Fix for bug #51136 "Crash in pthread_rwlock_rdlock on

TEMPORARY + HANDLER + LOCK + SP".

Server crashed when one: 
1) Opened HANDLER or acquired global read lock
2) Then locked one or several temporary tables with
   LOCK TABLES statement (but no base tables).
3) Then issued any statement causing commit (explicit 
   or implicit).
4) Issued statement which should have closed HANDLER
   or released global read lock.
   
The problem was that when entering LOCK TABLES mode in the
scenario described above we incorrectly set transactional
MDL sentinel to zero. As result during commit all metadata 
locks were released (including lock for open HANDLER or
global metadata shared lock). Indeed, attempt to release
metadata lock for the second time which happened during
HANLDER CLOSE or during release of GLR caused crash.

This patch fixes problem by changing MDL_context's
set_trans_sentinel() method to set sentinel to correct 
value (it should point to the most recent ticket).

mysql-test/include/handler.inc

@@ -1681,3 +1681,25 @@ handler t1 close;
 --echo # Clean-up.
 drop function f1;
 drop tables t1, t2;
+
+
+--echo #
+--echo # Test for bug #51136 "Crash in pthread_rwlock_rdlock on TEMPORARY +
+--echo #                      HANDLER + LOCK + SP".
+--echo # Also see additional coverage for this bug in flush.test.
+--echo #
+--disable_warnings
+drop tables if exists t1, t2;
+--enable_warnings
+create table t1 (i int);
+create temporary table t2 (j int);
+handler t1 open;
+lock table t2 read;
+--echo # This commit should not release any MDL locks.
+commit;
+unlock tables;
+--echo # The below statement crashed before the bug fix as it
+--echo # has attempted to release metadata lock which was
+--echo # already released by commit.
+handler t1 close;
+drop tables t1, t2;

mysql-test/r/flush.result

@@ -94,3 +94,20 @@ unlock tables;
 set global general_log= @old_general_log;
 set global read_only= @old_read_only;
 End of 5.1 tests
+#
+# Additional test for bug #51136 "Crash in pthread_rwlock_rdlock
+#                                 on TEMPORARY + HANDLER + LOCK + SP".
+# Also see the main test for this bug in include/handler.inc.
+#
+drop tables if exists t1, t2;
+create table t1 (i int);
+create temporary table t2 (j int);
+flush tables with read lock;
+lock table t2 read;
+# This commit should not release any MDL locks.
+commit;
+# The below statement crashed before the bug fix as it
+# has attempted to release global shared metadata lock
+# which was already released by commit.
+unlock tables;
+drop tables t1, t2;

mysql-test/r/handler_innodb.result

@@ -1667,3 +1667,21 @@ handler t1 close;
 # Clean-up.
 drop function f1;
 drop tables t1, t2;
+#
+# Test for bug #51136 "Crash in pthread_rwlock_rdlock on TEMPORARY +
+#                      HANDLER + LOCK + SP".
+# Also see additional coverage for this bug in flush.test.
+#
+drop tables if exists t1, t2;
+create table t1 (i int);
+create temporary table t2 (j int);
+handler t1 open;
+lock table t2 read;
+# This commit should not release any MDL locks.
+commit;
+unlock tables;
+# The below statement crashed before the bug fix as it
+# has attempted to release metadata lock which was
+# already released by commit.
+handler t1 close;
+drop tables t1, t2;

mysql-test/r/handler_myisam.result

@@ -1664,6 +1664,24 @@ handler t1 close;
 drop function f1;
 drop tables t1, t2;
 #
+# Test for bug #51136 "Crash in pthread_rwlock_rdlock on TEMPORARY +
+#                      HANDLER + LOCK + SP".
+# Also see additional coverage for this bug in flush.test.
+#
+drop tables if exists t1, t2;
+create table t1 (i int);
+create temporary table t2 (j int);
+handler t1 open;
+lock table t2 read;
+# This commit should not release any MDL locks.
+commit;
+unlock tables;
+# The below statement crashed before the bug fix as it
+# has attempted to release metadata lock which was
+# already released by commit.
+handler t1 close;
+drop tables t1, t2;
+#
 # BUG #46456: HANDLER OPEN + TRUNCATE + DROP (temporary) TABLE, crash 
 #
 CREATE TABLE t1 AS SELECT 1 AS f1;

mysql-test/t/flush.test

@@ -203,3 +203,24 @@ set global general_log= @old_general_log;
 set global read_only= @old_read_only;
 
 --echo End of 5.1 tests
+
+
+--echo #
+--echo # Additional test for bug #51136 "Crash in pthread_rwlock_rdlock
+--echo #                                 on TEMPORARY + HANDLER + LOCK + SP".
+--echo # Also see the main test for this bug in include/handler.inc.
+--echo #
+--disable_warnings
+drop tables if exists t1, t2;
+--enable_warnings
+create table t1 (i int);
+create temporary table t2 (j int);
+flush tables with read lock;
+lock table t2 read;
+--echo # This commit should not release any MDL locks.
+commit;
+--echo # The below statement crashed before the bug fix as it
+--echo # has attempted to release global shared metadata lock
+--echo # which was already released by commit.
+unlock tables;
+drop tables t1, t2;

sql/mdl.h

@@ -501,7 +501,7 @@ class MDL_context
 
   void set_trans_sentinel()
   {
-    m_trans_sentinel= mdl_savepoint();
+    m_trans_sentinel= m_tickets.front();
   }
   MDL_ticket *trans_sentinel() const { return m_trans_sentinel; }