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Commit6b802cf

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Avoid assertion failure with LISTEN in a serializable transaction.
If LISTEN is the only action in a serializable-mode transaction,and the session was not previously listening, and the notify queueis not empty, predicate.c reported an assertion failure. Thathappened because we'd acquire the transaction's initial snapshotduring PreCommit_Notify, which was called *after* predicate.cexpects any such snapshot to have been established.To fix, just swap the order of the PreCommit_Notify andPreCommit_CheckForSerializationFailure calls during CommitTransaction.This will imply holding the notify-insertion lock slightly longer,but the difference could only be meaningful in serializable mode,which is an expensive option anyway.It appears that this is just an assertion failure, with noconsequences in non-assert builds. A snapshot used only to scanthe notify queue could not have been involved in any serializationconflicts, so there would be nothing forPreCommit_CheckForSerializationFailure to do except assign it aprepareSeqNo and set the SXACT_FLAG_PREPARED flag. And given noconflicts, neither of those omissions affect the behavior ofReleasePredicateLocks. This admittedly once-over-lightly analysisis backed up by the lack of field reports of trouble.Per report from Mark Dilger. The bug is old, so back-patch to allsupported branches; but the new test case only goes back to 9.6,for lack of adequate isolationtester infrastructure before that.Discussion:https://postgr.es/m/3ac7f397-4d5f-be8e-f354-440020675694@gmail.comDiscussion:https://postgr.es/m/13881.1574557302@sss.pgh.pa.us
1 parent1974853 commit6b802cf

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3 files changed

+36
-10
lines changed

3 files changed

+36
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lines changed

‎src/backend/access/transam/xact.c‎

Lines changed: 10 additions & 9 deletions
Original file line numberDiff line numberDiff line change
@@ -2112,6 +2112,14 @@ CommitTransaction(void)
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/* close large objects before lower-level cleanup */
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AtEOXact_LargeObject(true);
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/*
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* Insert notifications sent by NOTIFY commands into the queue. This
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* should be late in the pre-commit sequence to minimize time spent
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* holding the notify-insertion lock. However, this could result in
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* creating a snapshot, so we must do it before serializable cleanup.
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*/
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PreCommit_Notify();
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/*
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* Mark serializable transaction as complete for predicate locking
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* purposes. This should be done as late as we can put it and still allow
@@ -2122,13 +2130,6 @@ CommitTransaction(void)
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if (!is_parallel_worker)
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PreCommit_CheckForSerializationFailure();
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/*
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* Insert notifications sent by NOTIFY commands into the queue. This
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* should be late in the pre-commit sequence to minimize time spent
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* holding the notify-insertion lock.
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*/
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PreCommit_Notify();
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/* Prevent cancel/die interrupt while cleaning up */
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HOLD_INTERRUPTS();
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@@ -2344,15 +2345,15 @@ PrepareTransaction(void)
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/* close large objects before lower-level cleanup */
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AtEOXact_LargeObject(true);
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/* NOTIFY requires no work at this point */
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/*
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* Mark serializable transaction as complete for predicate locking
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* purposes. This should be done as late as we can put it and still allow
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* errors to be raised for failure patterns found at commit.
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*/
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PreCommit_CheckForSerializationFailure();
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/* NOTIFY will be handled below */
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/*
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* Don't allow PREPARE TRANSACTION if we've accessed a temporary table in
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* this transaction. Having the prepared xact hold locks on another

‎src/test/isolation/expected/async-notify.out‎

Lines changed: 12 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1,4 +1,4 @@
1-
Parsed test spec with2 sessions
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Parsed test spec with3 sessions
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33
starting permutation: listenc notify1 notify2 notify3 notifyf
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step listenc: LISTEN c1; LISTEN c2;
@@ -83,6 +83,17 @@ listener: NOTIFY "c1" with payload "" from notifier
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listener: NOTIFY "c2" with payload "payload" from notifier
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listener: NOTIFY "c2" with payload "" from notifier
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starting permutation: l2listen l2begin notify1 lbegins llisten lcommit l2commit l2stop
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step l2listen: LISTEN c1;
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step l2begin: BEGIN;
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step notify1: NOTIFY c1;
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step lbegins: BEGIN ISOLATION LEVEL SERIALIZABLE;
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step llisten: LISTEN c1; LISTEN c2;
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step lcommit: COMMIT;
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step l2commit: COMMIT;
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listener2: NOTIFY "c1" with payload "" from notifier
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step l2stop: UNLISTEN *;
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starting permutation: llisten lbegin usage bignotify usage
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step llisten: LISTEN c1; LISTEN c2;
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step lbegin: BEGIN;

‎src/test/isolation/specs/async-notify.spec‎

Lines changed: 14 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -41,8 +41,18 @@ session "listener"
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step"llisten"{LISTENc1;LISTENc2;}
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step"lcheck"{SELECT1ASx;}
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step"lbegin"{ BEGIN;}
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step"lbegins"{ BEGINISOLATIONLEVELSERIALIZABLE;}
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step"lcommit"{COMMIT;}
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teardown{UNLISTEN *;}
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# In some tests we need a second listener, just to block the queue.
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session"listener2"
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step"l2listen"{LISTENc1;}
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step"l2begin"{ BEGIN;}
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step"l2commit"{COMMIT;}
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step"l2stop"{UNLISTEN *;}
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# Trivial cases.
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permutation"listenc""notify1""notify2""notify3""notifyf"
@@ -59,6 +69,10 @@ permutation "llisten" "notify1" "notify2" "notify3" "notifyf" "lcheck"
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# Again, with local delivery too.
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permutation"listenc""llisten""notify1""notify2""notify3""notifyf""lcheck"
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# Check for bug when initial listen is only action in a serializable xact,
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# and notify queue is not empty
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permutation"l2listen""l2begin""notify1""lbegins""llisten""lcommit""l2commit""l2stop"
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# Verify that pg_notification_queue_usage correctly reports a non-zero result,
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# after submitting notifications while another connection is listening for
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# those notifications and waiting inside an active transaction. We have to

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