Fixed main.flush_read_lock sporadic failure

With MDEV-19384 fixed FTWRL releases HANDLER locks early, which allows
concurrent threads to go. Test case may get stuck on FTWRL waiting for
LOCK TABLES.

mysql-test/main/flush_read_lock.result

@@ -1736,6 +1736,7 @@ SET DEBUG_SYNC= 'RESET';
 # MDEV-19384 Deadlock between FTWRL under open HANDLER, LOCK TABLE
 # and DROP DATABASE
 #
+SET DEBUG_SYNC= 'ftwrl_before_lock SIGNAL ready WAIT_FOR go';
 CREATE DATABASE mysqltest;
 CREATE TABLE mysqltest.t1(a INT);
 HANDLER mysqltest.t1 OPEN as t1;
@@ -1748,14 +1749,17 @@ SET DEBUG_SYNC= 'mdl_acquire_lock_wait SIGNAL ready';
 DROP DATABASE mysqltest;
 connect  con3,localhost,root,,;
 SET DEBUG_SYNC= 'now WAIT_FOR ready';
-disconnect con3;
 connection default;
 FLUSH TABLES WITH READ LOCK;
-UNLOCK TABLES;
-HANDLER t1 CLOSE;
+connection con3;
+SET DEBUG_SYNC= 'now WAIT_FOR ready';
+disconnect con3;
 connection con1;
-UNLOCK TABLES;
+SET DEBUG_SYNC= 'now SIGNAL go';
 disconnect con1;
+connection default;
+UNLOCK TABLES;
+HANDLER t1 CLOSE;
 connection con2;
 disconnect con2;
 connection default;

mysql-test/main/flush_read_lock.test

@@ -2100,6 +2100,7 @@ SET DEBUG_SYNC= 'RESET';
 --echo # and DROP DATABASE
 --echo #
 
+SET DEBUG_SYNC= 'ftwrl_before_lock SIGNAL ready WAIT_FOR go';
 CREATE DATABASE mysqltest;
 CREATE TABLE mysqltest.t1(a INT);
 HANDLER mysqltest.t1 OPEN as t1;
@@ -2114,18 +2115,24 @@ SET DEBUG_SYNC= 'mdl_acquire_lock_wait SIGNAL ready';
 
 connect (con3,localhost,root,,);
 SET DEBUG_SYNC= 'now WAIT_FOR ready';
-disconnect con3;
 
 connection default;
-FLUSH TABLES WITH READ LOCK;
-UNLOCK TABLES;
-HANDLER t1 CLOSE;
-connection con1;
+send FLUSH TABLES WITH READ LOCK;
+
+connection con3;
+SET DEBUG_SYNC= 'now WAIT_FOR ready';
+disconnect con3;
 
+connection con1;
 --error 0,ER_NO_SUCH_TABLE
 reap;
-UNLOCK TABLES;
+SET DEBUG_SYNC= 'now SIGNAL go';
 disconnect con1;
+
+connection default;
+reap;
+UNLOCK TABLES;
+HANDLER t1 CLOSE;
 connection con2;
 reap;
 disconnect con2;

sql/lock.cc

@@ -1034,6 +1034,7 @@ bool Global_read_lock::lock_global_read_lock(THD *thd)
       if another thread is trying to simultaneous drop the table
     */
     mysql_ha_cleanup_no_free(thd);
+    DEBUG_SYNC(thd, "ftwrl_before_lock");
 
     DBUG_ASSERT(! thd->mdl_context.is_lock_owner(MDL_key::BACKUP, "", "",
                                                  MDL_BACKUP_FTWRL1));