MDEV-28530: Revoking privileges from a non-existing user on a master breaks replication on the slave in the presence of replication filters

Problem:
========
Replication can break while applying a query log event if its
respective command errors on the primary, but is ignored by the
replication filter within Grant_tables on the replica. The bug
reported by MDEV-28530 shows this with REVOKE ALL PRIVILEGES using a
non-existent user. The primary will binlog the REVOKE command with
an error code, and the replica will think the command executed with
success because the replication filter will ignore the command while
accessing the Grant_tables classes. When the replica performs an
error check, it sees the difference between the error codes, and
replication breaks.

Solution:
========
If the replication filter check done by Grant_tables logic ignores
the tables, reset thd->slave_expected_error to 0 so that
Query_log_event::do_apply_event() can be made aware that the
underlying query was ignored when it compares errors.

Note that this bug also effects DROP USER if not all users exist
in the provided list, and the patch fixes and tests this case.

Reviewed By:
============
andrei.elkin@mariadb.com

1 files changed, 2 insertions, 0 deletions

diff --git a/sql/sql_class.h b/sql/sql_class.h
index 05b8f7eb443..d3e2b72b12d 100644
--- a/sql/sql_class.h
+++ b/sql/sql_class.h
@@ -3209,6 +3209,8 @@ public:
   /*
     In case of a slave, set to the error code the master got when executing
     the query. 0 if no error on the master.
+    The stored into variable master error code may get reset inside
+    execution stack when the event turns out to be ignored.
   */
   int	     slave_expected_error;
   enum_sql_command last_sql_command;  // Last sql_command exceuted in mysql_execute_command()