Refactor ReadMultiXactCounts() into GetMultiXactInfo()

This provides a single entry point to access some information about the
state of MultiXacts, able to return some data about multixacts offsets
and counts.  Originally this function was only able to return some
information about the number of multixacts and multixact members,
extended here to provide some data about the oldest multixact ID in use
and the oldest offset, if known.

This change has been proposed in a patch that aims at providing more
monitoring capabilities for multixacts, and it is useful on its own.
GetMultiXactInfo() is added to multixact.h, becoming available for
out-of-core code.

Extracted from a larger patch by the same author.

Author: Naga Appani <nagnrik@gmail.com>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CA+QeY+AAsYK6WvBW4qYzHz4bahHycDAY_q5ECmHkEV_eB9ckzg@mail.gmail.com

src/backend/access/transam/multixact.c

@@ -2852,31 +2852,43 @@ find_multixact_start(MultiXactId multi, MultiXactOffset *result)
 }
 
 /*
- * Determine how many multixacts, and how many multixact members, currently
- * exist.  Return false if unable to determine.
+ * GetMultiXactInfo
+ *
+ * Returns information about the current MultiXact state, as of:
+ * multixacts: Number of MultiXacts (nextMultiXactId - oldestMultiXactId)
+ * members: Number of member entries (nextOffset - oldestOffset)
+ * oldestMultiXactId: Oldest MultiXact ID still in use
+ * oldestOffset: Oldest offset still in use
+ *
+ * Returns false if unable to determine, the oldest offset being unknown.
  */
-static bool
-ReadMultiXactCounts(uint32 *multixacts, MultiXactOffset *members)
+bool
+GetMultiXactInfo(uint32 *multixacts, MultiXactOffset *members,
+				 MultiXactId *oldestMultiXactId, MultiXactOffset *oldestOffset)
 {
 	MultiXactOffset nextOffset;
-	MultiXactOffset oldestOffset;
-	MultiXactId oldestMultiXactId;
 	MultiXactId nextMultiXactId;
 	bool		oldestOffsetKnown;
 
 	LWLockAcquire(MultiXactGenLock, LW_SHARED);
 	nextOffset = MultiXactState->nextOffset;
-	oldestMultiXactId = MultiXactState->oldestMultiXactId;
+	*oldestMultiXactId = MultiXactState->oldestMultiXactId;
 	nextMultiXactId = MultiXactState->nextMXact;
-	oldestOffset = MultiXactState->oldestOffset;
+	*oldestOffset = MultiXactState->oldestOffset;
 	oldestOffsetKnown = MultiXactState->oldestOffsetKnown;
 	LWLockRelease(MultiXactGenLock);
 
 	if (!oldestOffsetKnown)
+	{
+		*members = 0;
+		*multixacts = 0;
+		*oldestMultiXactId = InvalidMultiXactId;
+		*oldestOffset = 0;
 		return false;
+	}
 
-	*members = nextOffset - oldestOffset;
-	*multixacts = nextMultiXactId - oldestMultiXactId;
+	*members = nextOffset - *oldestOffset;
+	*multixacts = nextMultiXactId - *oldestMultiXactId;
 	return true;
 }
 
@@ -2915,9 +2927,11 @@ MultiXactMemberFreezeThreshold(void)
 	uint32		victim_multixacts;
 	double		fraction;
 	int			result;
+	MultiXactId oldestMultiXactId;
+	MultiXactOffset oldestOffset;
 
 	/* If we can't determine member space utilization, assume the worst. */
-	if (!ReadMultiXactCounts(&multixacts, &members))
+	if (!GetMultiXactInfo(&multixacts, &members, &oldestMultiXactId, &oldestOffset))
 		return 0;
 
 	/* If member space utilization is low, no special action is required. */

src/include/access/multixact.h

@@ -111,6 +111,9 @@ extern bool MultiXactIdIsRunning(MultiXactId multi, bool isLockOnly);
 extern void MultiXactIdSetOldestMember(void);
 extern int	GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
 								  bool from_pgupgrade, bool isLockOnly);
+extern bool GetMultiXactInfo(uint32 *multixacts, MultiXactOffset *members,
+							 MultiXactId *oldestMultiXactId,
+							 MultiXactOffset *oldestOffset);
 extern bool MultiXactIdPrecedes(MultiXactId multi1, MultiXactId multi2);
 extern bool MultiXactIdPrecedesOrEquals(MultiXactId multi1,
 										MultiXactId multi2);