Enforce REPEATABLE READ since currently we export snap only once in xact

There is also no distribute predicate locking support, so SERIALIZABLE is not
the way to go too.

Author: arssher

contrib/postgres_fdw/connection.c

@@ -460,6 +460,10 @@ begin_remote_xact(ConnCacheEntry *entry)
 		elog(DEBUG3, "starting remote transaction on connection %p",
 			 entry->conn);
 
+		if (UseGlobalSnapshots && (!IsolationUsesXactSnapshot() ||
+								   IsolationIsSerializable()))
+			elog(ERROR, "Global snapshots support only REPEATABLE READ");
+
 		if (IsolationIsSerializable())
 			sql = "START TRANSACTION ISOLATION LEVEL SERIALIZABLE";
 		else

contrib/postgres_fdw/t/001_bank_coordinator.pl

@@ -12,6 +12,7 @@
 	log_checkpoints = true
 	postgres_fdw.use_global_snapshots = on
 	track_global_snapshots = on
+	default_transaction_isolation = 'REPEATABLE READ'
 ));
 $master->start;
 

contrib/postgres_fdw/t/002_bank_participant.pl

@@ -12,6 +12,7 @@
 	postgres_fdw.use_global_snapshots = on
 	global_snapshot_defer_time = 15
 	track_global_snapshots = on
+    default_transaction_isolation = 'REPEATABLE READ'
 ));
 $shard1->start;
 
@@ -22,6 +23,7 @@
 	postgres_fdw.use_global_snapshots = on
 	global_snapshot_defer_time = 15
 	track_global_snapshots = on
+	default_transaction_isolation = 'REPEATABLE READ'
 ));
 $shard2->start;