Fix for bug #50908 "Assertion `handler_tables_hash.records == 0'

failed in enter_locked_tables_mode".

Server was aborted due to assertion failure when one tried to 
execute statement requiring prelocking (i.e. firing triggers
or using stored functions) while having open HANDLERs.

The problem was that THD::enter_locked_tables_mode() method
which was called at the beginning of execution of prelocked 
statement assumed there are no open HANDLERs. It had to do 
so because corresponding THD::leave_locked_tables_mode()
method was unable to properly restore MDL sentinel when
leaving LOCK TABLES/prelocked mode in the presence of open 
HANDLERs.

This patch solves this problem by changing the latter method
to properly restore MDL sentinel and thus removing need for 
this assumption. As a side-effect, it lifts unjustified
limitation by allowing to keep HANDLERs open when entering 
LOCK TABLES mode.

Author: Dmitry Lenev

mysql-test/include/handler.inc

@@ -804,12 +804,10 @@ handler t1 open;
 handler t2 open;
 handler t3 open;
 --echo #
---echo # LOCK TABLES implicitly closes all handlers.
---echo #
-lock table t3 read;
---echo #
---echo # No HANDLER sql is available under lock tables anyway.
+--echo # No HANDLER sql is allowed under LOCK TABLES.
+--echo # But it does not implicitly closes all handlers.
 --echo #
+lock table t1 read;
 --error ER_LOCK_OR_ACTIVE_TRANSACTION
 handler t1 open;
 --error ER_LOCK_OR_ACTIVE_TRANSACTION
@@ -818,18 +816,18 @@ handler t1 read next;
 handler t2 close;
 --error ER_LOCK_OR_ACTIVE_TRANSACTION
 handler t3 open;
---echo # After UNLOCK TABLES no handlers are around, they were
---echo # implicitly closed.
+--echo # After UNLOCK TABLES handlers should be around and
+--echo # we should be able to continue reading through them.
 unlock tables;
-drop temporary table t3;
---error ER_UNKNOWN_TABLE
 handler t1 read next;
---error ER_UNKNOWN_TABLE
+handler t1 close;
+handler t2 read next;
 handler t2 close;
---error ER_UNKNOWN_TABLE
 handler t3 read next;
+handler t3 close;
+drop temporary table t3;
 --echo #
---echo # Other operations also implicitly close handler:
+--echo # Other operations that implicitly close handler:
 --echo #
 --echo # TRUNCATE
 --echo #
@@ -922,6 +920,93 @@ handler t1 read a prev;
 handler t1 close;
 unlock tables;
 --echo #
+--echo # Let us also check that these operations behave in similar
+--echo # way under LOCK TABLES.
+--echo #
+--echo # TRUNCATE under LOCK TABLES.
+--echo #
+handler t1 open;
+lock tables t1 write;
+truncate table t1;
+unlock tables;
+--error ER_UNKNOWN_TABLE
+handler t1 read next;
+handler t1 open;
+--echo #
+--echo # CREATE TRIGGER under LOCK TABLES.
+--echo #
+lock tables t1 write;
+create trigger t1_ai after insert on t1 for each row set @a=1;
+unlock tables;
+--error ER_UNKNOWN_TABLE
+handler t1 read next;
+--echo #
+--echo # DROP TRIGGER under LOCK TABLES.
+--echo #
+handler t1 open;
+lock tables t1 write;
+drop trigger t1_ai;
+unlock tables;
+--error ER_UNKNOWN_TABLE
+handler t1 read next;
+--echo #
+--echo # ALTER TABLE under LOCK TABLES.
+--echo # 
+handler t1 open;
+lock tables t1 write;
+alter table t1 drop column b;
+unlock tables;
+--error ER_UNKNOWN_TABLE
+handler t1 read next;
+--echo #
+--echo # ANALYZE TABLE under LOCK TABLES.
+--echo # 
+handler t1 open;
+lock tables t1 write;
+analyze table t1;
+unlock tables;
+--error ER_UNKNOWN_TABLE
+handler t1 read next;
+--echo #
+--echo # OPTIMIZE TABLE under LOCK TABLES.
+--echo # 
+handler t1 open;
+lock tables t1 write;
+optimize table t1;
+unlock tables;
+--error ER_UNKNOWN_TABLE
+handler t1 read next;
+--echo #
+--echo # REPAIR TABLE under LOCK TABLES.
+--echo # 
+handler t1 open;
+lock tables t1 write;
+repair table t1;
+unlock tables;
+--error ER_UNKNOWN_TABLE
+handler t1 read next;
+--echo #
+--echo # DROP TABLE under LOCK TABLES, naturally.
+--echo # 
+handler t1 open;
+lock tables t1 write;
+drop table t1;
+unlock tables;
+--error ER_UNKNOWN_TABLE
+handler t1 read next;
+create table t1 (a int, b int, key a (a));
+insert into t1 (a) values (1), (2), (3), (4), (5);
+--echo #
+--echo # FLUSH TABLE doesn't close the table but loses the position
+--echo # 
+handler t1 open;
+handler t1 read a prev;
+lock tables t1 write;
+flush table t1;
+unlock tables;
+handler t1 read a prev;
+handler t1 close;
+--echo #
 --echo # Explore the effect of HANDLER locks on concurrent DDL
 --echo #
 handler t1 open;
@@ -1433,6 +1518,8 @@ lock table not_exists_write read;
 --echo # We still have the read lock.
 --error ER_CANT_UPDATE_WITH_READLOCK
 drop table t1;
+handler t1 read next;
+handler t1 close;
 handler t1 open;
 select a from t2;
 handler t1 read next;
@@ -1542,3 +1629,55 @@ SELECT table_name, table_comment FROM information_schema.tables
 
 HANDLER t1 CLOSE;
 DROP TABLE t1;
+
+
+--echo #
+--echo # Test for bug #50908 "Assertion `handler_tables_hash.records == 0'
+--echo #                      failed in enter_locked_tables_mode".
+--echo #
+--disable_warnings
+drop tables if exists t1, t2;
+drop function if exists f1;
+--enable_warnings
+create table t1 (i int);
+insert into t1 values (1), (2);
+create table t2 (j int);
+insert into t2 values (1);
+create function f1() returns int return (select count(*) from t2);
+--echo # Check that open HANDLER survives statement executed in
+--echo # prelocked mode.
+handler t1 open;
+handler t1 read next;
+--echo # The below statement were aborted due to an assertion failure.
+select f1() from t2;
+handler t1 read next;
+handler t1 close;
+--echo # Check that the same happens under GLOBAL READ LOCK.
+flush tables with read lock;
+handler t1 open;
+handler t1 read next;
+select f1() from t2;
+handler t1 read next;
+unlock tables;
+handler t1 close;
+--echo # Now, check that the same happens if LOCK TABLES is executed.
+handler t1 open;
+handler t1 read next;
+lock table t2 read;
+select * from t2;
+unlock tables;
+handler t1 read next;
+handler t1 close;
+--echo # Finally, check scenario with GRL and LOCK TABLES.
+flush tables with read lock;
+handler t1 open;
+handler t1 read next;
+lock table t2 read;
+select * from t2;
+--echo # This unlocks both tables and GRL.
+unlock tables;
+handler t1 read next;
+handler t1 close;
+--echo # Clean-up.
+drop function f1;
+drop tables t1, t2;