Fixed hang in concurrent DROP TABLE and BACKUP LOCK BLOCK_DDL

The problem was that tdc_remove_referenced_share() did not take into
account that someone could push things into share->free_tables() even
if there is a MDL_EXCLUSIVE lock on the table.
This can happen if flush_tables() uses the table cache to flush a
a non transactional table to disk.

mysql-test/suite/mariabackup/deadlock_drop_table.result

+create or replace table t1 (a int primary key, b int, c int, key(b),key(c)) engine=myisam;
+insert into t1 (a) values(1);
+set debug_sync='RESET';
+connect  con1, localhost, root,,;
+connect  con2, localhost, root,,;
+connection default;
+backup stage start;
+backup stage flush;
+select * from t1;
+a	b	c
+1	NULL	NULL
+set debug_sync='after_purge_tables SIGNAL parked WAIT_FOR go';
+set debug_sync='before_tc_release_table SIGNAL parked2 WAIT_FOR go2';
+backup stage BLOCK_DDL;
+connection con1;
+set debug_sync='now WAIT_FOR parked';
+select * from t1;
+a	b	c
+1	NULL	NULL
+set debug_sync='now SIGNAL go';
+set debug_sync='now WAIT_FOR parked2';
+set debug_sync='before_wait_for_refs SIGNAL waiting WAIT_FOR go3';
+drop table t1;;
+connection con2;
+set debug_sync='now WAIT_FOR waiting';
+set debug_sync='now SIGNAL go2';
+set debug_sync='now SIGNAL go3';
+connection default;
+connection con1;
+connection default;
+disconnect con1;
+disconnect con2;
+set debug_sync='RESET';

mysql-test/suite/mariabackup/deadlock_drop_table.test

+--source include/have_debug.inc
+
+create or replace table t1 (a int primary key, b int, c int, key(b),key(c)) engine=myisam;
+insert into t1 (a) values(1);
+
+set debug_sync='RESET';
+connect (con1, localhost, root,,);
+connect (con2, localhost, root,,);
+connection default;
+
+backup stage start;
+backup stage flush;
+select * from t1;
+set debug_sync='after_purge_tables SIGNAL parked WAIT_FOR go';
+set debug_sync='before_tc_release_table SIGNAL parked2 WAIT_FOR go2';
+--send backup stage BLOCK_DDL
+--connection con1
+set debug_sync='now WAIT_FOR parked';
+select * from t1;
+set debug_sync='now SIGNAL go';
+set debug_sync='now WAIT_FOR parked2';
+set debug_sync='before_wait_for_refs SIGNAL waiting WAIT_FOR go3';
+--send drop table t1;
+--connection con2
+set debug_sync='now WAIT_FOR waiting';
+set debug_sync='now SIGNAL go2';
+set debug_sync='now SIGNAL go3';
+--connection default
+--reap
+--connection con1
+--reap
+connection default;
+disconnect con1;
+disconnect con2;
+set debug_sync='RESET';

sql/sql_base.cc

@@ -554,6 +554,7 @@ bool flush_tables(THD *thd, flush_tables_type flag)
   DBUG_ENTER("flush_tables");
 
   purge_tables();  /* Flush unused tables and shares */
+  DEBUG_SYNC(thd, "after_purge_tables");
 
   /*
     Loop over all shares and collect shares that have open tables
@@ -593,6 +594,7 @@ bool flush_tables(THD *thd, flush_tables_type flag)
     if (table)
     {
       (void) table->file->extra(HA_EXTRA_FLUSH);
+      DEBUG_SYNC(table->in_use, "before_tc_release_table");
       tc_release_table(table);
     }
     else

sql/table_cache.cc

@@ -1001,8 +1001,9 @@ void tdc_remove_referenced_share(THD *thd, TABLE_SHARE *share)
   DBUG_ASSERT(thd->mdl_context.is_lock_owner(MDL_key::TABLE, share->db.str,
                                              share->table_name.str,
                                              MDL_EXCLUSIVE));
-  share->tdc->flush_unused(false);
+  share->tdc->flush_unused(true);
   mysql_mutex_lock(&share->tdc->LOCK_table_share);
+  DEBUG_SYNC(thd, "before_wait_for_refs");
   share->tdc->wait_for_refs(1);
   DBUG_ASSERT(share->tdc->all_tables.is_empty());
   share->tdc->ref_count--;