From: Michael Paquier <michael@paquier.xyz>
Date: Tue, 30 Dec 2025 05:03:49 +0000 (+0900)
Subject: Change GetMultiXactInfo() to return the next multixact offset
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=9cf746a453c15ffdf652a6d50683bfd82e654950;p=thirdparty%2Fpostgresql.git

Change GetMultiXactInfo() to return the next multixact offset

This routine returned a number of members as a MultiXactOffset,
calculated based on the difference between the next-to-be-assigned
offset and the oldest offset.  However, this number is not actually an
offset but a number.

This type confusion comes from the original implementation of
MultiXactMemberFreezeThreshold(), in 53bb309d2d5a.  The number of
members is now defined as a uint64, large enough for MultiXactOffset.
This change will be used in a follow-up patch.

Reviewed-by: Naga Appani <nagnrik@gmail.com>
Discussion: https://postgr.es/m/aUyTvZMq2CLgNEB4@paquier.xyz
---

diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index 34956a5a663..0d6f594e2a0 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -2461,25 +2461,23 @@ find_multixact_start(MultiXactId multi, MultiXactOffset *result)
  *
  * Returns information about the current MultiXact state, as of:
  * multixacts: Number of MultiXacts (nextMultiXactId - oldestMultiXactId)
- * members: Number of member entries (nextOffset - oldestOffset)
+ * nextOffset: Next-to-be-assigned offset
  * oldestMultiXactId: Oldest MultiXact ID still in use
  * oldestOffset: Oldest offset still in use
  */
 void
-GetMultiXactInfo(uint32 *multixacts, MultiXactOffset *members,
+GetMultiXactInfo(uint32 *multixacts, MultiXactOffset *nextOffset,
 				 MultiXactId *oldestMultiXactId, MultiXactOffset *oldestOffset)
 {
-	MultiXactOffset nextOffset;
 	MultiXactId nextMultiXactId;
 
 	LWLockAcquire(MultiXactGenLock, LW_SHARED);
-	nextOffset = MultiXactState->nextOffset;
+	*nextOffset = MultiXactState->nextOffset;
 	*oldestMultiXactId = MultiXactState->oldestMultiXactId;
 	nextMultiXactId = MultiXactState->nextMXact;
 	*oldestOffset = MultiXactState->oldestOffset;
 	LWLockRelease(MultiXactGenLock);
 
-	*members = nextOffset - *oldestOffset;
 	*multixacts = nextMultiXactId - *oldestMultiXactId;
 }
 
@@ -2514,16 +2512,18 @@ GetMultiXactInfo(uint32 *multixacts, MultiXactOffset *members,
 int
 MultiXactMemberFreezeThreshold(void)
 {
-	MultiXactOffset members;
 	uint32		multixacts;
 	uint32		victim_multixacts;
 	double		fraction;
 	int			result;
 	MultiXactId oldestMultiXactId;
 	MultiXactOffset oldestOffset;
+	MultiXactOffset nextOffset;
+	uint64		members;
 
-	/* Read the current offsets and members usage. */
-	GetMultiXactInfo(&multixacts, &members, &oldestMultiXactId, &oldestOffset);
+	/* Read the current offsets and multixact usage. */
+	GetMultiXactInfo(&multixacts, &nextOffset, &oldestMultiXactId, &oldestOffset);
+	members = nextOffset - oldestOffset;
 
 	/* If member space utilization is low, no special action is required. */
 	if (members <= MULTIXACT_MEMBER_LOW_THRESHOLD)
diff --git a/src/include/access/multixact.h b/src/include/access/multixact.h
index 6433fe16364..d22abbb7251 100644
--- a/src/include/access/multixact.h
+++ b/src/include/access/multixact.h
@@ -109,7 +109,7 @@ extern bool MultiXactIdIsRunning(MultiXactId multi, bool isLockOnly);
 extern void MultiXactIdSetOldestMember(void);
 extern int	GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
 								  bool from_pgupgrade, bool isLockOnly);
-extern void GetMultiXactInfo(uint32 *multixacts, MultiXactOffset *members,
+extern void GetMultiXactInfo(uint32 *multixacts, MultiXactOffset *nextOffset,
 							 MultiXactId *oldestMultiXactId,
 							 MultiXactOffset *oldestOffset);
 extern bool MultiXactIdPrecedes(MultiXactId multi1, MultiXactId multi2);