Fix GetNewTransactionId()'s interaction with xidVacLimit.

Commit ad30805 switched to returning a FullTransactionId, but failed to load the potentially updated value in the case where xidVacLimit is reached and we release and reacquire the lock. Repair, closing bug #15727. While reviewing that commit, also fix the size computation used by EstimateTransactionStateSize() and switch to the mul_size() macro traditionally used in such expressions. Author: Thomas Munro Reported-by: Roman Zharkov Discussion: https://postgr.es/m/15727-0be246e7d852d229%40postgresql.org

Author: macdice

src/backend/access/transam/varsup.c

@@ -159,7 +159,8 @@ GetNewTransactionId(bool isSubXact)
 
 /* Re-acquire lock and start over */
 LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
-xid = XidFromFullTransactionId(ShmemVariableCache->nextFullXid);
+full_xid = ShmemVariableCache->nextFullXid;
+xid = XidFromFullTransactionId(full_xid);
 }
 
 /*

src/backend/access/transam/xact.c

@@ -5151,7 +5151,7 @@ EstimateTransactionStateSpace(void)
 nxids = add_size(nxids, s->nChildXids);
 }
 
-return add_size(size, sizeof(SerializedTransactionState) * nxids);
+return add_size(size, mul_size(sizeof(TransactionId), nxids));
 }
 
 /*