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Commitbdf7db8

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In generic WAL application and replay, ensure page "hole" is always zero.
The previous coding could allow the contents of the "hole" between pd_lowerand pd_upper to diverge during replay from what it had been when the updatewas originally applied. This would pose a problem if checksums were inuse, and in any case would complicate forensic comparisons between masterand slave servers. So force the "hole" to contain zeroes, both at initialapplication of a generically-logged action, and at replay.Alexander Korotkov, adjusted slightly by me
1 parent813b456 commitbdf7db8

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1 file changed

+37
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‎src/backend/access/transam/generic_xlog.c

Lines changed: 37 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -338,17 +338,31 @@ GenericXLogFinish(GenericXLogState *state)
338338
{
339339
PageData*pageData=&state->pages[i];
340340
Pagepage;
341+
PageHeaderpageHeader;
341342

342343
if (BufferIsInvalid(pageData->buffer))
343344
continue;
344345

345346
page=BufferGetPage(pageData->buffer,NULL,NULL,
346347
BGP_NO_SNAPSHOT_TEST);
348+
pageHeader= (PageHeader)pageData->image;
347349

348350
if (pageData->fullImage)
349351
{
350-
/* A full page image does not require anything special */
351-
memcpy(page,pageData->image,BLCKSZ);
352+
/*
353+
* A full-page image does not require us to supply any xlog
354+
* data. Just apply the image, being careful to zero the
355+
* "hole" between pd_lower and pd_upper in order to avoid
356+
* divergence between actual page state and what replay would
357+
* produce.
358+
*/
359+
memcpy(page,pageData->image,pageHeader->pd_lower);
360+
memset(page+pageHeader->pd_lower,0,
361+
pageHeader->pd_upper-pageHeader->pd_lower);
362+
memcpy(page+pageHeader->pd_upper,
363+
pageData->image+pageHeader->pd_upper,
364+
BLCKSZ-pageHeader->pd_upper);
365+
352366
XLogRegisterBuffer(i,pageData->buffer,
353367
REGBUF_FORCE_IMAGE |REGBUF_STANDARD);
354368
}
@@ -359,7 +373,15 @@ GenericXLogFinish(GenericXLogState *state)
359373
* associated with this page.
360374
*/
361375
computeDelta(pageData,page, (Page)pageData->image);
362-
memcpy(page,pageData->image,BLCKSZ);
376+
377+
/* Apply the image, with zeroed "hole" as above */
378+
memcpy(page,pageData->image,pageHeader->pd_lower);
379+
memset(page+pageHeader->pd_lower,0,
380+
pageHeader->pd_upper-pageHeader->pd_lower);
381+
memcpy(page+pageHeader->pd_upper,
382+
pageData->image+pageHeader->pd_upper,
383+
BLCKSZ-pageHeader->pd_upper);
384+
363385
XLogRegisterBuffer(i,pageData->buffer,REGBUF_STANDARD);
364386
XLogRegisterBufData(i,pageData->delta,pageData->deltaLen);
365387
}
@@ -395,6 +417,7 @@ GenericXLogFinish(GenericXLogState *state)
395417
BGP_NO_SNAPSHOT_TEST),
396418
pageData->image,
397419
BLCKSZ);
420+
/* We don't worry about zeroing the "hole" in this case */
398421
MarkBufferDirty(pageData->buffer);
399422
}
400423
END_CRIT_SECTION();
@@ -473,6 +496,7 @@ generic_redo(XLogReaderState *record)
473496
if (action==BLK_NEEDS_REDO)
474497
{
475498
Pagepage;
499+
PageHeaderpageHeader;
476500
char*blockDelta;
477501
SizeblockDeltaSize;
478502

@@ -481,6 +505,16 @@ generic_redo(XLogReaderState *record)
481505
blockDelta=XLogRecGetBlockData(record,block_id,&blockDeltaSize);
482506
applyPageRedo(page,blockDelta,blockDeltaSize);
483507

508+
/*
509+
* Since the delta contains no information about what's in the
510+
* "hole" between pd_lower and pd_upper, set that to zero to
511+
* ensure we produce the same page state that application of the
512+
* logged action by GenericXLogFinish did.
513+
*/
514+
pageHeader= (PageHeader)page;
515+
memset(page+pageHeader->pd_lower,0,
516+
pageHeader->pd_upper-pageHeader->pd_lower);
517+
484518
PageSetLSN(page,lsn);
485519
MarkBufferDirty(buffers[block_id]);
486520
}

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