Bug#19786309 - CRASH IN UNLOCK TABLES AFTER LOCKING AND TRUNCATING TEMPORARY TABLE.

Attempt to truncate temporary table using Blackhole storage and
locked by LOCK TABLES caused assertion failure and crashes.

Blackhole is a transaction-aware engine. While creating the temporary
table in transaction-aware engine, temporary table of type
"TRANSACTIONAL_TMP_TABLE" is created. For such temporary tables
a THR_LOCK lock is acquired by the LOCK TABLE operation. References
to them are also added into MYSQL_LOCK::table[] array. Also for
Blackhole engine, flag HTON_CAN_RECREATE is set.

While truncating temporary tables, no locks are taken and
recreate_temporary_table() is called for engines having
"HTON_CAN_RECREATE" in flag.

Function closefrm() is called from the recreate_temporary_table(),
to close the current temporary table. In closefrm(), the lock on
table expected is "F_UNLCK". In debug builds, assert condition on
this fails when lock of type "F_WRLCK" is acquired by LOCK TABLE
operation on temporary tables using Blackhole engine.

In non-debug builds closefrm() simply freed TABLE object leaving
dangling pointer to this object in MYSQL_LOCK::table[] array which
might lead to crashes later.

Fix:
---------
To fix this issue, we now unlock and remove table from MYSQL_LOCK::table[]
array before calling close_temporary_table() in recreate_temporary_table().
This is achieved by calling mysql_lock_remove() function for this table.

sql/sql_truncate.cc

-/* Copyright (c) 2010, 2014, Oracle and/or its affiliates. All rights reserved.
+/* Copyright (c) 2010, 2015, Oracle and/or its affiliates. All rights reserved.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -286,6 +286,12 @@ static bool recreate_temporary_table(THD *thd, TABLE *table)
 
   table->file->info(HA_STATUS_AUTO | HA_STATUS_NO_LOCK);
 
+  /*
+    If LOCK TABLES list is not empty and contains this table
+    then unlock the table and remove it from this list.
+  */
+  mysql_lock_remove(thd, thd->lock, table);
+
   /* Don't free share. */
   close_temporary_table(thd, table, FALSE, FALSE);