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commit    43b6f3ed8b135d8452e820e6489d90bd86c4ef4e
Author:   Tom Lane <tgl@sss.pgh.pa.us>
Date:     Fri Sep 04 16:40:28 2020 +0000
First released in: 14.0, 9.6.20, 10.15, 11.10, 12.5, 13.0

    Fix rare deadlock failure in create_am regression test.
    
    The "DROP ACCESS METHOD gist2" test will require locking the index
    to be dropped and then its table; while most ordinary operations
    lock a table first then its index.  While no concurrent test scripts
    should be touching fast_emp4000, autovacuum might chance to be
    processing that table when the DROP runs, resulting in a deadlock
    failure.  This is pretty rare but we see it in the buildfarm from
    time to time.
    
    To fix, acquire a lock on fast_emp4000 before issuing the DROP.
    
    Since the point of the exercise is mostly to prevent buildfarm
    failures, back-patch to 9.6 where this test was introduced.
    
    Discussion: https://postgr.es/m/839004.1599185607@sss.pgh.pa.us


 src/test/regress/expected/create_am.out | 5 +++++
 src/test/regress/sql/create_am.sql      | 5 +++++
 2 files changed, 10 insertions(+)

diff --git a/src/test/regress/expected/create_am.out b/src/test/regress/expected/create_am.out
index 47dd885c4e9..0b8ba6dbfab 100644
--- a/src/test/regress/expected/create_am.out
+++ b/src/test/regress/expected/create_am.out
@@ -97,5 +97,10 @@ ERROR:  cannot drop access method gist2 because other objects depend on it
 DETAIL:  index grect2ind2 depends on operator class box_ops for access method gist2
 HINT:  Use DROP ... CASCADE to drop the dependent objects too.
 -- Drop access method cascade
+-- To prevent a (rare) deadlock against autovacuum,
+-- we must lock the table that owns the index that will be dropped
+BEGIN;
+LOCK TABLE fast_emp4000;
 DROP ACCESS METHOD gist2 CASCADE;
 NOTICE:  drop cascades to index grect2ind2
+COMMIT;
diff --git a/src/test/regress/sql/create_am.sql b/src/test/regress/sql/create_am.sql
index 3e0ac104f3c..1aa338d3e67 100644
--- a/src/test/regress/sql/create_am.sql
+++ b/src/test/regress/sql/create_am.sql
@@ -65,4 +65,9 @@ ROLLBACK;
 DROP ACCESS METHOD gist2;
 
 -- Drop access method cascade
+-- To prevent a (rare) deadlock against autovacuum,
+-- we must lock the table that owns the index that will be dropped
+BEGIN;
+LOCK TABLE fast_emp4000;
 DROP ACCESS METHOD gist2 CASCADE;
+COMMIT;


[parent: 526df0a236df]