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, 27 insertions, 0 deletions

diff --git a/sql/sys_vars.cc b/sql/sys_vars.cc
index 08b4953b2e4..cd24ad38eb2 100644
--- a/sql/sys_vars.cc
+++ b/sql/sys_vars.cc
@@ -1538,6 +1538,33 @@ static Sys_var_gtid_binlog_state Sys_gtid_binlog_state(
        GLOBAL_VAR(opt_gtid_binlog_state_dummy), NO_CMD_LINE);
 
 
+static Sys_var_last_gtid Sys_last_gtid(
+       "last_gtid", "The GTID of the last commit (if binlogging was enabled), "
+       "or the empty string if none.",
+       READ_ONLY sys_var::ONLY_SESSION, NO_CMD_LINE);
+
+
+uchar *
+Sys_var_last_gtid::session_value_ptr(THD *thd, LEX_STRING *base)
+{
+  char buf[10+1+10+1+20+1];
+  String str(buf, sizeof(buf), system_charset_info);
+  char *p;
+  bool first= true;
+
+  str.length(0);
+  if ((thd->last_commit_gtid.seq_no > 0 &&
+       rpl_slave_state_tostring_helper(&str, &thd->last_commit_gtid, &first)) ||
+      !(p= thd->strmake(str.ptr(), str.length())))
+  {
+    my_error(ER_OUT_OF_RESOURCES, MYF(0));
+    return NULL;
+  }
+
+  return (uchar *)p;
+}
+
+
 static bool
 check_slave_parallel_threads(sys_var *self, THD *thd, set_var *var)
 {