From d24c5658a80c8f5037e9e1948de311d3f3350f12 Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Thu, 17 Jun 2021 14:50:42 +0300
Subject: [PATCH] Tidy up GetMultiXactIdMembers()'s behavior on error

One of the error paths left *members uninitialized. That's not a live
bug, because most callers don't look at *members when the function
returns -1, but let's be tidy. One caller, in heap_lock_tuple(), does
"if (members != NULL) pfree(members)", but AFAICS it never passes an
invalid '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, others
pfree() it if "nmembers >= 0", and others if "nmembers > 0". That's
not a live bug either, because the function should never return 0, but
add an Assert for that to make it more clear. I left the callers alone
for 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 it
clear that it's always set on return.

Also remove one unreachable return statement after ereport(ERROR), for
brevity and for consistency with the similar if-block right after it.

Author: Greg Nancarrow with the additional changes by me
Backpatch-through: 9.6, all supported versions
---
 src/backend/access/transam/multixact.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index daab546f29..b643564f16 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -1241,7 +1241,10 @@ GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
 	debug_elog3(DEBUG2, "GetMembers: asked for %u", multi);
 
 	if (!MultiXactIdIsValid(multi) || from_pgupgrade)
+	{
+		*members = NULL;
 		return -1;
+	}
 
 	/* See if the MultiXactId is in the local cache */
 	length = mXactCacheGetById(multi, members);
@@ -1292,13 +1295,10 @@ GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
 	LWLockRelease(MultiXactGenLock);
 
 	if (MultiXactIdPrecedes(multi, oldestMXact))
-	{
 		ereport(ERROR,
 				(errcode(ERRCODE_INTERNAL_ERROR),
 				 errmsg("MultiXactId %u does no longer exist -- apparent wraparound",
 						multi)));
-		return -1;
-	}
 
 	if (!MultiXactIdPrecedes(multi, nextMXact))
 		ereport(ERROR,
@@ -1398,7 +1398,6 @@ retry:
 	LWLockRelease(MultiXactOffsetSLRULock);
 
 	ptr = (MultiXactMember *) palloc(length * sizeof(MultiXactMember));
-	*members = ptr;
 
 	/* Now get the members themselves. */
 	LWLockAcquire(MultiXactMemberSLRULock, LW_EXCLUSIVE);
@@ -1443,6 +1442,9 @@ retry:
 
 	LWLockRelease(MultiXactMemberSLRULock);
 
+	/* A multixid with zero members should not happen */
+	Assert(truelength > 0);
+
 	/*
 	 * Copy the result into the local cache.
 	 */
@@ -1450,6 +1452,7 @@ retry:
 
 	debug_elog3(DEBUG2, "GetMembers: no cache for %s",
 				mxid_to_string(multi, truelength, ptr));
+	*members = ptr;
 	return truelength;
 }
 
@@ -1550,7 +1553,6 @@ mXactCacheGetById(MultiXactId multi, MultiXactMember **members)
 
 			size = sizeof(MultiXactMember) * entry->nmembers;
 			ptr = (MultiXactMember *) palloc(size);
-			*members = ptr;
 
 			memcpy(ptr, entry->members, size);
 
@@ -1566,6 +1568,7 @@ mXactCacheGetById(MultiXactId multi, MultiXactMember **members)
 			 */
 			dlist_move_head(&MXactCache, iter.cur);
 
+			*members = ptr;
 			return entry->nmembers;
 		}
 	}