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  1. MariaDB Server
  2. MDEV-24581

MDL race condition in trans_rollback_to_savepoint()




      In MariaDB Server 10.0.11, a change from MySQL 5.6.16 was applied.

      As far as I understand, the intention of the ha_rollback_to_savepoint_can_release_mdl() check is to ensure that no other transaction inside InnoDB is waiting for a lock on the table that the ROLLBACK TO SAVEPOINT would release. But, the check seems to be misplaced. We could have the following happen:

      1. innobase_rollback_to_savepoint_can_release_mdl() returns true, because the transaction only had placed implicit record locks. (Until MDEV-16232 is implemented, this is only possible if the transaction performed nothing else than INSERT operations).
      2. Another transaction starts to wait for a record lock that our transaction had implicitly locked. The implicit lock will be converted to an explicit one.
      3. ha_rollback_to_savepoint() will be executed. No explicit locks will be released, because we did not roll back the entire transaction.
      4. The MDL will be released in thd->mdl_context.rollback_to_savepoint(), even though the other transaction will be blocked inside InnoDB until our transaction has been committed or rolled back.

      Note: In innobase_rollback_to_savepoint_can_release_mdl() there is another race that would be fixed by the first hunk of the following untested patch:

      diff --git a/storage/innobase/handler/ha_innodb.cc b/storage/innobase/handler/ha_innodb.cc
      index 7cc4401d0ff..bca47f011a6 100644
      --- a/storage/innobase/handler/ha_innodb.cc
      +++ b/storage/innobase/handler/ha_innodb.cc
      @@ -4999,15 +4999,14 @@ innobase_rollback_to_savepoint_can_release_mdl(
       	DBUG_ASSERT(hton == innodb_hton_ptr);
       	trx_t*	trx = check_trx_exists(thd);
      +	trx_mutex_enter(trx);
      +	ulint n_locks = UT_LIST_GET_LEN(trx->lock.trx_locks) == 0;
      +	trx_mutex_exit(trx);
       	/* If transaction has not acquired any locks then it is safe
       	to release MDL after rollback to savepoint */
      -	if (UT_LIST_GET_LEN(trx->lock.trx_locks) == 0) {
      -		DBUG_RETURN(true);
      -	}
      -	DBUG_RETURN(false);
      +	DBUG_RETURN(!n_locks);
      diff --git a/sql/transaction.cc b/sql/transaction.cc
      index 543e0b7ad38..66b40d23e80 100644
      --- a/sql/transaction.cc
      +++ b/sql/transaction.cc
      @@ -718,10 +718,6 @@ bool trans_rollback_to_savepoint(THD *thd, LEX_STRING name)
           For backward-compatibility reasons we always release MDL if binary
           logging is off.
      -  bool mdl_can_safely_rollback_to_savepoint=
      -                (!(mysql_bin_log.is_open() && thd->variables.sql_log_bin) ||
      -                 ha_rollback_to_savepoint_can_release_mdl(thd));
         if (ha_rollback_to_savepoint(thd, sv))
           res= TRUE;
         else if (((thd->variables.option_bits & OPTION_KEEP_LOG) ||
      @@ -733,7 +729,8 @@ bool trans_rollback_to_savepoint(THD *thd, LEX_STRING name)
         thd->transaction.savepoints= sv;
      -  if (!res && mdl_can_safely_rollback_to_savepoint)
      +  if (!res && (!(mysql_bin_log.is_open() && thd->variables.sql_log_bin) ||
      +               ha_rollback_to_savepoint_can_release_mdl(thd)))

      Note: I did not review whether this could break something in other implementations of handlerton::savepoint_rollback_can_release_mdl.


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              marko Marko Mäkelä
              marko Marko Mäkelä
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