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Add a check to prevent overwriting valid data if smgrnblocks() gives a 

wrong answer, as has been seen to occur with a buggy Linux kernel.  Not
really our bug, but it's a simple test in a seldom-used control path,
so might as well have a defense.
This commit is contained in:
Tom Lane 2006-09-25 22:01:10 +00:00
parent 6575920fa5
commit ffae5cc5a6
1 changed files with 21 additions and 4 deletions
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25
src/backend/storage/buffer/bufmgr.c
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@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.210 2006/09/17 22:16:22 tgl Exp $
* $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.211 2006/09/25 22:01:10 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -175,9 +175,26 @@ ReadBuffer(Relation reln, BlockNumber blockNum)
/*
* 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
* This can happen because mdread doesn't complain about reads beyond
* EOF --- which is arguably bogus, but changing it seems tricky ---
* and so a previous attempt to read a block just beyond EOF could
* have left a "valid" zero-filled buffer. Unfortunately, we have
* also seen this case occurring because of buggy Linux kernels that
* sometimes return an lseek(SEEK_END) result that doesn't account for
* a recent write. In that situation, the pre-existing buffer would
* contain valid data that we don't want to overwrite. Since the
* legitimate cases should always have left a zero-filled buffer,
* complain if not PageIsNew.
*/
bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);
if (!PageIsNew((PageHeader) bufBlock))
ereport(ERROR,
(errmsg("unexpected data beyond EOF in block %u of relation \"%s\"",
blockNum, RelationGetRelationName(reln)),
errhint("This has been seen to occur with buggy kernels; consider updating your system.")));
/*
* 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.