MDEV-5509: Seconds_behind_master incorrect in parallel replication

The problem was a race between the SQL driver thread and the worker threads.
The SQL driver thread would set rli->last_master_timestamp to zero to
mark that it has caught up with the master, while the worker threads would
set it to the timestamp of the executed event. This can happen out-of-order
in parallel replication, causing the "caught up" status to be overwritten
and Seconds_Behind_Master to wrongly grow when the slave is idle.

To fix, introduce a separate flag rli->sql_thread_caught_up to mark that the
SQL driver thread is caught up. This avoids issues with worker threads
overwriting the SQL driver thread status. In parallel replication, we then
make SHOW SLAVE STATUS check in addition that all worker threads are idle
before showing Seconds_Behind_Master as 0 due to slave idle.

1 files changed, 6 insertions, 0 deletions

diff --git a/sql/rpl_rli.h b/sql/rpl_rli.h
index ff2ffd0b366..6716718d354 100644
--- a/sql/rpl_rli.h
+++ b/sql/rpl_rli.h
@@ -221,6 +221,12 @@ public:
   bool sql_force_rotate_relay;
 
   time_t last_master_timestamp;
+  /*
+    The SQL driver thread sets this true while it is waiting at the end of the
+    relay log for more events to arrive. SHOW SLAVE STATUS uses this to report
+    Seconds_Behind_Master as zero while the SQL thread is so waiting.
+  */
+  bool sql_thread_caught_up;
 
   void clear_until_condition();