Add checks to DefineQueryRewrite() to prohibit attaching rules to relations

that aren't RELKIND_RELATION or RELKIND_VIEW, and to disallow attaching rules
to system relations unless allowSystemTableMods is on.  This is to make the
behavior of CREATE RULE more like CREATE TRIGGER, which disallows the
comparable cases.  Per discussion of bug #4808.

src/backend/rewrite/rewriteDefine.c

@@ -8,13 +8,14 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/rewrite/rewriteDefine.c,v 1.136 2009/01/27 12:40:15 petere Exp $
+ *	  $PostgreSQL: pgsql/src/backend/rewrite/rewriteDefine.c,v 1.137 2009/05/13 22:32:55 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
 
 #include "access/heapam.h"
+#include "catalog/catalog.h"
 #include "catalog/dependency.h"
 #include "catalog/indexing.h"
 #include "catalog/namespace.h"
@@ -242,6 +243,22 @@ DefineQueryRewrite(char *rulename,
 	 */
 	event_relation = heap_open(event_relid, AccessExclusiveLock);
 
+	/*
+	 * Verify relation is of a type that rules can sensibly be applied to.
+	 */
+	if (event_relation->rd_rel->relkind != RELKIND_RELATION &&
+		event_relation->rd_rel->relkind != RELKIND_VIEW)
+		ereport(ERROR,
+				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				 errmsg("\"%s\" is not a table or view",
+						RelationGetRelationName(event_relation))));
+
+	if (!allowSystemTableMods && IsSystemRelation(event_relation))
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+				 errmsg("permission denied: \"%s\" is a system catalog",
+						RelationGetRelationName(event_relation))));
+
 	/*
 	 * Check user has permission to apply rules to this relation.
 	 */