Don't process multi xmax in FreezeMultiXactId() if it is already marked as invalid.

Hello!

The src/backend/access/heap/README.tuplock says about HEAP_XMAX_INVALID bit
that "Any tuple with this bit set does not have a valid value stored in XMAX."

Found that FreezeMultiXactId() tries to process such an invalid multi xmax
and may looks for an update xid in the pg_multixact for it.

Maybe not do this work in FreezeMultiXactId() and exit immediately if the
bit HEAP_XMAX_INVALID was already set?

For instance, like that:

master
@@ -6215,6 +6215,15 @@ FreezeMultiXactId(MultiXactId multi, uint16 t_infomask,
/* We should only be called in Multis */
Assert(t_infomask & HEAP_XMAX_IS_MULTI);

+ /* Xmax is already marked as invalid */
+ if (MultiXactIdIsValid(multi) &&
+ (t_infomask & HEAP_XMAX_INVALID))
+ {
+ *flags |= FRM_INVALIDATE_XMAX;
+ pagefrz->freeze_required = true;
+ return InvalidTransactionId;
+ }
+
if (!MultiXactIdIsValid(multi) ||
HEAP_LOCKED_UPGRADED(t_infomask))

With the best regards,

--
Anton A. Melnikov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company