MDEV-6551: Some replication errors are ignored if slave_parallel_threads > 0

The problem occured when using parallel replication, and an error occured that
caused the SQL thread to stop when the IO thread had already reached a
following binlog file from the master (or otherwise performed a relay log
rotation).

In this case, the Rotate Event at the end of the relay log file could still be
executed, even though an earlier event in that relay log file had gotten an
error. This would cause the position to be incorrectly updated, so that upon
restart of the SQL thread, the event that had failed would be silently skipped
and ignored, causing replication corruption.

Fixed by checking before executing Rotate Event, whether an earlier event
has failed. If so, the Rotate Event is not executed, just dequeued, same as
for other normal events following a failing event.

1 files changed, 9 insertions, 1 deletions

diff --git a/sql/rpl_parallel.h b/sql/rpl_parallel.h
index 415259cd3c4..c7e15528e97 100644
--- a/sql/rpl_parallel.h
+++ b/sql/rpl_parallel.h
@@ -72,7 +72,15 @@ struct rpl_parallel_thread {
   rpl_parallel_entry *current_entry;
   struct queued_event {
     queued_event *next;
-    Log_event *ev;
+    /*
+      queued_event can hold either an event to be executed, or just a binlog
+      position to be updated without any associated event.
+    */
+    enum queued_event_t { QUEUED_EVENT, QUEUED_POS_UPDATE } typ;
+    union {
+      Log_event *ev;                            /* QUEUED_EVENT */
+      rpl_parallel_entry *entry_for_queued;     /* QUEUED_POS_UPDATE */
+    };
     rpl_group_info *rgi;
     inuse_relaylog *ir;
     ulonglong future_event_relay_log_pos;