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Commit304160c

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Fix ReadBuffer() to correctly handle the case where it's trying to extend
the relation but it finds a pre-existing valid buffer. The buffer does notcorrespond to any page known to the kernel, so we *must* do smgrextend toensure that the space becomes allocated. The 7.x branches all do thiscorrectly, 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 benot-BM_VALID, which is not so.)
1 parent6d41b14 commit304160c

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1 file changed

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‎src/backend/storage/buffer/bufmgr.c

Lines changed: 40 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
99
*
1010
* IDENTIFICATION
11-
* $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.201 2005/12/29 18:08:05 tgl Exp $
11+
* $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.202 2006/01/06 00:04:20 tgl Exp $
1212
*
1313
*-------------------------------------------------------------------------
1414
*/
@@ -164,13 +164,47 @@ ReadBuffer(Relation reln, BlockNumber blockNum)
164164
/* if it was already in the buffer pool, we're done */
165165
if (found)
166166
{
167-
/* Just need to update stats before we exit */
168-
pgstat_count_buffer_hit(&reln->pgstat_info,reln);
167+
if (!isExtend)
168+
{
169+
/* Just need to update stats before we exit */
170+
pgstat_count_buffer_hit(&reln->pgstat_info,reln);
171+
172+
if (VacuumCostActive)
173+
VacuumCostBalance+=VacuumCostPageHit;
169174

170-
if (VacuumCostActive)
171-
VacuumCostBalance+=VacuumCostPageHit;
175+
returnBufferDescriptorGetBuffer(bufHdr);
176+
}
172177

173-
returnBufferDescriptorGetBuffer(bufHdr);
178+
/*
179+
* We get here only in the corner case where we are trying to extend
180+
* the relation but we found a pre-existing buffer marked BM_VALID.
181+
* (This can happen because mdread doesn't complain about reads
182+
* beyond EOF --- which is arguably bogus, but changing it seems
183+
* tricky.) We *must* do smgrextend before succeeding, else the
184+
* page will not be reserved by the kernel, and the next P_NEW call
185+
* will decide to return the same page. Clear the BM_VALID bit,
186+
* do the StartBufferIO call that BufferAlloc didn't, and proceed.
187+
*/
188+
if (isLocalBuf)
189+
{
190+
/* Only need to adjust flags */
191+
Assert(bufHdr->flags&BM_VALID);
192+
bufHdr->flags &= ~BM_VALID;
193+
}
194+
else
195+
{
196+
/*
197+
* Loop to handle the very small possibility that someone
198+
* re-sets BM_VALID between our clearing it and StartBufferIO
199+
* inspecting it.
200+
*/
201+
do {
202+
LockBufHdr(bufHdr);
203+
Assert(bufHdr->flags&BM_VALID);
204+
bufHdr->flags &= ~BM_VALID;
205+
UnlockBufHdr(bufHdr);
206+
}while (!StartBufferIO(bufHdr, true));
207+
}
174208
}
175209

176210
/*

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