MDEV-12669 Circular foreign keys cause a loop and OOM upon LOCK TABLE

table_already_fk_prelocked() was looking for a table in the wrong
list (not the complete list of prelocked tables, but only in its tail,
starting from the current table - which is always empty for the last
added table), so for circular FKs it kept adding same tables to the list
indefinitely.

mysql-test/suite/innodb/r/foreign_key.result

@@ -150,3 +150,22 @@ unique_constraint_name
 PRIMARY
 DROP TABLE t2;
 DROP TABLE t1;
+SET FOREIGN_KEY_CHECKS=0;
+CREATE TABLE staff (
+staff_id TINYINT UNSIGNED NOT NULL AUTO_INCREMENT,
+store_id TINYINT UNSIGNED NOT NULL,
+PRIMARY KEY  (staff_id),
+KEY idx_fk_store_id (store_id),
+CONSTRAINT fk_staff_store FOREIGN KEY (store_id) REFERENCES store (store_id) ON DELETE RESTRICT ON UPDATE CASCADE
+) ENGINE=InnoDB;
+CREATE TABLE store (
+store_id TINYINT UNSIGNED NOT NULL AUTO_INCREMENT,
+manager_staff_id TINYINT UNSIGNED NOT NULL,
+PRIMARY KEY  (store_id),
+UNIQUE KEY idx_unique_manager (manager_staff_id),
+CONSTRAINT fk_store_staff FOREIGN KEY (manager_staff_id) REFERENCES staff (staff_id) ON DELETE RESTRICT ON UPDATE CASCADE
+) ENGINE=InnoDB;
+SET FOREIGN_KEY_CHECKS=DEFAULT;
+LOCK TABLE staff WRITE;
+UNLOCK TABLES;
+DROP TABLES staff, store;

mysql-test/suite/innodb/t/foreign_key.test

@@ -113,3 +113,27 @@ WHERE table_name = 't2';
 
 DROP TABLE t2;
 DROP TABLE t1;
+
+#
+# MDEV-12669 Circular foreign keys cause a loop and OOM upon LOCK TABLE
+#
+SET FOREIGN_KEY_CHECKS=0;
+CREATE TABLE staff (
+  staff_id TINYINT UNSIGNED NOT NULL AUTO_INCREMENT,
+  store_id TINYINT UNSIGNED NOT NULL,
+  PRIMARY KEY  (staff_id),
+  KEY idx_fk_store_id (store_id),
+  CONSTRAINT fk_staff_store FOREIGN KEY (store_id) REFERENCES store (store_id) ON DELETE RESTRICT ON UPDATE CASCADE
+) ENGINE=InnoDB;
+CREATE TABLE store (
+  store_id TINYINT UNSIGNED NOT NULL AUTO_INCREMENT,
+  manager_staff_id TINYINT UNSIGNED NOT NULL,
+  PRIMARY KEY  (store_id),
+  UNIQUE KEY idx_unique_manager (manager_staff_id),
+  CONSTRAINT fk_store_staff FOREIGN KEY (manager_staff_id) REFERENCES staff (staff_id) ON DELETE RESTRICT ON UPDATE CASCADE
+) ENGINE=InnoDB;
+SET FOREIGN_KEY_CHECKS=DEFAULT;
+
+LOCK TABLE staff WRITE;
+UNLOCK TABLES;
+DROP TABLES staff, store;

sql/sql_base.cc

@@ -4255,8 +4255,9 @@ handle_table(THD *thd, Query_tables_list *prelocking_ctx,
         else
           lock_type= TL_READ;
 
-        if (table_already_fk_prelocked(table_list, fk->foreign_db,
-                                       fk->foreign_table, lock_type))
+        if (table_already_fk_prelocked(prelocking_ctx->query_tables,
+                                       fk->foreign_db, fk->foreign_table,
+                                       lock_type))
           continue;
 
         TABLE_LIST *tl= (TABLE_LIST *) thd->alloc(sizeof(TABLE_LIST));