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Update obsolete comment: it's no longer the case that mdread() will allow 

reads beyond EOF, except by special coercion.
This commit is contained in:
Tom Lane 2007-06-18 00:47:20 +00:00
parent 011b51cb7e
commit de6a6383a7
1 changed files with 9 additions and 10 deletions
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19
src/backend/storage/buffer/bufmgr.c
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@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.220 2007/05/30 20:11:58 tgl Exp $
* $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.221 2007/06/18 00:47:20 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -223,15 +223,14 @@ ReadBuffer_common(Relation reln, BlockNumber blockNum, bool zeroPage,
* 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 ---
* 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.
* EOF (when zero_damaged_pages is ON) and so a previous attempt to
* read a block 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 case should
* always have left a zero-filled buffer, complain if not PageIsNew.
*/
bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);
if (!PageIsNew((PageHeader) bufBlock))