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Commitae1525f

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Fix bogus when-to-deregister-from-listener-array logic.
Since a backend adds itself to the global listener array duringExec_ListenPreCommit, it's inappropriate for it to remove itself duringExec_UnlistenCommit or Exec_UnlistenAllCommit --- that leads to failurewhen committing a transaction that did UNLISTEN then LISTEN, since we endup not registered though we should be. (This leads to missing laternotifications, or to Assert failures in assert-enabled builds.) Insteaddeal with deregistering at the bottom of AtCommit_Notify, when we know thefinal state of the listenChannels list.Also, simplify the representation of registration status by replacing thetransient backendHasExecutedInitialListen flag with an amRegisteredListenerflag.Per report from Greg Sabino Mullane. Back-patch to 9.0, where the problemwas introduced during the LISTEN/NOTIFY rewrite.
1 parent639f5ad commitae1525f

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+22
-38
lines changed

1 file changed

+22
-38
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‎src/backend/commands/async.c

Lines changed: 22 additions & 38 deletions
Original file line numberDiff line numberDiff line change
@@ -349,12 +349,12 @@ static volatile sig_atomic_t notifyInterruptOccurred = 0;
349349
/* True if we've registered an on_shmem_exit cleanup */
350350
staticboolunlistenExitRegistered= false;
351351

352+
/* True if we're currently registered as a listener in asyncQueueControl */
353+
staticboolamRegisteredListener= false;
354+
352355
/* has this backend sent notifications in the current transaction? */
353356
staticboolbackendHasSentNotifications= false;
354357

355-
/* has this backend executed its first LISTEN in the current transaction? */
356-
staticboolbackendHasExecutedInitialListen= false;
357-
358358
/* GUC parameter */
359359
boolTrace_notify= false;
360360

@@ -745,6 +745,7 @@ static void
745745
Async_UnlistenOnExit(intcode,Datumarg)
746746
{
747747
Exec_UnlistenAllCommit();
748+
asyncQueueUnregister();
748749
}
749750

750751
/*
@@ -789,8 +790,6 @@ PreCommit_Notify(void)
789790
if (Trace_notify)
790791
elog(DEBUG1,"PreCommit_Notify");
791792

792-
Assert(backendHasExecutedInitialListen== false);
793-
794793
/* Preflight for any pending listen/unlisten actions */
795794
foreach(p,pendingActions)
796795
{
@@ -913,11 +912,9 @@ AtCommit_Notify(void)
913912
}
914913
}
915914

916-
/*
917-
* If we did an initial LISTEN, listenChannels now has the entry, so we no
918-
* longer need or want the flag to be set.
919-
*/
920-
backendHasExecutedInitialListen= false;
915+
/* If no longer listening to anything, get out of listener array */
916+
if (amRegisteredListener&&listenChannels==NIL)
917+
asyncQueueUnregister();
921918

922919
/* And clean up */
923920
ClearPendingActionsAndNotifies();
@@ -935,19 +932,12 @@ Exec_ListenPreCommit(void)
935932
* Nothing to do if we are already listening to something, nor if we
936933
* already ran this routine in this transaction.
937934
*/
938-
if (listenChannels!=NIL||backendHasExecutedInitialListen)
935+
if (amRegisteredListener)
939936
return;
940937

941938
if (Trace_notify)
942939
elog(DEBUG1,"Exec_ListenPreCommit(%d)",MyProcPid);
943940

944-
/*
945-
* We need this variable to detect an aborted initial LISTEN. In that case
946-
* we would set up our pointer but not listen on any channel. This flag
947-
* gets cleared in AtCommit_Notify or AtAbort_Notify().
948-
*/
949-
backendHasExecutedInitialListen= true;
950-
951941
/*
952942
* Before registering, make sure we will unlisten before dying. (Note:
953943
* this action does not get undone if we abort later.)
@@ -971,6 +961,9 @@ Exec_ListenPreCommit(void)
971961
QUEUE_BACKEND_PID(MyBackendId)=MyProcPid;
972962
LWLockRelease(AsyncQueueLock);
973963

964+
/* Now we are listed in the global array, so remember we're listening */
965+
amRegisteredListener= true;
966+
974967
/*
975968
* Try to move our pointer forward as far as possible. This will skip over
976969
* already-committed notifications. Still, we could get notifications that
@@ -1043,10 +1036,6 @@ Exec_UnlistenCommit(const char *channel)
10431036
* We do not complain about unlistening something not being listened;
10441037
* should we?
10451038
*/
1046-
1047-
/* If no longer listening to anything, get out of listener array */
1048-
if (listenChannels==NIL)
1049-
asyncQueueUnregister();
10501039
}
10511040

10521041
/*
@@ -1062,8 +1051,6 @@ Exec_UnlistenAllCommit(void)
10621051

10631052
list_free_deep(listenChannels);
10641053
listenChannels=NIL;
1065-
1066-
asyncQueueUnregister();
10671054
}
10681055

10691056
/*
@@ -1181,6 +1168,9 @@ asyncQueueUnregister(void)
11811168

11821169
Assert(listenChannels==NIL);/* else caller error */
11831170

1171+
if (!amRegisteredListener)/* nothing to do */
1172+
return;
1173+
11841174
LWLockAcquire(AsyncQueueLock,LW_SHARED);
11851175
/* check if entry is valid and oldest ... */
11861176
advanceTail= (MyProcPid==QUEUE_BACKEND_PID(MyBackendId))&&
@@ -1189,6 +1179,9 @@ asyncQueueUnregister(void)
11891179
QUEUE_BACKEND_PID(MyBackendId)=InvalidPid;
11901180
LWLockRelease(AsyncQueueLock);
11911181

1182+
/* mark ourselves as no longer listed in the global array */
1183+
amRegisteredListener= false;
1184+
11921185
/* If we were the laziest backend, try to advance the tail pointer */
11931186
if (advanceTail)
11941187
asyncQueueAdvanceTail();
@@ -1545,21 +1538,12 @@ void
15451538
AtAbort_Notify(void)
15461539
{
15471540
/*
1548-
* If we LISTEN but then roll back the transactionwe have set our pointer
1549-
*but havenot made any entry in listenChannels. In that case, remove our
1550-
*pointer again.
1541+
* If we LISTEN but then roll back the transactionafter PreCommit_Notify,
1542+
*we haveregistered as a listener but have not made any entry in
1543+
*listenChannels. In that case, deregister again.
15511544
*/
1552-
if (backendHasExecutedInitialListen)
1553-
{
1554-
/*
1555-
* Checking listenChannels should be redundant but it can't hurt doing
1556-
* it for safety reasons.
1557-
*/
1558-
if (listenChannels==NIL)
1559-
asyncQueueUnregister();
1560-
1561-
backendHasExecutedInitialListen= false;
1562-
}
1545+
if (amRegisteredListener&&listenChannels==NIL)
1546+
asyncQueueUnregister();
15631547

15641548
/* And clean up */
15651549
ClearPendingActionsAndNotifies();

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