Fix bug that could try to freeze running multixacts.

Commits 801c2dc7 and 801c2dc7 made it possible for vacuum to
try to freeze a multixact that is still running.  That was
prevented by a check, but raised an error.  Repair.

Back-patch all the way.

Author: Nathan Bossart, Jeremy Schneider
Reported-by: Jeremy Schneider
Reviewed-by: Jim Nasby, Thomas Munro
Discussion: https://postgr.es/m/DAFB8AFF-2F05-4E33-AD7F-FF8B0F760C17%40amazon.com

src/backend/commands/vacuum.c

@@ -893,6 +893,7 @@ vacuum_set_xid_limits(Relation rel,
 	int			effective_multixact_freeze_max_age;
 	TransactionId limit;
 	TransactionId safeLimit;
+	MultiXactId oldestMxact;
 	MultiXactId mxactLimit;
 	MultiXactId safeMxactLimit;
 
@@ -970,7 +971,8 @@ vacuum_set_xid_limits(Relation rel,
 	Assert(mxid_freezemin >= 0);
 
 	/* compute the cutoff multi, being careful to generate a valid value */
-	mxactLimit = GetOldestMultiXactId() - mxid_freezemin;
+	oldestMxact = GetOldestMultiXactId();
+	mxactLimit = oldestMxact - mxid_freezemin;
 	if (mxactLimit < FirstMultiXactId)
 		mxactLimit = FirstMultiXactId;
 
@@ -984,7 +986,11 @@ vacuum_set_xid_limits(Relation rel,
 		ereport(WARNING,
 				(errmsg("oldest multixact is far in the past"),
 				 errhint("Close open transactions with multixacts soon to avoid wraparound problems.")));
-		mxactLimit = safeMxactLimit;
+		/* Use the safe limit, unless an older mxact is still running */
+		if (MultiXactIdPrecedes(oldestMxact, safeMxactLimit))
+			mxactLimit = oldestMxact;
+		else
+			mxactLimit = safeMxactLimit;
 	}
 
 	*multiXactCutoff = mxactLimit;