MDEV-20867 - Perform careful review of "Server crashes with BACKUP STAGE and FLUSH TABLE table_name"

Reverted original patch (c2e0a0b).

For consistency with "LOCK TABLE <table_name> READ" and "FLUSH TABLES
WITH READ LOCK", which are forbidden under "BACKUP STAGE", forbid "FLUSH
TABLE <table_name> FOR EXPORT" and "FLUSH TABLE <table_name> WITH READ
LOCK" as well.

It'd allow consistent fixes for problems like MDEV-18643.

mysql-test/main/backup_interaction.result

@@ -521,26 +521,8 @@ 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;
+ERROR HY000: Can't execute the command as you have a BACKUP STAGE active
+FLUSH TABLE t1 WITH READ LOCK;
+ERROR HY000: Can't execute the command as you have a BACKUP STAGE active
 BACKUP STAGE END;
 DROP TABLE t1;

mysql-test/main/backup_interaction.test

@@ -509,30 +509,9 @@ BACKUP STAGE END;
 
 CREATE OR REPLACE TABLE t1 (pk INT PRIMARY KEY, f INT);
 BACKUP STAGE START;
+--error ER_BACKUP_LOCK_IS_ACTIVE
 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;
+--error ER_BACKUP_LOCK_IS_ACTIVE
+FLUSH TABLE t1 WITH READ LOCK;
 BACKUP STAGE END;
 DROP TABLE t1;

sql/mdl.cc

@@ -3166,19 +3166,14 @@ void MDL_context::set_transaction_duration_for_all_locks()
 
   DBUG_ASSERT(m_tickets[MDL_STATEMENT].is_empty());
 
-  /* 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]);
+  m_tickets[MDL_TRANSACTION].swap(m_tickets[MDL_EXPLICIT]);
 
   Ticket_iterator it_ticket(m_tickets[MDL_EXPLICIT]);
 
   while ((ticket= it_ticket++))
   {
-    if (ticket->get_key()->mdl_namespace() != MDL_key::BACKUP)
-    {
-      m_tickets[MDL_EXPLICIT].remove(ticket);
-      m_tickets[MDL_TRANSACTION].push_front(ticket);
-    }
+    m_tickets[MDL_EXPLICIT].remove(ticket);
+    m_tickets[MDL_TRANSACTION].push_front(ticket);
   }
 
 #ifndef DBUG_OFF

sql/sql_class.cc

@@ -5580,6 +5580,7 @@ void THD::leave_locked_tables_mode()
 {
   if (locked_tables_mode == LTM_LOCK_TABLES)
   {
+    DBUG_ASSERT(current_backup_stage == BACKUP_FINISHED);
     /*
       When leaving LOCK TABLES mode we have to change the duration of most
       of the metadata locks being held, except for HANDLER and GRL locks,

sql/sql_reload.cc

@@ -525,6 +525,12 @@ bool flush_tables_with_read_lock(THD *thd, TABLE_LIST *all_tables)
     goto error;
   }
 
+  if (thd->current_backup_stage != BACKUP_FINISHED)
+  {
+    my_error(ER_BACKUP_LOCK_IS_ACTIVE, MYF(0));
+    return true;
+  }
+
   if (thd->lex->type & REFRESH_READ_LOCK)
   {
     /*