MDEV-26: Global transaction ID.

Fix problems related to reconnect. When we need to reconnect (ie. explict
stop/start of just the IO thread by user, or automatic reconnect due to
loosing network connection with the master), it is a bit complex to correctly
resume at the right point without causing duplicate or missing events in the
relay log. The previous code had multiple problems in this regard.

With this patch, the problem is solved as follows. The IO thread keeps track
(in memory) of which GTID was last queued to the relay log. If it needs to
reconnect, it resumes at that GTID position. It also counts number of events
received within the last, possibly partial, event group, and skips the same
number of events after a reconnect, so that events already enqueued before the
reconnect are not duplicated.

(There is no need to keep any persistent state; whenever we restart slave
threads after both of them being stopped (such as after server restart), we
erase the relay logs and start over from the last GTID applied by SQL thread.
But while the SQL thread is running, this patch is needed to get correct relay
log).

1 files changed, 4 insertions, 0 deletions

diff --git a/sql/rpl_gtid.h b/sql/rpl_gtid.h
index 40d51568357..4d5302020bf 100644
--- a/sql/rpl_gtid.h
+++ b/sql/rpl_gtid.h
@@ -93,6 +93,8 @@ struct rpl_slave_state
   int record_gtid(THD *thd, const rpl_gtid *gtid, uint64 sub_id,
                   bool in_transaction, bool in_statement);
   uint64 next_subid(uint32 domain_id);
+  int iterate(int (*cb)(rpl_gtid *, void *), void *data,
+              rpl_gtid *extra_gtids, uint32 num_extra);
   int tostring(String *dest, rpl_gtid *extra_gtids, uint32 num_extra);
   bool domain_to_gtid(uint32 domain_id, rpl_gtid *out_gtid);
   int load(THD *thd, char *state_from_master, size_t len, bool reset,
@@ -178,8 +180,10 @@ struct slave_connection_state
   slave_connection_state();
   ~slave_connection_state();
 
+  void reset() { my_hash_reset(&hash); }
   int load(char *slave_request, size_t len);
   int load(const rpl_gtid *gtid_list, uint32 count);
+  int load(rpl_slave_state *state, rpl_gtid *extra_gtids, uint32 num_extra);
   rpl_gtid *find(uint32 domain_id);
   int update(const rpl_gtid *in_gtid);
   void remove(const rpl_gtid *gtid);