MDEV-13407 innodb.drop_table_background failed in buildbot with "Tablespace for table exists"

The InnoDB background DROP TABLE queue is something that we should
really remove, but are unable to until we remove dict_operation_lock
so that DDL and DML operations can be combined in a single transaction.

Because the queue is not persistent, it is not crash-safe. In stable
versions of MariaDB, we can only try harder to drop all enqueued
tables before server shutdown.

row_mysql_drop_t::table_id: Replaces table_name.

row_drop_tables_for_mysql_in_background():
Do not remove the entry from the list as long as the table exists.
In this way, the table should eventually be dropped.

Author: dr-m

storage/innobase/row/row0mysql.cc

@@ -73,7 +73,7 @@ UNIV_INTERN ibool	row_rollback_on_timeout	= FALSE;
 
 /** Chain node of the list of tables to drop in the background. */
 struct row_mysql_drop_t{
-	char*				table_name;	/*!< table name */
+	table_id_t			table_id;	/*!< table id */
 	UT_LIST_NODE_T(row_mysql_drop_t)row_mysql_drop_list;
 							/*!< list chain node */
 };
@@ -136,19 +136,6 @@ row_mysql_is_system_table(
 	       || 0 == strcmp(name + 6, "db"));
 }
 
-/*********************************************************************//**
-If a table is not yet in the drop list, adds the table to the list of tables
-which the master thread drops in background. We need this on Unix because in
-ALTER TABLE MySQL may call drop table even if the table has running queries on
-it. Also, if there are running foreign key checks on the table, we drop the
-table lazily.
-@return	TRUE if the table was not yet in the drop list, and was added there */
-static
-ibool
-row_add_table_to_background_drop_list(
-/*==================================*/
-	const char*	name);	/*!< in: table name */
-
 /*******************************************************************//**
 Delays an INSERT, DELETE or UPDATE operation if the purge is lagging. */
 static
@@ -2727,7 +2714,7 @@ row_drop_tables_for_mysql_in_background(void)
 	mutex_enter(&row_drop_list_mutex);
 
 	ut_a(row_mysql_drop_list_inited);
-
+next:
 	drop = UT_LIST_GET_FIRST(row_mysql_drop_list);
 
 	n_tables = UT_LIST_GET_LEN(row_mysql_drop_list);
@@ -2740,62 +2727,39 @@ row_drop_tables_for_mysql_in_background(void)
 		return(n_tables + n_tables_dropped);
 	}
 
-	DBUG_EXECUTE_IF("row_drop_tables_in_background_sleep",
-		os_thread_sleep(5000000);
-	);
-
-	table = dict_table_open_on_name(drop->table_name, FALSE, FALSE,
-					DICT_ERR_IGNORE_NONE);
-
-	if (table == NULL) {
-		/* If for some reason the table has already been dropped
-		through some other mechanism, do not try to drop it */
+	table = dict_table_open_on_id(drop->table_id, FALSE,
+				      DICT_TABLE_OP_NORMAL);
 
-		goto already_dropped;
+	if (!table) {
+		n_tables_dropped++;
+		mutex_enter(&row_drop_list_mutex);
+		UT_LIST_REMOVE(row_mysql_drop_list, row_mysql_drop_list, drop);
+		MONITOR_DEC(MONITOR_BACKGROUND_DROP_TABLE);
+		ut_free(drop);
+		goto next;
 	}
 
+	ut_a(!table->can_be_evicted);
+
 	if (!table->to_be_dropped) {
-		/* There is a scenario: the old table is dropped
-		just after it's added into drop list, and new
-		table with the same name is created, then we try
-		to drop the new table in background. */
 		dict_table_close(table, FALSE, FALSE);
 
-		goto already_dropped;
+		mutex_enter(&row_drop_list_mutex);
+		UT_LIST_REMOVE(row_mysql_drop_list, row_mysql_drop_list, drop);
+		UT_LIST_ADD_LAST(row_mysql_drop_list, row_mysql_drop_list,
+				 drop);
+		goto next;
 	}
 
-	ut_a(!table->can_be_evicted);
-
 	dict_table_close(table, FALSE, FALSE);
 
 	if (DB_SUCCESS != row_drop_table_for_mysql_in_background(
-		    drop->table_name)) {
+		    table->name)) {
 		/* If the DROP fails for some table, we return, and let the
 		main thread retry later */
-
 		return(n_tables + n_tables_dropped);
 	}
 
-	n_tables_dropped++;
-
-already_dropped:
-	mutex_enter(&row_drop_list_mutex);
-
-	UT_LIST_REMOVE(row_mysql_drop_list, row_mysql_drop_list, drop);
-
-	MONITOR_DEC(MONITOR_BACKGROUND_DROP_TABLE);
-
-	ut_print_timestamp(stderr);
-	fputs("  InnoDB: Dropped table ", stderr);
-	ut_print_name(stderr, NULL, TRUE, drop->table_name);
-	fputs(" in background drop queue.\n", stderr);
-
-	mem_free(drop->table_name);
-
-	mem_free(drop);
-
-	mutex_exit(&row_drop_list_mutex);
-
 	goto loop;
 }
 
@@ -2827,14 +2791,13 @@ which the master thread drops in background. We need this on Unix because in
 ALTER TABLE MySQL may call drop table even if the table has running queries on
 it. Also, if there are running foreign key checks on the table, we drop the
 table lazily.
-@return	TRUE if the table was not yet in the drop list, and was added there */
+@return	whether background DROP TABLE was scheduled for the first time */
 static
-ibool
-row_add_table_to_background_drop_list(
-/*==================================*/
-	const char*	name)	/*!< in: table name */
+bool
+row_add_table_to_background_drop_list(table_id_t table_id)
 {
 	row_mysql_drop_t*	drop;
+	bool			added = true;
 
 	mutex_enter(&row_drop_list_mutex);
 
@@ -2845,31 +2808,21 @@ row_add_table_to_background_drop_list(
 	     drop != NULL;
 	     drop = UT_LIST_GET_NEXT(row_mysql_drop_list, drop)) {
 
-		if (strcmp(drop->table_name, name) == 0) {
-			/* Already in the list */
-
-			mutex_exit(&row_drop_list_mutex);
-
-			return(FALSE);
+		if (drop->table_id == table_id) {
+			added = false;
+			goto func_exit;
 		}
 	}
 
-	drop = static_cast<row_mysql_drop_t*>(
-		mem_alloc(sizeof(row_mysql_drop_t)));
-
-	drop->table_name = mem_strdup(name);
+	drop = static_cast<row_mysql_drop_t*>(ut_malloc(sizeof *drop));
+	drop->table_id = table_id;
 
 	UT_LIST_ADD_LAST(row_mysql_drop_list, row_mysql_drop_list, drop);
 
 	MONITOR_INC(MONITOR_BACKGROUND_DROP_TABLE);
-
-	/*	fputs("InnoDB: Adding table ", stderr);
-	ut_print_name(stderr, trx, TRUE, drop->table_name);
-	fputs(" to background drop list\n", stderr); */
-
+func_exit:
 	mutex_exit(&row_drop_list_mutex);
-
-	return(TRUE);
+	return added;
 }
 
 /*********************************************************************//**
@@ -4043,7 +3996,7 @@ row_drop_table_for_mysql(
 
 
 	DBUG_EXECUTE_IF("row_drop_table_add_to_background",
-		row_add_table_to_background_drop_list(table->name);
+		row_add_table_to_background_drop_list(table->id);
 		err = DB_SUCCESS;
 		goto funct_exit;
 	);
@@ -4055,33 +4008,22 @@ row_drop_table_for_mysql(
 	checks take an IS or IX lock on the table. */
 
 	if (table->n_foreign_key_checks_running > 0) {
-
-		const char*	save_tablename = table->name;
-		ibool		added;
-
-		added = row_add_table_to_background_drop_list(save_tablename);
-
-		if (added) {
+		if (row_add_table_to_background_drop_list(table->id)) {
 			ut_print_timestamp(stderr);
 			fputs("  InnoDB: You are trying to drop table ",
 			      stderr);
-			ut_print_name(stderr, trx, TRUE, save_tablename);
+			ut_print_name(stderr, trx, TRUE, table->name);
 			fputs("\n"
 			      "InnoDB: though there is a"
 			      " foreign key check running on it.\n"
 			      "InnoDB: Adding the table to"
 			      " the background drop queue.\n",
 			      stderr);
-
-			/* We return DB_SUCCESS to MySQL though the drop will
-			happen lazily later */
-
-			err = DB_SUCCESS;
-		} else {
-			/* The table is already in the background drop list */
-			err = DB_ERROR;
 		}
 
+		/* We return DB_SUCCESS to MySQL though the drop will
+		happen lazily later */
+		err = DB_SUCCESS;
 		goto funct_exit;
 	}
 
@@ -4103,11 +4045,7 @@ row_drop_table_for_mysql(
 		lock_remove_all_on_table(table, TRUE);
 		ut_a(table->n_rec_locks == 0);
 	} else if (table->n_ref_count > 0 || table->n_rec_locks > 0) {
-		ibool	added;
-
-		added = row_add_table_to_background_drop_list(table->name);
-
-		if (added) {
+		if (row_add_table_to_background_drop_list(table->id)) {
 			ut_print_timestamp(stderr);
 			fputs("  InnoDB: Warning: MySQL is"
 			      " trying to drop table ", stderr);