Somebody seems to have thought they could get away without checking for

rsinfo->expectedDesc == NULL in deflist_to_tuplestore(), but that doesn't
look very safe to me.  Noted in passing while studying problem report
from Greg Davidson.

src/backend/foreign/foreign.c

@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
  *
  * IDENTIFICATION
- *		  $PostgreSQL: pgsql/src/backend/foreign/foreign.c,v 1.4 2009/06/11 14:48:57 momjian Exp $
+ *		  $PostgreSQL: pgsql/src/backend/foreign/foreign.c,v 1.5 2009/06/11 16:14:18 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -279,7 +279,8 @@ deflist_to_tuplestore(ReturnSetInfo *rsinfo, List *options)
 		ereport(ERROR,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 				 errmsg("set-valued function called in context that cannot accept a set")));
-	if (!(rsinfo->allowedModes & SFRM_Materialize))
+	if (!(rsinfo->allowedModes & SFRM_Materialize) ||
+		rsinfo->expectedDesc == NULL)
 		ereport(ERROR,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 				 errmsg("materialize mode required, but it is not allowed in this context")));