS price < PREV(price)
+);
+
+-- using NEXT
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (START UPDOWN)
+ DEFINE
+ START AS TRUE,
+ UPDOWN AS price > PREV(price) AND price > NEXT(price)
+);
+
+-- using AFTER MATCH SKIP TO NEXT ROW
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ AFTER MATCH SKIP TO NEXT ROW
+ INITIAL
+ PATTERN (START UPDOWN)
+ DEFINE
+ START AS TRUE,
+ UPDOWN AS price > PREV(price) AND price > NEXT(price)
+);
+
+-- match everything
+
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ORDER BY tdate
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ AFTER MATCH SKIP PAST LAST ROW
+ INITIAL
+ PATTERN (A+)
+ DEFINE
+ A AS TRUE
+);
+
+-- backtracking with reclassification of rows
+-- using AFTER MATCH SKIP PAST LAST ROW
+SELECT company, tdate, price, first_value(tdate) OVER w, last_value(tdate) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ORDER BY tdate
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ AFTER MATCH SKIP PAST LAST ROW
+ INITIAL
+ PATTERN (A+ B+)
+ DEFINE
+ A AS price > 100,
+ B AS price > 100
+);
+
+-- backtracking with reclassification of rows
+-- using AFTER MATCH SKIP TO NEXT ROW
+SELECT company, tdate, price, first_value(tdate) OVER w, last_value(tdate) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ORDER BY tdate
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ AFTER MATCH SKIP TO NEXT ROW
+ INITIAL
+ PATTERN (A+ B+)
+ DEFINE
+ A AS price > 100,
+ B AS price > 100
+);
+
+-- ROWS BETWEEN CURRENT ROW AND offset FOLLOWING
+SELECT company, tdate, price, first_value(tdate) OVER w, last_value(tdate) OVER w,
+ count(*) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ORDER BY tdate
+ ROWS BETWEEN CURRENT ROW AND 2 FOLLOWING
+ AFTER MATCH SKIP PAST LAST ROW
+ PATTERN (START UP+ DOWN+)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+
+--
+-- Aggregates
+--
+
+-- using AFTER MATCH SKIP PAST LAST ROW
+SELECT company, tdate, price,
+ first_value(price) OVER w,
+ last_value(price) OVER w,
+ max(price) OVER w,
+ min(price) OVER w,
+ sum(price) OVER w,
+ avg(price) OVER w,
+ count(price) OVER w
+FROM stock
+WINDOW w AS (
+PARTITION BY company
+ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+AFTER MATCH SKIP PAST LAST ROW
+INITIAL
+PATTERN (START UP+ DOWN+)
+DEFINE
+START AS TRUE,
+UP AS price > PREV(price),
+DOWN AS price < PREV(price)
+);
+
+-- using AFTER MATCH SKIP TO NEXT ROW
+SELECT company, tdate, price,
+ first_value(price) OVER w,
+ last_value(price) OVER w,
+ max(price) OVER w,
+ min(price) OVER w,
+ sum(price) OVER w,
+ avg(price) OVER w,
+ count(price) OVER w
+FROM stock
+WINDOW w AS (
+PARTITION BY company
+ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+AFTER MATCH SKIP TO NEXT ROW
+INITIAL
+PATTERN (START UP+ DOWN+)
+DEFINE
+START AS TRUE,
+UP AS price > PREV(price),
+DOWN AS price < PREV(price)
+);
+
+-- JOIN case
+CREATE TEMP TABLE t1 (i int, v1 int);
+CREATE TEMP TABLE t2 (j int, v2 int);
+INSERT INTO t1 VALUES(1,10);
+INSERT INTO t1 VALUES(1,11);
+INSERT INTO t1 VALUES(1,12);
+INSERT INTO t2 VALUES(2,10);
+INSERT INTO t2 VALUES(2,11);
+INSERT INTO t2 VALUES(2,12);
+
+SELECT * FROM t1, t2 WHERE t1.v1 <= 11 AND t2.v2 <= 11;
+
+SELECT *, count(*) OVER w FROM t1, t2
+WINDOW w AS (
+ PARTITION BY t1.i
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (A)
+ DEFINE
+ A AS v1 <= 11 AND v2 <= 11
+);
+
+-- WITH case
+WITH wstock AS (
+ SELECT * FROM stock WHERE tdate < '2023-07-08'
+)
+SELECT tdate, price,
+first_value(tdate) OVER w,
+count(*) OVER w
+ FROM wstock
+ WINDOW w AS (
+ PARTITION BY company
+ ORDER BY tdate
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (START UP+ DOWN+)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+
+--
+-- Error cases
+--
+
+-- row pattern definition variable name must not appear more than once
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ ORDER BY tdate
+ INITIAL
+ PATTERN (START UP+ DOWN+)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price),
+ UP AS price > PREV(price)
+);
+
+-- pattern variable name must appear in DEFINE
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ORDER BY tdate
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (START UP+ DOWN+ END)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+
+-- FRAME must start at current row when row patttern recognition is used
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ORDER BY tdate
+ ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
+ INITIAL
+ PATTERN (START UP+ DOWN+)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
+
+-- SEEK is not supported
+SELECT company, tdate, price, first_value(price) OVER w, last_value(price) OVER w
+ FROM stock
+ WINDOW w AS (
+ PARTITION BY company
+ ORDER BY tdate
+ ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
+ AFTER MATCH SKIP TO NEXT ROW
+ SEEK
+ PATTERN (START UP+ DOWN+)
+ DEFINE
+ START AS TRUE,
+ UP AS price > PREV(price),
+ DOWN AS price < PREV(price)
+);
--
2.25.1
----Next_Part(Thu_Mar_28_19_59_25_2024_076)--
Content-Type: Text/X-Patch; charset=us-ascii
Content-Transfer-Encoding: 7bit
Content-Disposition: inline;
filename="v15-0008-Allow-to-print-raw-parse-tree.patch"
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v2] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index fb6a4914b06..1da073f0aaa 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index d3febfe58f1..075194a8af8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1844,6 +1844,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index 6d304f32fb0..d346aefe9b6 100644
--- a/meson.build
+++ b/meson.build
@@ -2694,6 +2694,7 @@ decl_checks = [
['strlcpy', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 563f20374ff..b41a5f9dcca 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#if HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#if HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#if HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 339268dc8ef..e7b8e023829 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -80,6 +80,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of `memset_s', and to 0 if you
don't. */
#undef HAVE_DECL_MEMSET_S
base-commit: 6831cd9e3b082d7b830c3196742dd49e3540c49b
--
2.49.0
--7u2oub7w3nl66bkx--
^ permalink raw reply [nested|flat] 195+ messages in thread
* [PATCH v1] Use open file description locks for data directory lockfile
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
0 siblings, 0 replies; 195+ messages in thread
From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)
When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.
To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.
This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:
* Portability issues. Open file description lock was a non-POSIX extension in
Linux and similar flock is from BSD standard. But looks like everybody agrees
that such locks make more sense than a typical advisory locks, and
F_OFD_SETLK made its way into POSIX.1 2024 [1].
* Issues with NFS. The current state of things here looks like this:
- NFSv3 doesn't implement open file description locks, they're converted to
advisory locks instead. Advisory locks are subject to namespace isolation,
meaning that processes in different PID namespaces will not see each other
advisory lock, and it's still possible to run multiple postgres
instances on the same data directory.
- NFSv4 uses a lease system for locking, I haven't found any mention of
conversion to advisory locks neither in the man page nor in RFC [2].
To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530
---
configure | 14 ++++
configure.ac | 3 +
meson.build | 1 +
src/backend/utils/init/miscinit.c | 107 +++++++++++++++++++++++++-----
src/include/pg_config.h.in | 4 ++
5 files changed, 111 insertions(+), 18 deletions(-)
diff --git a/configure b/configure
index 14ad0a5006f..b176ac39799 100755
--- a/configure
+++ b/configure
@@ -16177,6 +16177,20 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+ ac_have_decl=1
+else
+ ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
if test "x$ac_cv_func_explicit_bzero" = xyes; then :
$as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 01b3bbc1be8..d6cf1f27771 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1838,6 +1838,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
# This is probably only present on macOS, but may as well check always
AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
AC_REPLACE_FUNCS(m4_normalize([
explicit_bzero
getopt
diff --git a/meson.build b/meson.build
index d7c5193d4ce..ad9ecad829f 100644
--- a/meson.build
+++ b/meson.build
@@ -2667,6 +2667,7 @@ decl_checks = [
['strnlen', 'string.h'],
['strsep', 'string.h'],
['timingsafe_bcmp', 'string.h'],
+ ['F_OFD_SETLK', 'fcntl.h'],
]
# Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index fec79992c8d..78bb7df543e 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -68,6 +68,11 @@ static List *lock_files = NIL;
static Latch LocalLatchData;
+#ifdef HAVE_DECL_F_OFD_SETLK
+/* File descriptor for data directory lock file. */
+static int DataDirLockFD;
+#endif
+
/* ----------------------------------------------------------------
* ignoring system indexes support stuff
*
@@ -1117,6 +1122,45 @@ RestoreClientConnectionInfo(char *conninfo)
*-------------------------------------------------------------------------
*/
+/*
+ * Flock the data directory lockfile.
+ *
+ * Lock the data directory lockfile with an open file description lock. If the
+ * lock is already taken, it's a hard stop. It's only a best effort test, and
+ * any other errors are ignored. On succes the file descriptor is duplicated,
+ * to make sure there will be at least one open copy of it to keep the lock.
+ *
+ * filename is used only for reporting purposes.
+ */
+static void
+FlockDataDirLockFile(int fd, const char *filename)
+{
+
+#ifdef HAVE_DECL_F_OFD_SETLK
+ struct flock lock;
+
+ lock.l_type = F_WRLCK;
+ lock.l_whence = SEEK_SET;
+ lock.l_start = 0;
+ lock.l_len = 0;
+ lock.l_pid = 0;
+
+ if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+ {
+ if (errno == EAGAIN)
+ ereport(FATAL,
+ (errcode(ERRCODE_LOCK_FILE_EXISTS),
+ errmsg("cannot lock the lock file \"%s\"", filename),
+ errhint("Another server is starting.")));
+ else
+ elog(WARNING, "Failed locking file \"%s\", %m", filename);
+ }
+ else
+ DataDirLockFD = dup(fd);
+#endif
+
+}
+
/*
* proc_exit callback to remove lockfiles.
*/
@@ -1125,6 +1169,11 @@ UnlinkLockFiles(int status, Datum arg)
{
ListCell *l;
+#ifdef HAVE_DECL_F_OFD_SETLK
+ /* Close the file descriptor, which keeps the open file description lock */
+ close(DataDirLockFD);
+#endif
+
foreach(l, lock_files)
{
char *curfile = (char *) lfirst(l);
@@ -1171,22 +1220,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
const char *envvar;
/*
- * If the PID in the lockfile is our own PID or our parent's or
- * grandparent's PID, then the file must be stale (probably left over from
- * a previous system boot cycle). We need to check this because of the
- * likelihood that a reboot will assign exactly the same PID as we had in
- * the previous reboot, or one that's only one or two counts larger and
- * hence the lockfile's PID now refers to an ancestor shell process. We
- * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
- * via the environment variable PG_GRANDPARENT_PID; this is so that
- * launching the postmaster via pg_ctl can be just as reliable as
- * launching it directly. There is no provision for detecting
- * further-removed ancestor processes, but if the init script is written
- * carefully then all but the immediate parent shell will be root-owned
- * processes and so the kill test will fail with EPERM. Note that we
- * cannot get a false negative this way, because an existing postmaster
- * would surely never launch a competing postmaster or pg_ctl process
- * directly.
+ * If we find an already existing lockfile containing our own PID,
+ * there are few options:
+ *
+ * - There is another process, that we don't see due to PID namespace
+ * isolation, which is already running in this data directory.
+ *
+ * To prevent two concurrent processes working with the same data
+ * directory, we first try to lock the lockfile exclusively.
+ *
+ * - The file must be stale, probably left over from a previous system boot
+ * cycle. The same if the lockfile contains our parent's or grandparent's
+ * PID.
+ *
+ * We need to check this because of the likelihood that a reboot will
+ * assign exactly the same PID as we had in the previous reboot, or one
+ * that's only one or two counts larger and hence the lockfile's PID now
+ * refers to an ancestor shell process. We allow pg_ctl to pass down its
+ * parent shell PID (our grandparent PID) via the environment variable
+ * PG_GRANDPARENT_PID; this is so that launching the postmaster via
+ * pg_ctl can be just as reliable as launching it directly. There is no
+ * provision for detecting further-removed ancestor processes, but if the
+ * init script is written carefully then all but the immediate parent
+ * shell will be root-owned processes and so the kill test will fail with
+ * EPERM. Note that we cannot get a false negative this way, because an
+ * existing postmaster would surely never launch a competing postmaster
+ * or pg_ctl process directly.
*/
my_pid = getpid();
@@ -1222,7 +1281,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
*/
fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
if (fd >= 0)
- break; /* Success; exit the retry loop */
+ {
+ /* Success; lock and exit the retry loop */
+ FlockDataDirLockFile(fd, filename);
+ break;
+ }
/*
* Couldn't create the pid file. Probably it already exists.
@@ -1236,8 +1299,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
/*
* Read the file to get the old owner's PID. Note race condition
* here: file might have been deleted since we tried to create it.
+ *
+ * We're going to use the same fd for flock, and want to create a write
+ * lock for the latter one. Since both fd and the lock have to be of
+ * the same type, open the file for read and write.
*/
- fd = open(filename, O_RDONLY, pg_file_create_mode);
+ fd = open(filename, O_RDWR, pg_file_create_mode);
if (fd < 0)
{
if (errno == ENOENT)
@@ -1247,6 +1314,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
errmsg("could not open lock file \"%s\": %m",
filename)));
}
+
+ /* Try to lock the file. We stop here, if it's already locked. */
+ FlockDataDirLockFile(fd, filename);
+
pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
ereport(FATAL,
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 92fcc5f3063..c19a50f108e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -83,6 +83,10 @@
don't. */
#undef HAVE_DECL_F_FULLFSYNC
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+ don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
/* Define to 1 if you have the declaration of
`LLVMCreateGDBRegistrationListener', and to 0 if you don't. */
#undef HAVE_DECL_LLVMCREATEGDBREGISTRATIONLISTENER
base-commit: 44f49511b7940adf3be4337d4feb2de38fe92297
--
2.49.0
--n6t6hie3zne7u6vd--
^ permalink raw reply [nested|flat] 195+ messages in thread
end of thread, other threads:[~2025-12-18 17:21 UTC | newest]
Thread overview: 195+ messages (download: mbox mbox.gz follow: Atom feed)
-- links below jump to the message on this page --
2024-03-28 10:30 [PATCH v15 7/8] Row pattern recognition patch (tests). Tatsuo Ishii <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v2] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v1] Use open file description locks for data directory lockfile Dmitrii Dolgov <[email protected]>
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