Reject non-ON-SELECT rules that are named "_RETURN".

DefineQueryRewrite() has long required that ON SELECT rules be named
"_RETURN".  But we overlooked the converse case: we should forbid
non-ON-SELECT rules that are named "_RETURN".  In particular this
prevents using CREATE OR REPLACE RULE to overwrite a view's _RETURN
rule with some other kind of rule, thereby breaking the view.

Per bug #17646 from Kui Liu.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/17646-70c93cfa40365776@postgresql.org

src/backend/rewrite/rewriteDefine.c

@@ -524,6 +524,18 @@ DefineQueryRewrite(char *rulename,
 								RelationGetDescr(event_relation),
 								false, false);
 		}
+
+		/*
+		 * And finally, if it's not an ON SELECT rule then it must *not* be
+		 * named _RETURN.  This prevents accidentally or maliciously replacing
+		 * a view's ON SELECT rule with some other kind of rule.
+		 */
+		if (strcmp(rulename, ViewSelectRuleName) == 0)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+					 errmsg("non-view rule for \"%s\" must not be named \"%s\"",
+							RelationGetRelationName(event_relation),
+							ViewSelectRuleName)));
 	}
 
 	/*