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Commitd24c565

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Tidy up GetMultiXactIdMembers()'s behavior on error
One of the error paths left *members uninitialized. That's not a livebug, because most callers don't look at *members when the functionreturns -1, but let's be tidy. One caller, in heap_lock_tuple(), does"if (members != NULL) pfree(members)", but AFAICS it never passes aninvalid 'multi' value so it should not reach that error case.The callers are also a bit inconsistent in their expectations.heap_lock_tuple() pfrees the 'members' array if it's not-NULL, otherspfree() it if "nmembers >= 0", and others if "nmembers > 0". That'snot a live bug either, because the function should never return 0, butadd an Assert for that to make it more clear. I left the callers alonefor now.I also moved the line where we set *nmembers. It wasn't wrong before,but I like to do that right next to the 'return' statement, to make itclear that it's always set on return.Also remove one unreachable return statement after ereport(ERROR), forbrevity and for consistency with the similar if-block right after it.Author: Greg Nancarrow with the additional changes by meBackpatch-through: 9.6, all supported versions
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‎src/backend/access/transam/multixact.c

Lines changed: 8 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -1241,7 +1241,10 @@ GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
12411241
debug_elog3(DEBUG2,"GetMembers: asked for %u",multi);
12421242

12431243
if (!MultiXactIdIsValid(multi)||from_pgupgrade)
1244+
{
1245+
*members=NULL;
12441246
return-1;
1247+
}
12451248

12461249
/* See if the MultiXactId is in the local cache */
12471250
length=mXactCacheGetById(multi,members);
@@ -1292,13 +1295,10 @@ GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
12921295
LWLockRelease(MultiXactGenLock);
12931296

12941297
if (MultiXactIdPrecedes(multi,oldestMXact))
1295-
{
12961298
ereport(ERROR,
12971299
(errcode(ERRCODE_INTERNAL_ERROR),
12981300
errmsg("MultiXactId %u does no longer exist -- apparent wraparound",
12991301
multi)));
1300-
return-1;
1301-
}
13021302

13031303
if (!MultiXactIdPrecedes(multi,nextMXact))
13041304
ereport(ERROR,
@@ -1398,7 +1398,6 @@ GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
13981398
LWLockRelease(MultiXactOffsetSLRULock);
13991399

14001400
ptr= (MultiXactMember*)palloc(length*sizeof(MultiXactMember));
1401-
*members=ptr;
14021401

14031402
/* Now get the members themselves. */
14041403
LWLockAcquire(MultiXactMemberSLRULock,LW_EXCLUSIVE);
@@ -1443,13 +1442,17 @@ GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
14431442

14441443
LWLockRelease(MultiXactMemberSLRULock);
14451444

1445+
/* A multixid with zero members should not happen */
1446+
Assert(truelength>0);
1447+
14461448
/*
14471449
* Copy the result into the local cache.
14481450
*/
14491451
mXactCachePut(multi,truelength,ptr);
14501452

14511453
debug_elog3(DEBUG2,"GetMembers: no cache for %s",
14521454
mxid_to_string(multi,truelength,ptr));
1455+
*members=ptr;
14531456
returntruelength;
14541457
}
14551458

@@ -1550,7 +1553,6 @@ mXactCacheGetById(MultiXactId multi, MultiXactMember **members)
15501553

15511554
size=sizeof(MultiXactMember)*entry->nmembers;
15521555
ptr= (MultiXactMember*)palloc(size);
1553-
*members=ptr;
15541556

15551557
memcpy(ptr,entry->members,size);
15561558

@@ -1566,6 +1568,7 @@ mXactCacheGetById(MultiXactId multi, MultiXactMember **members)
15661568
*/
15671569
dlist_move_head(&MXactCache,iter.cur);
15681570

1571+
*members=ptr;
15691572
returnentry->nmembers;
15701573
}
15711574
}

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