Fix for bug #50784 "MDL: Assertion `m_tickets.is_empty() ||

m_tickets.front() == m_trans_sentinel'".

Debug build of server crashed due to assert failure in MDL
subsystem when one tried to execute multi-table REPAIR or
OPTIMIZE in autocommit=0 mode.

The assert failure occured when multi-table REPAIR or OPTIMIZE
started processing of second table from its table list and
tried to acquire upgradable metadata lock on this table.
The cause of the assert failure were MDL locks left over from
processing of previous table. It turned out that in autocommit=0
mode close_thread_tables() which happens at the end of table
processing doesn't release metadata locks.

This fix solves problem by releasing locks explicitly using
MDL_context::release_trans_locks() call.

mysql-test/r/repair.result

@@ -169,3 +169,17 @@ test.t1	repair	Error	Table 't1' was locked with a READ lock and can't be updated
 test.t1	repair	status	Operation failed
 UNLOCK TABLES;
 DROP TABLE t1;
+#
+# Test for bug #50784 "MDL: Assertion `m_tickets.is_empty() ||
+#                      m_tickets.front() == m_trans_sentinel'"
+#
+drop tables if exists t1, t2;
+create table t1 (i int);
+create table t2 (j int);
+set @@autocommit= 0;
+repair table t1, t2;
+Table	Op	Msg_type	Msg_text
+test.t1	repair	status	OK
+test.t2	repair	status	OK
+set @@autocommit= default;
+drop tables t1, t2;

mysql-test/t/repair.test

@@ -173,3 +173,18 @@ REPAIR TABLE t1;
 
 UNLOCK TABLES;
 DROP TABLE t1;
+
+
+--echo #
+--echo # Test for bug #50784 "MDL: Assertion `m_tickets.is_empty() ||
+--echo #                      m_tickets.front() == m_trans_sentinel'"
+--echo #
+--disable_warnings
+drop tables if exists t1, t2;
+--enable_warnings
+create table t1 (i int);
+create table t2 (j int);
+set @@autocommit= 0;
+repair table t1, t2;
+set @@autocommit= default;
+drop tables t1, t2;

sql/sql_table.cc

@@ -5030,6 +5030,7 @@ static bool mysql_admin_table(THD* thd, TABLE_LIST* tables,
     trans_commit_stmt(thd);
     trans_commit_implicit(thd);
     close_thread_tables(thd);
+    thd->mdl_context.release_transactional_locks();
     table->table=0;				// For query cache
 
     /*
@@ -5059,6 +5060,7 @@ static bool mysql_admin_table(THD* thd, TABLE_LIST* tables,
   trans_rollback_stmt(thd);
   trans_rollback(thd);
   close_thread_tables(thd);			// Shouldn't be needed
+  thd->mdl_context.release_transactional_locks();
   if (table)
     table->table=0;
   DBUG_RETURN(TRUE);