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Commita50d976

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Wrap multixact/members correctly during extension
In the 9.2 code for extending multixact/members, the logic was verysimple because the number of entries in a members page was a properdivisor of 2^32, and thus at 2^32 wraparound the logic for page switchwas identical than at any other page boundary. In commit0ac5ad5 Ifailed to realize this and introduced code that was not able to go overthe 2^32 boundary. Fix that by ensuring that when we reach the lastpage of the last segment we correctly zero the initial page of theinitial segment, using correct uint32-wraparound-safe arithmetic.Noticed while investigating bug #8673 reported by Serge Negodyuck, asdiagnosed by Andres Freund.
1 parent722acf5 commita50d976

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+19
-5
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1 file changed

+19
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‎src/backend/access/transam/multixact.c

Lines changed: 19 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -2259,7 +2259,6 @@ ExtendMultiXactMember(MultiXactOffset offset, int nmembers)
22592259
{
22602260
intflagsoff;
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intflagsbit;
2262-
intdifference;
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/*
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* Only zero when at first entry of a page.
@@ -2280,10 +2279,25 @@ ExtendMultiXactMember(MultiXactOffset offset, int nmembers)
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LWLockRelease(MultiXactMemberControlLock);
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}
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2283-
/* Advance to next page (OK if nmembers goes negative) */
2284-
difference=MULTIXACT_MEMBERS_PER_PAGE-offset %MULTIXACT_MEMBERS_PER_PAGE;
2285-
offset+=difference;
2286-
nmembers-=difference;
2282+
/*
2283+
* Advance to next page, taking care to properly handle the wraparound
2284+
* case. OK if nmembers goes negative.
2285+
*/
2286+
if ((unsignedint) (offset+nmembers)<offset)
2287+
{
2288+
uint32difference=offset+MULTIXACT_MEMBERS_PER_PAGE;
2289+
2290+
nmembers-= (unsignedint) (MULTIXACT_MEMBERS_PER_PAGE-difference);
2291+
offset=0;
2292+
}
2293+
else
2294+
{
2295+
intdifference;
2296+
2297+
difference=MULTIXACT_MEMBERS_PER_PAGE-offset %MULTIXACT_MEMBERS_PER_PAGE;
2298+
nmembers-=difference;
2299+
offset+=difference;
2300+
}
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}
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}
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