MDEV-17595 - ALTER TABLE ADD FOREIGN KEY crash

ALTER TABLE ... ADD FOREIGN KEY may trigger assertion failure when
it has LOCK=EXCLUSIVE clause or concurrent FLUSH TABLES is being
executed.

In both cases being altered table is marked as flushed, which forces
subsequent attempt to open parent table to re-open. Which in turn is
not allowed while transaction is running.

Rather than opening parent table, just take appropriate MDL lock.

Also removed table_already_fk_prelocked() check: MDL itself has much
better methods to handle duplicate locks. E.g. the former won't acquire
MDL_SHARED_NO_WRITE if it already has MDL_SHARED_READ.

mysql-test/suite/innodb/r/foreign-keys.result

@@ -87,3 +87,16 @@ drop table t3;
 drop table t2;
 drop table t1;
 set debug_sync='reset';
+#
+# MDEV-17595 - Server crashes in copy_data_between_tables or
+#              Assertion `thd->transaction.stmt.is_empty() ||
+#              (thd->state_flags & Open_tables_state::BACKUPS_AVAIL)'
+#              fails in close_tables_for_reopen upon concurrent
+#              ALTER TABLE and FLUSH
+#
+CREATE TABLE t1 (a INT, KEY(a)) ENGINE=InnoDB;
+INSERT INTO t1 VALUES(1),(2);
+CREATE TABLE t2 (b INT, KEY(b)) ENGINE=InnoDB;
+INSERT INTO t2 VALUES(2);
+ALTER TABLE t2 ADD FOREIGN KEY(b) REFERENCES t1(a), LOCK=EXCLUSIVE;
+DROP TABLE t2, t1;

mysql-test/suite/innodb/t/foreign-keys.test

@@ -111,3 +111,18 @@ drop table t3;
 drop table t2;
 drop table t1;
 set debug_sync='reset';
+
+
+--echo #
+--echo # MDEV-17595 - Server crashes in copy_data_between_tables or
+--echo #              Assertion `thd->transaction.stmt.is_empty() ||
+--echo #              (thd->state_flags & Open_tables_state::BACKUPS_AVAIL)'
+--echo #              fails in close_tables_for_reopen upon concurrent
+--echo #              ALTER TABLE and FLUSH
+--echo #
+CREATE TABLE t1 (a INT, KEY(a)) ENGINE=InnoDB;
+INSERT INTO t1 VALUES(1),(2);
+CREATE TABLE t2 (b INT, KEY(b)) ENGINE=InnoDB;
+INSERT INTO t2 VALUES(2);
+ALTER TABLE t2 ADD FOREIGN KEY(b) REFERENCES t1(a), LOCK=EXCLUSIVE;
+DROP TABLE t2, t1;

sql/sql_table.cc

@@ -9074,6 +9074,8 @@ bool mysql_alter_table(THD *thd,char *new_db, char *new_name,
       new_table->file->get_foreign_key_list(thd, &fk_list);
       while ((fk= fk_list_it++))
       {
+        MDL_request mdl_request;
+
         if (lower_case_table_names)
         {
          char buf[NAME_LEN];
@@ -9085,20 +9087,14 @@ bool mysql_alter_table(THD *thd,char *new_db, char *new_name,
          len = my_casedn_str(files_charset_info, buf);
          thd->make_lex_string(fk->referenced_table, buf, len);
         }
-        if (table_already_fk_prelocked(table_list, fk->referenced_db,
-                                       fk->referenced_table, TL_READ_NO_INSERT))
-          continue;
 
-        TABLE_LIST *tl= (TABLE_LIST *) thd->alloc(sizeof(TABLE_LIST));
-        tl->init_one_table_for_prelocking(fk->referenced_db->str, fk->referenced_db->length,
-                           fk->referenced_table->str, fk->referenced_table->length,
-                           NULL, TL_READ_NO_INSERT, false, NULL, 0,
-                           &thd->lex->query_tables_last);
+        mdl_request.init(MDL_key::TABLE,
+                         fk->referenced_db->str, fk->referenced_table->str,
+                         MDL_SHARED_NO_WRITE, MDL_TRANSACTION);
+        if (thd->mdl_context.acquire_lock(&mdl_request,
+                                          thd->variables.lock_wait_timeout))
+          goto err_new_table_cleanup;
       }
-
-      if (open_tables(thd, &table_list->next_global, &tables_opened, 0,
-                      &alter_prelocking_strategy))
-        goto err_new_table_cleanup;
     }
   }
   /*