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, 34 insertions, 3 deletions

diff --git a/sql/rpl_mi.h b/sql/rpl_mi.h
index 916bd0dae02..38daed0e260 100644
--- a/sql/rpl_mi.h
+++ b/sql/rpl_mi.h
@@ -61,6 +61,10 @@ typedef struct st_mysql MYSQL;
 class Master_info : public Slave_reporting_capability
 {
  public:
+  enum enum_using_gtid {
+    USE_GTID_NO= 0, USE_GTID_CURRENT_POS= 1, USE_GTID_SLAVE_POS= 2
+  };
+
   Master_info(LEX_STRING *connection_name, bool is_slave_recovery);
   ~Master_info();
   bool shall_ignore_server_id(ulong s_id);
@@ -70,6 +74,7 @@ class Master_info : public Slave_reporting_capability
     /* If malloc() in initialization failed */
     return connection_name.str == 0;
   }
+  static const char *using_gtid_astext(enum enum_using_gtid arg);
 
   /* the variables below are needed because we can change masters on the fly */
   char master_log_name[FN_REFLEN+6]; /* Room for multi-*/
@@ -135,9 +140,35 @@ class Master_info : public Slave_reporting_capability
     Note that you can not change the numeric values of these, they are used
     in master.info.
   */
-  enum {
-    USE_GTID_NO= 0, USE_GTID_CURRENT_POS= 1, USE_GTID_SLAVE_POS= 2
-  } using_gtid;
+  enum enum_using_gtid using_gtid;
+
+  /*
+    This GTID position records how far we have fetched into the relay logs.
+    This is used to continue fetching when the IO thread reconnects to the
+    master.
+
+    (Full slave stop/start does not use it, as it resets the relay logs).
+  */
+  slave_connection_state gtid_current_pos;
+  /*
+    If events_queued_since_last_gtid is non-zero, it is the number of events
+    queued so far in the relaylog of a GTID-prefixed event group.
+    It is zero when no partial event group has been queued at the moment.
+  */
+  uint64 events_queued_since_last_gtid;
+  /*
+    The GTID of the partially-queued event group, when
+    events_queued_since_last_gtid is non-zero.
+  */
+  rpl_gtid last_queued_gtid;
+  /*
+    When slave IO thread needs to reconnect, gtid_reconnect_event_skip_count
+    counts number of events to skip from the first GTID-prefixed event group,
+    to avoid duplicating events in the relay log.
+  */
+  uint64 gtid_reconnect_event_skip_count;
+  /* gtid_event_seen is false until we receive first GTID event from master. */
+  bool gtid_event_seen;
 };
 int init_master_info(Master_info* mi, const char* master_info_fname,
 		     const char* slave_info_fname,