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, 28 insertions, 0 deletions

diff --git a/sql/item_func.cc b/sql/item_func.cc
index 5d9abbb0d8c..b2af80e6d96 100644
--- a/sql/item_func.cc
+++ b/sql/item_func.cc
@@ -3989,6 +3989,34 @@ err:
 }
 
 
+longlong Item_master_gtid_wait::val_int()
+{
+  DBUG_ASSERT(fixed == 1);
+  longlong result= 0;
+
+  if (args[0]->null_value)
+  {
+    null_value= 1;
+    return 0;
+  }
+
+  null_value=0;
+#ifdef HAVE_REPLICATION
+  THD* thd= current_thd;
+  longlong timeout_us;
+  String *gtid_pos = args[0]->val_str(&value);
+
+  if (arg_count==2 && !args[1]->null_value)
+    timeout_us= (longlong)(1e6*args[1]->val_real());
+  else
+    timeout_us= (longlong)-1;
+
+  result= rpl_global_gtid_waiting.wait_for_pos(thd, gtid_pos, timeout_us);
+#endif
+  return result;
+}
+
+
 /**
   Enables a session to wait on a condition until a timeout or a network
   disconnect occurs.