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Fix overly strict assertion in SummarizeOldestCommittedSxact(). There's a

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race condition where SummarizeOldestCommittedSxact() is called even though
another backend already cleared out all finished sxact entries. That's OK,
RegisterSerializableTransactionInt() can just retry getting a news xact
slot from the available-list when that happens.

Reported by YAMAMOTO Takashi, bug #5918.
This commit is contained in:
Heikki Linnakangas 2011-03-08 21:01:29 +02:00
parent 93d888232e
commit 46c333a963
1 changed files with 10 additions and 4 deletions
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14
src/backend/storage/lmgr/predicate.c
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@ -1314,15 +1314,21 @@ SummarizeOldestCommittedSxact(void)
LWLockAcquire(SerializableFinishedListLock, LW_EXCLUSIVE); LWLockAcquire(SerializableFinishedListLock, LW_EXCLUSIVE);
#ifdef TEST_OLDSERXID /*
* This function is only called if there are no sxact slots available.
* Some of them must belong to old, already-finished transactions, so
* there should be something in FinishedSerializableTransactions list
* that we can summarize. However, there's a race condition: while we
* were not holding any locks, a transaction might have ended and cleaned
* up all the finished sxact entries already, freeing up their sxact
* slots. In that case, we have nothing to do here. The caller will find
* one of the slots released by the other backend when it retries.
*/
if (SHMQueueEmpty(FinishedSerializableTransactions)) if (SHMQueueEmpty(FinishedSerializableTransactions))
{ {
LWLockRelease(SerializableFinishedListLock); LWLockRelease(SerializableFinishedListLock);
return; return;
} }
#else
Assert(!SHMQueueEmpty(FinishedSerializableTransactions));
#endif
/* /*
* Grab the first sxact off the finished list -- this will be the earliest * Grab the first sxact off the finished list -- this will be the earliest