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commit    7fedd79b71c212d6e83dbe9af75c4ae3c7b89753
Author:   Tom Lane <tgl@sss.pgh.pa.us>
Date:     Wed Feb 12 19:52:29 2014 +0000
First released in: 9.4.0, 8.4.20, 9.0.16, 9.1.12, 9.2.7, 9.3.3

    In XLogReadBufferExtended, don't assume P_NEW yields consecutive pages.
    
    In a database that's not yet reached consistency, it's possible that some
    segments of a relation are not full-size but are not the last ones either.
    Because of the way smgrnblocks() works, asking for a new page with P_NEW
    will fill in the last not-full-size segment --- and if that makes it full
    size, the apparent EOF of the relation will increase by more than one page,
    so that the next P_NEW request will yield a page past the next consecutive
    one.  This breaks the relation-extension logic in XLogReadBufferExtended,
    possibly allowing a page update to be applied to some page far past where
    it was intended to go.  This appears to be the explanation for reports of
    table bloat on replication slaves compared to their masters, and probably
    explains some corrupted-slave reports as well.
    
    Fix the loop to check the page number it actually got, rather than merely
    Assert()'ing that dead reckoning got it to the desired place.  AFAICT,
    there are no other places that make assumptions about exactly which page
    they'll get from P_NEW.
    
    Problem identified by Greg Stark, though this is not the same as his
    proposed patch.
    
    It's been like this for a long time, so back-patch to all supported
    branches.


 src/backend/access/transam/xlogutils.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/src/backend/access/transam/xlogutils.c b/src/backend/access/transam/xlogutils.c
index a059b60d1aa..8ffe1b0f84b 100644
--- a/src/backend/access/transam/xlogutils.c
+++ b/src/backend/access/transam/xlogutils.c
@@ -314,15 +314,21 @@ XLogReadBufferExtended(RelFileNode rnode, ForkNumber forknum,
 		/* we do this in recovery only - no rel-extension lock needed */
 		Assert(InRecovery);
 		buffer = InvalidBuffer;
-		while (blkno >= lastblock)
+		do
 		{
 			if (buffer != InvalidBuffer)
 				ReleaseBuffer(buffer);
 			buffer = ReadBufferWithoutRelcache(rnode, false, forknum,
 											   P_NEW, mode, NULL);
-			lastblock++;
 		}
-		Assert(BufferGetBlockNumber(buffer) == blkno);
+		while (BufferGetBlockNumber(buffer) < blkno);
+		/* Handle the corner case that P_NEW returns non-consecutive pages */
+		if (BufferGetBlockNumber(buffer) != blkno)
+		{
+			ReleaseBuffer(buffer);
+			buffer = ReadBufferWithoutRelcache(rnode, false, forknum, blkno,
+											   mode, NULL);
+		}
 	}
 
 	if (mode == RBM_NORMAL)


[parent: e1e7642bd3bd]