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Fix ReadBuffer() to correctly handle the case where it's trying to extend

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the relation but it finds a pre-existing valid buffer.  The buffer does not
correspond to any page known to the kernel, so we *must* do smgrextend to
ensure that the space becomes allocated.  The 7.x branches all do this
correctly, but the corner case got lost somewhere during 8.0 bufmgr rewrites.
(My fault no doubt :-( ... I think I assumed that such a buffer must be
not-BM_VALID, which is not so.)
This commit is contained in:
Tom Lane 2006-01-06 00:04:20 +00:00
parent 6d41b144e9
commit 304160c3e2
1 changed files with 40 additions and 6 deletions
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36
src/backend/storage/buffer/bufmgr.c
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@ -8,7 +8,7 @@
* *
* *
* IDENTIFICATION * IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.201 2005/12/29 18:08:05 tgl Exp $ * $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.202 2006/01/06 00:04:20 tgl Exp $
* *
*------------------------------------------------------------------------- *-------------------------------------------------------------------------
*/ */
@ -163,6 +163,8 @@ ReadBuffer(Relation reln, BlockNumber blockNum)
/* if it was already in the buffer pool, we're done */ /* if it was already in the buffer pool, we're done */
if (found) if (found)
{
if (!isExtend)
{ {
/* Just need to update stats before we exit */ /* Just need to update stats before we exit */
pgstat_count_buffer_hit(&reln->pgstat_info, reln); pgstat_count_buffer_hit(&reln->pgstat_info, reln);
@ -173,6 +175,38 @@ ReadBuffer(Relation reln, BlockNumber blockNum)
return BufferDescriptorGetBuffer(bufHdr); return BufferDescriptorGetBuffer(bufHdr);
} }
/*
* We get here only in the corner case where we are trying to extend
* the relation but we found a pre-existing buffer marked BM_VALID.
* (This can happen because mdread doesn't complain about reads
* beyond EOF --- which is arguably bogus, but changing it seems
* tricky.) We *must* do smgrextend before succeeding, else the
* page will not be reserved by the kernel, and the next P_NEW call
* will decide to return the same page. Clear the BM_VALID bit,
* do the StartBufferIO call that BufferAlloc didn't, and proceed.
*/
if (isLocalBuf)
{
/* Only need to adjust flags */
Assert(bufHdr->flags & BM_VALID);
bufHdr->flags &= ~BM_VALID;
}
else
{
/*
* Loop to handle the very small possibility that someone
* re-sets BM_VALID between our clearing it and StartBufferIO
* inspecting it.
*/
do {
LockBufHdr(bufHdr);
Assert(bufHdr->flags & BM_VALID);
bufHdr->flags &= ~BM_VALID;
UnlockBufHdr(bufHdr);
} while (!StartBufferIO(bufHdr, true));
}
}
/* /*
* if we have gotten to this point, we have allocated a buffer for the * if we have gotten to this point, we have allocated a buffer for the
* page but its contents are not yet valid. IO_IN_PROGRESS is set for it, * page but its contents are not yet valid. IO_IN_PROGRESS is set for it,