From 304160c3e270e6db75e109c149799d09b6f1461b Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Fri, 6 Jan 2006 00:04:20 +0000
Subject: [PATCH] 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 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.)
---
src/backend/storage/buffer/bufmgr.c | 46 +++++++++++++++++++++++++----
1 file changed, 40 insertions(+), 6 deletions(-)
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index a8a37e5ca3c..11e83812d0c 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -8,7 +8,7 @@
*
*
* 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 $
*
*-------------------------------------------------------------------------
*/
@@ -164,13 +164,47 @@ ReadBuffer(Relation reln, BlockNumber blockNum)
/* if it was already in the buffer pool, we're done */
if (found)
{
- /* Just need to update stats before we exit */
- pgstat_count_buffer_hit(&reln->pgstat_info, reln);
+ if (!isExtend)
+ {
+ /* Just need to update stats before we exit */
+ pgstat_count_buffer_hit(&reln->pgstat_info, reln);
+
+ if (VacuumCostActive)
+ VacuumCostBalance += VacuumCostPageHit;
- if (VacuumCostActive)
- VacuumCostBalance += VacuumCostPageHit;
+ 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));
+ }
}
/*
--
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