The sql_slave_skip_counter is important to be able to recover replication from
certain errors. Often, an appropriate solution is to set
sql_slave_skip_counter to skip over a problem event. But setting
sql_slave_skip_counter produced an error in GTID mode, with a suggestion to
instead set @@gtid_slave_pos to point past the problem event. This however is
not always possible; for example, in case of an INCIDENT event, that event
does not have any GTID to assign to @@gtid_slave_pos.
With this patch, sql_slave_skip_counter now works in GTID mode the same was as
in non-GTID mode. When set, that many initial events are skipped when the SQL
thread starts, plus as many extra events are needed to completely skip any
partially skipped event group. The GTID position is updated to point past the
skipped event(s).
If replication breaks in GTID mode, it is not trivial to determine the GTID of
the failing event group. This is a problem, as such GTID is needed eg. to
explicitly set @@gtid_slave_pos to skip to after that event group, or to
compare errors on different servers, etc.
Fix by ensuring that relevant slave errors logged to the error log include the
GTID of the event group containing the problem event.
