Clamp result of MultiXactMemberFreezeThreshold

The purpose of the function is to reduce the effective
autovacuum_multixact_freeze_max_age if the multixact members SLRU is
approaching wraparound, to make multixid freezing more aggressive.
The returned value should therefore never be greater than plain
autovacuum_multixact_freeze_max_age.

Reviewed-by: Robert Haas
Discussion: https://www.postgresql.org/message-id/85fb354c-f89f-4d47-b3a2-3cbd461c90a3@iki.fi
Backpatch-through: 12, all supported versions

src/backend/access/transam/multixact.c

@@ -2828,6 +2828,7 @@ MultiXactMemberFreezeThreshold(void)
 	uint32		multixacts;
 	uint32		victim_multixacts;
 	double		fraction;
+	int			result;
 
 	/* If we can't determine member space utilization, assume the worst. */
 	if (!ReadMultiXactCounts(&multixacts, &members))
@@ -2849,7 +2850,13 @@ MultiXactMemberFreezeThreshold(void)
 	/* fraction could be > 1.0, but lowest possible freeze age is zero */
 	if (victim_multixacts > multixacts)
 		return 0;
-	return multixacts - victim_multixacts;
+	result = multixacts - victim_multixacts;
+
+	/*
+	 * Clamp to autovacuum_multixact_freeze_max_age, so that we never make
+	 * autovacuum less aggressive than it would otherwise be.
+	 */
+	return Min(result, autovacuum_multixact_freeze_max_age);
 }
 
 typedef struct mxtruncinfo