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Commit0d11fed

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Avoid transaction-commit race condition while receiving a NOTIFY message.
Use TransactionIdIsInProgress, then TransactionIdDidCommit, to distinguishwhether a NOTIFY message's originating transaction is in progress,committed, or aborted. The previous coding could accept a message from atransaction that was still in-progress according to the PGPROC array;if the client were fast enough at starting a new transaction, it might failto see table rows added/updated by the message-sending transaction. Whichof course would usually be the point of receiving the message. We notedthis type of race condition long ago in tqual.c, but async.c overlooked it.The race condition probably cannot occur unless there are multiple NOTIFYsenders in action, since an individual backend doesn't send NOTIFY signalsuntil well after it's done committing. But if two senders commit in closesuccession, it's certainly possible that we could see the second sender'smessage within the race condition window while responding to the signalfrom the first one.Per bug #9557 from Marko Tiikkaja. This patch is slightly more invasivethan what he proposed, since it removes the now-redundantTransactionIdDidAbort call.Back-patch to 9.0, where the current NOTIFY implementation was introduced.
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‎src/backend/commands/async.c

Lines changed: 24 additions & 18 deletions
Original file line numberDiff line numberDiff line change
@@ -126,6 +126,7 @@
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#include"miscadmin.h"
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#include"storage/ipc.h"
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#include"storage/lmgr.h"
129+
#include"storage/procarray.h"
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#include"storage/procsignal.h"
130131
#include"storage/sinval.h"
131132
#include"tcop/tcopprot.h"
@@ -1980,7 +1981,27 @@ asyncQueueProcessPageEntries(QueuePosition *current,
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/* Ignore messages destined for other databases */
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if (qe->dboid==MyDatabaseId)
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{
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if (TransactionIdDidCommit(qe->xid))
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if (TransactionIdIsInProgress(qe->xid))
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{
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/*
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* The source transaction is still in progress, so we can't
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* process this message yet. Break out of the loop, but first
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* back up *current so we will reprocess the message next
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* time. (Note: it is unlikely but not impossible for
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* TransactionIdDidCommit to fail, so we can't really avoid
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* this advance-then-back-up behavior when dealing with an
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* uncommitted message.)
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*
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* Note that we must test TransactionIdIsInProgress before we
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* test TransactionIdDidCommit, else we might return a message
1997+
* from a transaction that is not yet visible to snapshots;
1998+
* compare the comments at the head of tqual.c.
1999+
*/
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*current=thisentry;
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reachedStop= true;
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break;
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}
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elseif (TransactionIdDidCommit(qe->xid))
19842005
{
19852006
/* qe->data is the null-terminated channel name */
19862007
char*channel=qe->data;
@@ -1993,27 +2014,12 @@ asyncQueueProcessPageEntries(QueuePosition *current,
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NotifyMyFrontEnd(channel,payload,qe->srcPid);
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}
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}
1996-
elseif (TransactionIdDidAbort(qe->xid))
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{
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/*
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* If the source transaction aborted, we just ignore its
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* notifications.
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*/
2002-
}
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else
20042018
{
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/*
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* The transaction has neither committed nor aborted so far,
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* so we can't process its message yet. Break out of the
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* loop, but first back up *current so we will reprocess the
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* message next time. (Note: it is unlikely but not
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* impossible for TransactionIdDidCommit to fail, so we can't
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* really avoid this advance-then-back-up behavior when
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* dealing with an uncommitted message.)
2020+
* The source transaction aborted or crashed, so we just
2021+
* ignore its notifications.
20132022
*/
2014-
*current=thisentry;
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reachedStop= true;
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break;
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}
20182024
}
20192025

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