MDEV-4937: sql_slave_skip_counter does not work with GTID

As a side-effect of purge_relay_logs(), sql_slave_skip_counter
was silently ignored in GTID mode.

But sql_slave_skip_counter in fact is not a good match with GTID.
And it is not really needed either, as users can explicitly set
@@gtid_slave_pos to skip specific GTIDs, in a way that matches
well how GTID replication works.

So with this patch, we give an error on attempts to set
sql_slave_skip_counter when using GTID, with a suggestion to use
gtid_slave_pos instead, if needed.

1 files changed, 6 insertions, 1 deletions

diff --git a/sql/sys_vars.cc b/sql/sys_vars.cc
index 0495ee3e240..69d93968f9f 100644
--- a/sql/sys_vars.cc
+++ b/sql/sys_vars.cc
@@ -3988,13 +3988,18 @@ bool update_multi_source_variable(sys_var *self_var, THD *thd,
 
 static bool update_slave_skip_counter(sys_var *self, THD *thd, Master_info *mi)
 {
+  if (mi->using_gtid != Master_info::USE_GTID_NO)
+  {
+    my_error(ER_SLAVE_SKIP_NOT_IN_GTID, MYF(0));
+    return true;
+  }
   if (mi->rli.slave_running)
   {
     my_error(ER_SLAVE_MUST_STOP, MYF(0), mi->connection_name.length,
              mi->connection_name.str);
     return true;
   }
-  /* The value was stored temporarly in thd */
+  /* The value was stored temporarily in thd */
   mi->rli.slave_skip_counter= thd->variables.slave_skip_counter;
   return false;
 }