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Wrap multixact/members correctly during extension 

In the 9.2 code for extending multixact/members, the logic was very
simple because the number of entries in a members page was a proper
divisor of 2^32, and thus at 2^32 wraparound the logic for page switch
was identical than at any other page boundary.  In commit 0ac5ad5134 I
failed to realize this and introduced code that was not able to go over
the 2^32 boundary.  Fix that by ensuring that when we reach the last
page of the last segment we correctly zero the initial page of the
initial segment, using correct uint32-wraparound-safe arithmetic.

Noticed while investigating bug #8673 reported by Serge Negodyuck, as
diagnosed by Andres Freund.
This commit is contained in:
Alvaro Herrera 2014-01-02 18:17:07 -03:00
parent 722acf51a0
commit a50d976254
1 changed files with 19 additions and 5 deletions
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24
src/backend/access/transam/multixact.c
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@ -2259,7 +2259,6 @@ ExtendMultiXactMember(MultiXactOffset offset, int nmembers)
{
int flagsoff;
int flagsbit;
int difference;
/*
* Only zero when at first entry of a page.
@ -2280,10 +2279,25 @@ ExtendMultiXactMember(MultiXactOffset offset, int nmembers)
LWLockRelease(MultiXactMemberControlLock);
}
/* Advance to next page (OK if nmembers goes negative) */
difference = MULTIXACT_MEMBERS_PER_PAGE - offset % MULTIXACT_MEMBERS_PER_PAGE;
offset += difference;
nmembers -= difference;
/*
* Advance to next page, taking care to properly handle the wraparound
* case. OK if nmembers goes negative.
*/
if ((unsigned int) (offset + nmembers) < offset)
{
uint32 difference = offset + MULTIXACT_MEMBERS_PER_PAGE;
nmembers -= (unsigned int) (MULTIXACT_MEMBERS_PER_PAGE - difference);
offset = 0;
}
else
{
int difference;
difference = MULTIXACT_MEMBERS_PER_PAGE - offset % MULTIXACT_MEMBERS_PER_PAGE;
nmembers -= difference;
offset += difference;
}
}
}