MDEV-6321: close_temporary_tables() in format description event not serialised correctly

When a master server starts up, it logs a special format_description event at
the start of a new binlog to mark that is has restarted. This is used by a
slave to drop all temporary tables - this is needed in case the master crashed
and did not have a chance to send explicit DROP TEMPORARY TABLE statements to
the slave.

In parallel replication, we need to be careful when dropping the temporary
tables - we need to be sure that no prior events are still executing that
might be using the temporary tables to be dropped, _and_ that no following
events have started executing that might have created new temporary tables
that should not be dropped.

This was not handled correctly, which could cause errors about access to not
existing temporary tables or even crashes. This patch implements that such
format_description events cause serialisation of event execution; all prior
events are executed to completion first, then the format_description event is
executed, dropping temporary tables, then following events are queued for
execution.

Master restarts should be sufficiently infrequent that the resulting loss of
parallelism should be of minimal impact.

1 files changed, 1 insertions, 0 deletions

diff --git a/sql/rpl_parallel.h b/sql/rpl_parallel.h
index c7e15528e97..d59205cc72a 100644
--- a/sql/rpl_parallel.h
+++ b/sql/rpl_parallel.h
@@ -239,6 +239,7 @@ struct rpl_parallel {
   void wait_for_done(THD *thd, Relay_log_info *rli);
   void stop_during_until();
   bool workers_idle();
+  void wait_for_workers_idle(THD *thd);
   int do_event(rpl_group_info *serial_rgi, Log_event *ev, ulonglong event_size);
 };