postgres.git / summary / log / commit / refs
commit 0dbfb8520f609f93f4fd05810be6f5bec601911f
Author: Masahiko Sawada <msawada@postgresql.org>
Date: Tue Jul 28 19:33:47 2026 +0000
Fix logical decoding of empty prepared transactions.
A two-phase transaction that is assigned an XID but produces no change
to be decoded -- for example, one that only acquires row locks via
SELECT ... FOR SHARE -- has no base snapshot in the reorder
buffer. ReorderBufferReplay() already skips such a transaction at
PREPARE time and never invokes the begin_prepare/change/prepare
callbacks for it, but ReorderBufferFinishPrepared() still called the
commit_prepared (or rollback_prepared) callback. As a result a
spurious COMMIT/ROLLBACK PREPARED was sent to the output plugin with
no preceding PREPARE. For the built-in subscriber this breaks
replication (the apply worker fails to find the prepared transaction),
and test_decoding could even crash.
Fix this by detecting an empty transaction (base_snapshot == NULL) in
ReorderBufferFinishPrepared() and cleaning it up without invoking the
commit/rollback prepared callbacks, mirroring the existing empty
transaction handling in ReorderBufferReplay().
On v18 and newer versions, commit 072ee847ad4 changed
ReorderBufferPrepare() to send the prepare whenever it had not already
been sent, which also fires for empty transactions and emits a
spurious PREPARE. On those branches ReorderBufferPrepare() is
therefore additionally guarded with base_snapshot != NULL. This guard
and the Assert(!rbtxn_sent_prepare()) added in
ReorderBufferFinishPrepared(), are not necessary on v17 and older
versions: there ReorderBufferPrepare() only sends a prepare for
concurrently-aborted transactions (which never applies to an empty
transaction) and the RBTXN_SENT_PREPARE flag does not exist.
Back-patch to v14, where decoding of two-phase transactions was
introduced.
Bug: #19556
Reported-by: Alexander Kozhemyakin <a.kozhemyakin@postgrespro.ru>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/19556-daa6d7ea65054d48@postgresql.org
Backpatch-through: 14
contrib/test_decoding/expected/twophase.out | 25 +++++++++++++++
contrib/test_decoding/sql/twophase.sql | 12 ++++++++
src/backend/replication/logical/reorderbuffer.c | 18 +++++++++++
src/test/subscription/t/021_twophase.pl | 41 +++++++++++++++++++++++++
4 files changed, 96 insertions(+)
diff --git a/contrib/test_decoding/expected/twophase.out b/contrib/test_decoding/expected/twophase.out
index 22d128d431f..4d16d50560b 100644
--- a/contrib/test_decoding/expected/twophase.out
+++ b/contrib/test_decoding/expected/twophase.out
@@ -227,6 +227,31 @@ SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'inc
COMMIT PREPARED 'test_toast_table_access'
(1 row)
+-- Test that an empty prepared transaction should not be decoded, whether it
+-- is committed or rolled back.
+BEGIN;
+SELECT * FROM test_prepared1 WHERE id = 1 FOR SHARE;
+ id | data
+----+------
+ 1 |
+(1 row)
+
+PREPARE TRANSACTION 'test_empty_transaction';
+COMMIT PREPARED 'test_empty_transaction';
+BEGIN;
+SELECT * FROM test_prepared1 WHERE id = 1 FOR SHARE;
+ id | data
+----+------
+ 1 |
+(1 row)
+
+PREPARE TRANSACTION 'test_empty_transaction';
+ROLLBACK PREPARED 'test_empty_transaction';
+SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
+ data
+------
+(0 rows)
+
-- Test 8:
-- cleanup and make sure results are also empty
DROP TABLE test_prepared1;
diff --git a/contrib/test_decoding/sql/twophase.sql b/contrib/test_decoding/sql/twophase.sql
index 0ff6ede1f38..0a569ee531d 100644
--- a/contrib/test_decoding/sql/twophase.sql
+++ b/contrib/test_decoding/sql/twophase.sql
@@ -125,6 +125,18 @@ COMMIT PREPARED 'test_toast_table_access';
-- consume commit prepared
SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1', 'stream-changes', '1');
+-- Test that an empty prepared transaction should not be decoded, whether it
+-- is committed or rolled back.
+BEGIN;
+SELECT * FROM test_prepared1 WHERE id = 1 FOR SHARE;
+PREPARE TRANSACTION 'test_empty_transaction';
+COMMIT PREPARED 'test_empty_transaction';
+BEGIN;
+SELECT * FROM test_prepared1 WHERE id = 1 FOR SHARE;
+PREPARE TRANSACTION 'test_empty_transaction';
+ROLLBACK PREPARED 'test_empty_transaction';
+SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
+
-- Test 8:
-- cleanup and make sure results are also empty
DROP TABLE test_prepared1;
diff --git a/src/backend/replication/logical/reorderbuffer.c b/src/backend/replication/logical/reorderbuffer.c
index b190b4712bb..20fed05ec79 100644
--- a/src/backend/replication/logical/reorderbuffer.c
+++ b/src/backend/replication/logical/reorderbuffer.c
@@ -2865,6 +2865,24 @@ ReorderBufferFinishPrepared(ReorderBuffer *rb, TransactionId xid,
txn->xact_time.prepare_time, txn->origin_id, txn->origin_lsn);
}
+ /*
+ * If this transaction has no snapshot, it didn't make any changes to the
+ * database, so there's nothing to decode. Note that
+ * ReorderBufferCommitChild will have transferred any snapshots from
+ * subtransactions if there were any.
+ */
+ if (txn->base_snapshot == NULL)
+ {
+ Assert(txn->ninvalidations == 0);
+
+ /*
+ * Removing this txn before a commit might result in the computation
+ * of an incorrect restart_lsn. See SnapBuildProcessRunningXacts.
+ */
+ ReorderBufferCleanupTXN(rb, txn);
+ return;
+ }
+
txn->final_lsn = commit_lsn;
txn->end_lsn = end_lsn;
txn->xact_time.commit_time = commit_time;
diff --git a/src/test/subscription/t/021_twophase.pl b/src/test/subscription/t/021_twophase.pl
index bd04a1f36a1..534215cb7d7 100644
--- a/src/test/subscription/t/021_twophase.pl
+++ b/src/test/subscription/t/021_twophase.pl
@@ -297,6 +297,47 @@ $result = $node_subscriber->safe_psql('postgres',
"SELECT count(*) FROM pg_prepared_xacts;");
is($result, qq(0), 'transaction is aborted on subscriber');
+###############################
+# Test that an empty prepared transaction is not replicated.
+#
+# A transaction that is assigned an XID but makes no change decoded by logical
+# replication (here, via a row lock) must not be sent to the subscriber.
+# Otherwise the subscriber would receive a PREPARE with no preceding BEGIN
+# PREPARE and error out, breaking replication.
+###############################
+
+# An empty prepared transaction that is committed.
+$node_publisher->safe_psql(
+ 'postgres', "
+ BEGIN;
+ SELECT a FROM tab_full WHERE a = 1 FOR SHARE;
+ PREPARE TRANSACTION 'test_empty_prepared';
+ COMMIT PREPARED 'test_empty_prepared';");
+
+# An empty prepared transaction that is rolled back.
+$node_publisher->safe_psql(
+ 'postgres', "
+ BEGIN;
+ SELECT a FROM tab_full WHERE a = 1 FOR SHARE;
+ PREPARE TRANSACTION 'test_empty_prepared';
+ ROLLBACK PREPARED 'test_empty_prepared';");
+
+# A subsequent normal change must still replicate. Reaching catchup confirms
+# the apply worker was not stalled by the empty prepared transactions above.
+$node_publisher->safe_psql('postgres', "INSERT INTO tab_full VALUES (31);");
+$node_publisher->wait_for_catchup($appname);
+
+# The empty transactions must not have been prepared on the subscriber.
+$result = $node_subscriber->safe_psql('postgres',
+ "SELECT count(*) FROM pg_prepared_xacts;");
+is($result, qq(0), 'empty prepared transaction is not replicated');
+
+# The subsequent change is visible, so replication is healthy.
+$result = $node_subscriber->safe_psql('postgres',
+ "SELECT count(*) FROM tab_full WHERE a = 31;");
+is($result, qq(1),
+ 'replication continues after an empty prepared transaction');
+
###############################
# copy_data=false and two_phase
###############################
[parent: 7123abab7768]