Disallow dropping rules on system tables by default

This was previously not covered by allow_system_table_mods, but now it
is.  The impact in practice is probably low, but this makes it
consistent with most other DDL commands.
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/ee9df1af-c0d8-7c82-5be7-39ce4e3b0a9d%402ndquadrant.com

src/backend/rewrite/rewriteRemove.c

@@ -18,6 +18,7 @@
 #include "access/htup_details.h"
 #include "access/sysattr.h"
 #include "access/table.h"
+#include "catalog/catalog.h"
 #include "catalog/dependency.h"
 #include "catalog/indexing.h"
 #include "catalog/namespace.h"
@@ -28,6 +29,7 @@
 #include "utils/fmgroids.h"
 #include "utils/inval.h"
 #include "utils/lsyscache.h"
+#include "utils/rel.h"
 #include "utils/syscache.h"
 
 /*
@@ -72,6 +74,12 @@ RemoveRewriteRuleById(Oid ruleOid)
 	eventRelationOid = ((Form_pg_rewrite) GETSTRUCT(tuple))->ev_class;
 	event_relation = table_open(eventRelationOid, AccessExclusiveLock);
 
+	if (!allowSystemTableMods && IsSystemRelation(event_relation))
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+				 errmsg("permission denied: \"%s\" is a system catalog",
+						RelationGetRelationName(event_relation))));
+
 	/*
 	 * Now delete the pg_rewrite tuple for the rule
 	 */

src/test/modules/unsafe_tests/expected/alter_system_table.out

@@ -81,7 +81,16 @@ CREATE RULE r1 AS ON INSERT TO pg_description DO INSTEAD NOTHING;
 ERROR:  permission denied: "pg_description" is a system catalog
 ALTER RULE r1 ON pg_description RENAME TO r2;
 ERROR:  permission denied: "pg_description" is a system catalog
---DROP RULE r2 ON pg_description;
+-- now make one to test dropping:
+SET allow_system_table_mods TO on;
+CREATE RULE r2 AS ON INSERT TO pg_description DO INSTEAD NOTHING;
+RESET allow_system_table_mods;
+DROP RULE r2 ON pg_description;
+ERROR:  permission denied: "pg_description" is a system catalog
+-- cleanup:
+SET allow_system_table_mods TO on;
+DROP RULE r2 ON pg_description;
+RESET allow_system_table_mods;
 SET allow_system_table_mods = on;
 -- create new table in pg_catalog
 BEGIN;

src/test/modules/unsafe_tests/sql/alter_system_table.sql

@@ -79,7 +79,15 @@ ALTER TRIGGER t1 ON pg_description RENAME TO t2;
 -- rules
 CREATE RULE r1 AS ON INSERT TO pg_description DO INSTEAD NOTHING;
 ALTER RULE r1 ON pg_description RENAME TO r2;
---DROP RULE r2 ON pg_description;
+-- now make one to test dropping:
+SET allow_system_table_mods TO on;
+CREATE RULE r2 AS ON INSERT TO pg_description DO INSTEAD NOTHING;
+RESET allow_system_table_mods;
+DROP RULE r2 ON pg_description;
+-- cleanup:
+SET allow_system_table_mods TO on;
+DROP RULE r2 ON pg_description;
+RESET allow_system_table_mods;
 
 
 SET allow_system_table_mods = on;