Bug #51355 handler stmt cause assertion in

           bool MDL_context::try_acquire_lock(MDL_request*)

This assert was triggered in the following way:
1) HANDLER OPEN t1 from connection 1
2) DROP TABLE t1 from connection 2. This will block due to the metadata lock
held by the open handler in connection 1.
3) DML statement (e.g. INSERT) from connection 1. This will close the table
opened by the HANDLER in 1) and release its metadata lock. This is done due
to the pending exclusive metadata lock from 2). 
4) DROP TABLE t1 from connection 2 now completes and removes table t1.
5) HANDLER READ from connection 1. Since the handler table was closed in 3),
the handler code will try to reopen the table. First a new metadata lock on
t1 will be granted before the command fails since the table was removed in 4).
6) HANDLER READ from connection 1. This caused the assert.

The reason for the assert was that the MDL_request's pointer to the lock
ticket was not reset when the statement failed. HANDLER READ then tried to
acquire a lock using the same MDL_request object, triggering the assert.
This bug was only noticeable on debug builds and did not cause any problems
on release builds.

This patch fixes the problem by assuring that the pointer to the metadata 
lock ticket is reset when reopening of handler tables fails.

Test case added to handler.inc

mysql-test/include/handler.inc

@@ -1704,3 +1704,56 @@ unlock tables;
 --echo # already released by commit.
 handler t1 close;
 drop tables t1, t2;
+
+
+--echo #
+--echo # Bug#51355 handler stmt cause assertion in
+--echo #           bool MDL_context::try_acquire_lock(MDL_request*)
+--echo #
+
+--disable_warnings
+DROP TABLE IF EXISTS t1;
+--enable_warnings
+
+connect(con51355, localhost, root);
+
+--echo # Connection default
+connection default;
+CREATE TABLE t1(id INT, KEY id(id));
+HANDLER t1 OPEN;
+
+--echo # Connection con51355
+connection con51355;
+--echo # Sending:
+--send DROP TABLE t1
+
+--echo # Connection default
+connection default;
+--echo # This I_S query will cause the handler table to be closed and
+--echo # the metadata lock to be released. This will allow DROP TABLE
+--echo # to proceed. Waiting for the table to be removed.
+let $wait_condition=
+  SELECT COUNT(*) = 0 FROM information_schema.tables WHERE table_name = "t1";
+--source include/wait_condition.inc
+
+--echo # Connection con51355
+connection con51355;
+--echo # Reaping: DROP TABLE t1
+--reap
+
+--echo # Connection default
+connection default;
+--error ER_NO_SUCH_TABLE
+HANDLER t1 READ id NEXT;
+# This caused an assertion
+--error ER_NO_SUCH_TABLE
+HANDLER t1 READ id NEXT;
+
+HANDLER t1 CLOSE;
+--echo # Connection con51355
+connection con51355;
+disconnect con51355;
+--source include/wait_until_disconnected.inc
+--echo # Connection default
+connection default;
+

mysql-test/r/handler_innodb.result

@@ -1685,3 +1685,28 @@ unlock tables;
 # already released by commit.
 handler t1 close;
 drop tables t1, t2;
+#
+# Bug#51355 handler stmt cause assertion in
+#           bool MDL_context::try_acquire_lock(MDL_request*)
+#
+DROP TABLE IF EXISTS t1;
+# Connection default
+CREATE TABLE t1(id INT, KEY id(id));
+HANDLER t1 OPEN;
+# Connection con51355
+# Sending:
+DROP TABLE t1;
+# Connection default
+# This I_S query will cause the handler table to be closed and
+# the metadata lock to be released. This will allow DROP TABLE
+# to proceed. Waiting for the table to be removed.
+# Connection con51355
+# Reaping: DROP TABLE t1
+# Connection default
+HANDLER t1 READ id NEXT;
+ERROR 42S02: Table 'test.t1' doesn't exist
+HANDLER t1 READ id NEXT;
+ERROR 42S02: Table 'test.t1' doesn't exist
+HANDLER t1 CLOSE;
+# Connection con51355
+# Connection default

mysql-test/r/handler_myisam.result

@@ -1682,6 +1682,31 @@ unlock tables;
 handler t1 close;
 drop tables t1, t2;
 #
+# Bug#51355 handler stmt cause assertion in
+#           bool MDL_context::try_acquire_lock(MDL_request*)
+#
+DROP TABLE IF EXISTS t1;
+# Connection default
+CREATE TABLE t1(id INT, KEY id(id));
+HANDLER t1 OPEN;
+# Connection con51355
+# Sending:
+DROP TABLE t1;
+# Connection default
+# This I_S query will cause the handler table to be closed and
+# the metadata lock to be released. This will allow DROP TABLE
+# to proceed. Waiting for the table to be removed.
+# Connection con51355
+# Reaping: DROP TABLE t1
+# Connection default
+HANDLER t1 READ id NEXT;
+ERROR 42S02: Table 'test.t1' doesn't exist
+HANDLER t1 READ id NEXT;
+ERROR 42S02: Table 'test.t1' doesn't exist
+HANDLER t1 CLOSE;
+# Connection con51355
+# Connection default
+#
 # BUG #46456: HANDLER OPEN + TRUNCATE + DROP (temporary) TABLE, crash 
 #
 CREATE TABLE t1 AS SELECT 1 AS f1;

sql/sql_handler.cc

@@ -311,7 +311,11 @@ bool mysql_ha_open(THD *thd, TABLE_LIST *tables, bool reopen)
     if (!reopen)
       my_hash_delete(&thd->handler_tables_hash, (uchar*) hash_tables);
     else
+    {
       hash_tables->table= NULL;
+      /* Safety, cleanup the pointer to satisfy MDL assertions. */
+      hash_tables->mdl_request.ticket= NULL;
+    }
     DBUG_PRINT("exit",("ERROR"));
     DBUG_RETURN(TRUE);
   }