Avoid consuming an XID during vac_truncate_clog().

vac_truncate_clog() uses its own transaction ID as the comparison point in
a sanity check that no database's datfrozenxid has already wrapped around
"into the future".  That was probably fine when written, but in a lazy
vacuum we won't have assigned an XID, so calling GetCurrentTransactionId()
causes an XID to be assigned when otherwise one would not be.  Most of the
time that's not a big problem ... but if we are hard up against the
wraparound limit, consuming XIDs during antiwraparound vacuums is a very
bad thing.

Instead, use ReadNewTransactionId(), which not only avoids this problem
but is in itself a better comparison point to test whether wraparound
has already occurred.

Report and patch by Alexander Korotkov.  Back-patch to all versions.

Report: <CAPpHfdspOkmiQsxh-UZw2chM6dRMwXAJGEmmbmqYR=yvM7-s6A@mail.gmail.com>

src/backend/commands/vacuum.c

@@ -802,7 +802,7 @@ vac_update_datfrozenxid(void)
 static void
 vac_truncate_clog(TransactionId frozenXID)
 {
-	TransactionId myXID = GetCurrentTransactionId();
+	TransactionId nextXID = ReadNewTransactionId();
 	Relation	relation;
 	HeapScanDesc scan;
 	HeapTuple	tuple;
@@ -834,7 +834,7 @@ vac_truncate_clog(TransactionId frozenXID)
 
 		Assert(TransactionIdIsNormal(dbform->datfrozenxid));
 
-		if (TransactionIdPrecedes(myXID, dbform->datfrozenxid))
+		if (TransactionIdPrecedes(nextXID, dbform->datfrozenxid))
 			frozenAlreadyWrapped = true;
 		else if (TransactionIdPrecedes(dbform->datfrozenxid, frozenXID))
 		{