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RestoreBkpBlocks was in the habit of zeroing and refilling the target buffer; which was perfectly safe when the code was written, but is unsafe during Hot Standby operation. The reason is that we have coding rules that allow backends to continue accessing a tuple in a heap relation while holding only a pin on its buffer. Such a backend could see transiently zeroed data, if WAL replay had occasion to change other data on the page. This has been shown to be the cause of bug #6425 from Duncan Rance (who deserves kudos for developing a sufficiently-reproducible test case) as well as Bridget Frey's re-report of bug #6200. It most likely explains the original report as well, though we don't yet have confirmation of that. To fix, change the code so that only bytes that are supposed to change will change, even transiently. This actually saves cycles in RestoreBkpBlocks, since it's not writing the same bytes twice. Also fix seq_redo, which has the same disease, though it has to work a bit harder to meet the requirement. So far as I can tell, no other WAL replay routines have this type of bug. In particular, the index-related replay routines, which would certainly be broken if they had to meet the same standard, are not at risk because we do not have coding rules that allow access to an index page when not holding a buffer lock on it. Back-patch to 9.0 where Hot Standby was added. |
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