MDEV-18154 Deadlock and assertion upon no-op ALTER under LOCK TABLES

1. Fix DBUG_ASSERT(!table->pos_in_locked_tables) in tc_release_table();
2. Fix access of prematurely freed MDL_ticket: don't close ticket if table was not closed;
3. Fix deadlock after erroneous ALTER.

mysql_alter_table() leaves dirty table->m_needs_reopen in case of
error exit which then incorrectly treated by mysql_lock_tables().

mysql-test/main/alter_table.result

@@ -2539,3 +2539,21 @@ test.t1	analyze	status	OK
 set @@use_stat_tables= @save_use_stat_tables;
 set @@optimizer_use_condition_selectivity= @save_optimizer_use_condition_selectivity;
 drop table t1;
+#
+# MDEV-18154 Deadlock and assertion upon no-op ALTER under LOCK TABLES
+#
+create or replace table t1 (pk int, i int, primary key (pk)) engine myisam;
+create or replace view v1 as select * from t1;
+lock table v1 read, t1 write;
+alter table t1 change f1 f2 int;
+ERROR 42S22: Unknown column 'f1' in 't1'
+set max_statement_time= 1;
+alter table t1 add column if not exists i int after pk;
+Warnings:
+Note	1060	Duplicate column name 'i'
+set max_statement_time= 0;
+drop table t1;
+drop view v1;
+#
+# End of 10.3 tests
+#

mysql-test/main/alter_table.test

@@ -2057,3 +2057,21 @@ analyze table t1;
 set @@use_stat_tables= @save_use_stat_tables;
 set @@optimizer_use_condition_selectivity= @save_optimizer_use_condition_selectivity;
 drop table t1;
+
+--echo #
+--echo # MDEV-18154 Deadlock and assertion upon no-op ALTER under LOCK TABLES
+--echo #
+create or replace table t1 (pk int, i int, primary key (pk)) engine myisam;
+create or replace view v1 as select * from t1;
+lock table v1 read, t1 write;
+--error ER_BAD_FIELD_ERROR
+alter table t1 change f1 f2 int;
+set max_statement_time= 1;
+alter table t1 add column if not exists i int after pk;
+set max_statement_time= 0;
+drop table t1;
+drop view v1;
+
+--echo #
+--echo # End of 10.3 tests
+--echo #

sql/sql_base.cc

@@ -2433,6 +2433,7 @@ unlink_all_closed_tables(THD *thd, MYSQL_LOCK *lock, size_t reopen_count)
       DBUG_ASSERT(thd->open_tables == m_reopen_array[reopen_count]);
 
       thd->open_tables->pos_in_locked_tables->table= NULL;
+      thd->open_tables->pos_in_locked_tables= NULL;
 
       close_thread_table(thd, &thd->open_tables);
     }

sql/sql_table.cc

@@ -7837,6 +7837,7 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
   Create_field *def;
   Field **f_ptr,*field;
   MY_BITMAP *dropped_fields= NULL; // if it's NULL - no dropped fields
+  bool save_reopen= table->m_needs_reopen;
   DBUG_ENTER("mysql_prepare_alter_table");
 
   /*
@@ -8516,7 +8517,9 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
   alter_info->create_list.swap(new_create_list);
   alter_info->key_list.swap(new_key_list);
   alter_info->check_constraint_list.swap(new_constraint_list);
+  DBUG_RETURN(rc);
 err:
+  table->m_needs_reopen= save_reopen;
   DBUG_RETURN(rc);
 }
 
@@ -10195,7 +10198,8 @@ do_continue:;
     tables and release the exclusive metadata lock.
   */
   thd->locked_tables_list.unlink_all_closed_tables(thd, NULL, 0);
-  thd->mdl_context.release_all_locks_for_name(mdl_ticket);
+  if (!table_list->table)
+    thd->mdl_context.release_all_locks_for_name(mdl_ticket);
   DBUG_RETURN(true);
 }