Server crashes with BACKUP STAGE and FLUSH TABLE table_name

Fixes MDEV-18067, MDEV-18068 and MDEV-18069

The problem was that FLUSH TABLES table_name combined with UNLOCK TABLES
calls MDL_context::set_transaction_duration_for_all_locks(), which
changed backup_locks from MDL_EXPLICT to MDL_TRANSACTION.

Fixed by ensuring that set_transaction_duration_for_all_locks() doesn't
touch BACKUP locks.

mysql-test/main/backup_interaction.result

@@ -518,3 +518,29 @@ TRUNCATE t_temporary_innodb;
 ALTER TABLE t_temporary_innodb ADD COLUMN col2 INT;
 ALTER TABLE t_temporary_innodb ADD KEY idx(col2);
 BACKUP STAGE END;
+CREATE OR REPLACE TABLE t1 (pk INT PRIMARY KEY, f INT);
+BACKUP STAGE START;
+FLUSH TABLE t1 FOR EXPORT;
+UNLOCK TABLES;
+BACKUP STAGE END;
+DROP TABLE t1;
+CREATE TABLE t1 (a INT);
+BACKUP STAGE START;
+FLUSH TABLE t1 FOR EXPORT;
+UNLOCK TABLES;
+connect  con1,localhost,root,,test;
+BACKUP STAGE START;
+connection default;
+BACKUP STAGE END;
+connection con1;
+BACKUP STAGE END;
+disconnect con1;
+connection default;
+DROP TABLE t1;
+CREATE TABLE t1 (a INT);
+BACKUP STAGE START;
+FLUSH TABLES t1 WITH READ LOCK;
+UNLOCK TABLES;
+BACKUP STAGE BLOCK_COMMIT;
+BACKUP STAGE END;
+DROP TABLE t1;

mysql-test/main/backup_interaction.test

@@ -501,3 +501,38 @@ TRUNCATE t_temporary_innodb;
 ALTER TABLE t_temporary_innodb ADD COLUMN col2 INT;
 ALTER TABLE t_temporary_innodb ADD KEY idx(col2);
 BACKUP STAGE END;
+
+#
+# MDEV-18067, MDEV-18068 and MDEV-18069
+# Server crashes with BACKUP STAGE and FLUSH TABLE table_name
+#
+
+CREATE OR REPLACE TABLE t1 (pk INT PRIMARY KEY, f INT);
+BACKUP STAGE START;
+FLUSH TABLE t1 FOR EXPORT;
+UNLOCK TABLES;
+BACKUP STAGE END;
+DROP TABLE t1;
+
+CREATE TABLE t1 (a INT);
+BACKUP STAGE START;
+FLUSH TABLE t1 FOR EXPORT;
+UNLOCK TABLES;
+--connect (con1,localhost,root,,test)
+--send BACKUP STAGE START
+--connection default
+BACKUP STAGE END;
+--connection con1
+reap;
+BACKUP STAGE END;
+--disconnect con1
+--connection default
+DROP TABLE t1;
+
+CREATE TABLE t1 (a INT);
+BACKUP STAGE START;
+FLUSH TABLES t1 WITH READ LOCK;
+UNLOCK TABLES;
+BACKUP STAGE BLOCK_COMMIT;
+BACKUP STAGE END;
+DROP TABLE t1;

sql/mdl.cc

@@ -3166,14 +3166,19 @@ void MDL_context::set_transaction_duration_for_all_locks()
 
   DBUG_ASSERT(m_tickets[MDL_STATEMENT].is_empty());
 
-  m_tickets[MDL_TRANSACTION].swap(m_tickets[MDL_EXPLICIT]);
+  /* Don't swap locks if this thread is running backup stages */
+  if (current_thd->current_backup_stage == BACKUP_FINISHED)
+    m_tickets[MDL_TRANSACTION].swap(m_tickets[MDL_EXPLICIT]);
 
   Ticket_iterator it_ticket(m_tickets[MDL_EXPLICIT]);
 
   while ((ticket= it_ticket++))
   {
-    m_tickets[MDL_EXPLICIT].remove(ticket);
-    m_tickets[MDL_TRANSACTION].push_front(ticket);
+    if (ticket->get_key()->mdl_namespace() != MDL_key::BACKUP)
+    {
+      m_tickets[MDL_EXPLICIT].remove(ticket);
+      m_tickets[MDL_TRANSACTION].push_front(ticket);
+    }
   }
 
 #ifndef DBUG_OFF