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Commit2dc2e4e

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Avoid improbable PANIC during heap_update, redux.
Commit34f581c intended to ensure that RelationGetBufferForTuplewould acquire a visibility-map page pin in case the otherBuffer'sall-visible bit had become set since we last had lock on that page.But I missed a case: when we're extending the relation, VM concernswere dealt with only in the relatively-less-likely case that wefail to conditionally lock the otherBuffer. I think I'd believedthat we couldn't need to worry about it if the conditional locksucceeds, which is true for the target buffer; but the otherBufferwas unlocked for awhile so its bit might be set anyway. So we needto do the GetVisibilityMapPins dance, and then also recheck thepage's free space, in both cases.Per report from Jaime Casanova. Back-patch to v12 as the previouspatch was (although there's still no evidence that the bug isreachable pre-v14).Discussion:https://postgr.es/m/E1lWLjP-00006Y-Ml@gemulon.postgresql.org
1 parent0e497ea commit2dc2e4e

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1 file changed

+23
-18
lines changed
  • src/backend/access/heap

1 file changed

+23
-18
lines changed

‎src/backend/access/heap/hio.c

Lines changed: 23 additions & 18 deletions
Original file line numberDiff line numberDiff line change
@@ -678,29 +678,34 @@ RelationGetBufferForTuple(Relation relation, Size len,
678678
LockBuffer(buffer,BUFFER_LOCK_UNLOCK);
679679
LockBuffer(otherBuffer,BUFFER_LOCK_EXCLUSIVE);
680680
LockBuffer(buffer,BUFFER_LOCK_EXCLUSIVE);
681+
}
681682

682-
/*
683-
* Because the buffers were unlocked for a while, it's possible,
684-
* although unlikely, that an all-visible flag became set or that
685-
* somebody used up the available space in the new page. We can
686-
*useGetVisibilityMapPins to deal with the first case. In the
687-
* second case, just retry from start.
688-
*/
689-
GetVisibilityMapPins(relation,otherBuffer,buffer,
690-
otherBlock,targetBlock,vmbuffer_other,
691-
vmbuffer);
683+
/*
684+
* Because the buffers were unlocked for a while, it's possible,
685+
* although unlikely, that an all-visible flag became set or that
686+
* somebody used up the available space in the new page. We can use
687+
* GetVisibilityMapPins to deal with the first case. In the second
688+
* case, just retry from start.
689+
*/
690+
GetVisibilityMapPins(relation,otherBuffer,buffer,
691+
otherBlock,targetBlock,vmbuffer_other,
692+
vmbuffer);
692693

693-
if (len>PageGetHeapFreeSpace(page))
694-
{
695-
LockBuffer(otherBuffer,BUFFER_LOCK_UNLOCK);
696-
UnlockReleaseBuffer(buffer);
694+
/*
695+
* Note that we have to check the available space even if our
696+
* conditional lock succeeded, because GetVisibilityMapPins might've
697+
* transiently released lock on the target buffer to acquire a VM pin
698+
* for the otherBuffer.
699+
*/
700+
if (len>PageGetHeapFreeSpace(page))
701+
{
702+
LockBuffer(otherBuffer,BUFFER_LOCK_UNLOCK);
703+
UnlockReleaseBuffer(buffer);
697704

698-
gotoloop;
699-
}
705+
gotoloop;
700706
}
701707
}
702-
703-
if (len>PageGetHeapFreeSpace(page))
708+
elseif (len>PageGetHeapFreeSpace(page))
704709
{
705710
/* We should not get here given the test at the top */
706711
elog(PANIC,"tuple is too big: size %zu",len);

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