MDEV-4984: Implement MASTER_GTID_WAIT() and @@LAST_GTID.

MASTER_GTID_WAIT() is similar to MASTER_POS_WAIT(), but works with a
GTID position rather than an old-style filename/offset.

@@LAST_GTID gives the GTID assigned to the last transaction written
into the binlog.

Together, the two can be used by applications to obtain the GTID of
an update on the master, and then do a MASTER_GTID_WAIT() for that
position on any read slave where it is important to get results that
are caught up with the master at least to the point of the update.

The implementation of MASTER_GTID_WAIT() is implemented in a way
that tries to minimise the performance impact on the SQL threads,
even in the presense of many waiters on single GTID positions (as
from @@LAST_GTID).

1 files changed, 50 insertions, 0 deletions

diff --git a/sql/sys_vars.h b/sql/sys_vars.h
index 3cc4da32811..6a84fc5fbc2 100644
--- a/sql/sys_vars.h
+++ b/sql/sys_vars.h
@@ -2211,3 +2211,53 @@ public:
   }
   uchar *global_value_ptr(THD *thd, LEX_STRING *base);
 };
+
+
+/**
+  Class for @@session.last_gtid.
+*/
+class Sys_var_last_gtid: public sys_var
+{
+public:
+  Sys_var_last_gtid(const char *name_arg,
+          const char *comment, int flag_args, CMD_LINE getopt)
+    : sys_var(&all_sys_vars, name_arg, comment, flag_args, 0, getopt.id,
+              getopt.arg_type, SHOW_CHAR, 0, NULL, VARIABLE_NOT_IN_BINLOG,
+              NULL, NULL, NULL)
+  {
+    option.var_type= GET_STR;
+  }
+  bool do_check(THD *thd, set_var *var)
+  {
+    DBUG_ASSERT(false);
+    return true;
+  }
+  bool session_update(THD *thd, set_var *var)
+  {
+    DBUG_ASSERT(false);
+    return true;
+  }
+  bool global_update(THD *thd, set_var *var)
+  {
+    DBUG_ASSERT(false);
+    return true;
+  }
+  bool check_update_type(Item_result type) {
+    DBUG_ASSERT(false);
+    return false;
+  }
+  void session_save_default(THD *thd, set_var *var)
+  {
+    DBUG_ASSERT(false);
+  }
+  void global_save_default(THD *thd, set_var *var)
+  {
+    DBUG_ASSERT(false);
+  }
+  uchar *session_value_ptr(THD *thd, LEX_STRING *base);
+  uchar *global_value_ptr(THD *thd, LEX_STRING *base)
+  {
+    DBUG_ASSERT(false);
+    return NULL;
+  }
+};