Fix ALTER TRIGGER ... RENAME, broken in WITH OIDS removal.

I (Andres) broke this in 578b2297.

Author: Rushabh Lathia
Discussion: https://postgr.es/m/CAGPqQf04PywZX3sVQaF6H=oLiW9GJncRW+=e78vTy4MokEWcZw@mail.gmail.com

src/backend/commands/trigger.c

@@ -1721,14 +1721,14 @@ renametrig(RenameStmt *stmt)
 								NULL, 2, key);
 	if (HeapTupleIsValid(tuple = systable_getnext(tgscan)))
 	{
-		Form_pg_trigger trigform = (Form_pg_trigger) GETSTRUCT(tuple);
-
-		tgoid = trigform->oid;
+		Form_pg_trigger trigform;
 
 		/*
 		 * Update pg_trigger tuple with new tgname.
 		 */
 		tuple = heap_copytuple(tuple);	/* need a modifiable copy */
+		trigform = (Form_pg_trigger) GETSTRUCT(tuple);
+		tgoid = trigform->oid;
 
 		namestrcpy(&trigform->tgname,
 				   stmt->newname);

src/test/regress/expected/triggers.out

@@ -434,7 +434,21 @@ SELECT pg_get_triggerdef(oid, true) FROM pg_trigger WHERE tgrelid = 'main_table'
  CREATE TRIGGER modified_any BEFORE UPDATE OF a ON main_table FOR EACH ROW WHEN (old.* IS DISTINCT FROM new.*) EXECUTE PROCEDURE trigger_func('modified_any')
 (1 row)
 
-DROP TRIGGER modified_a ON main_table;
+-- Test RENAME TRIGGER
+ALTER TRIGGER modified_a ON main_table RENAME TO modified_modified_a;
+SELECT count(*) FROM pg_trigger WHERE tgrelid = 'main_table'::regclass AND tgname = 'modified_a';
+ count 
+-------
+     0
+(1 row)
+
+SELECT count(*) FROM pg_trigger WHERE tgrelid = 'main_table'::regclass AND tgname = 'modified_modified_a';
+ count 
+-------
+     1
+(1 row)
+
+DROP TRIGGER modified_modified_a ON main_table;
 DROP TRIGGER modified_any ON main_table;
 DROP TRIGGER insert_a ON main_table;
 DROP TRIGGER delete_a ON main_table;

src/test/regress/sql/triggers.sql

@@ -284,7 +284,13 @@ SELECT * FROM main_table ORDER BY a, b;
 SELECT pg_get_triggerdef(oid, true) FROM pg_trigger WHERE tgrelid = 'main_table'::regclass AND tgname = 'modified_a';
 SELECT pg_get_triggerdef(oid, false) FROM pg_trigger WHERE tgrelid = 'main_table'::regclass AND tgname = 'modified_a';
 SELECT pg_get_triggerdef(oid, true) FROM pg_trigger WHERE tgrelid = 'main_table'::regclass AND tgname = 'modified_any';
-DROP TRIGGER modified_a ON main_table;
+
+-- Test RENAME TRIGGER
+ALTER TRIGGER modified_a ON main_table RENAME TO modified_modified_a;
+SELECT count(*) FROM pg_trigger WHERE tgrelid = 'main_table'::regclass AND tgname = 'modified_a';
+SELECT count(*) FROM pg_trigger WHERE tgrelid = 'main_table'::regclass AND tgname = 'modified_modified_a';
+
+DROP TRIGGER modified_modified_a ON main_table;
 DROP TRIGGER modified_any ON main_table;
 DROP TRIGGER insert_a ON main_table;
 DROP TRIGGER delete_a ON main_table;