Error if AssignTransactionId is called by Reader.

QE_READER and QE_ENTRY_DB_SINGLETON are not expected to allocate transaction
IDs. They are just helper sub-processes to WRITER and should be using its xid.

src/backend/access/transam/xact.c

@@ -496,6 +496,13 @@ AssignTransactionId(TransactionState s)
 	Assert(!TransactionIdIsValid(s->transactionId));
 	Assert(s->state == TRANS_INPROGRESS);
 
+	if (DistributedTransactionContext == DTX_CONTEXT_QE_READER ||
+		DistributedTransactionContext == DTX_CONTEXT_QE_ENTRY_DB_SINGLETON)
+	{
+		elog(ERROR, "AssignTransactionId() called by %s process",
+			 DtxContextToString(DistributedTransactionContext));
+	}
+
 	/*
 	 * Ensure parent(s) have XIDs, so that a child always has an XID later
 	 * than its parent.  Musn't recurse here, or we might get a stack overflow