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.

mysql-test/suite/rpl/r/rpl_gtid_basic.result

@@ -61,7 +61,13 @@ include/stop_slave.inc
 INSERT INTO t1 VALUES (5, "m1a");
 INSERT INTO t2 VALUES (5, "i1a");
 CHANGE MASTER TO master_host = '127.0.0.1', master_port = MASTER_PORT,
+MASTER_USE_GTID=SLAVE_POS;
+SET GLOBAL sql_slave_skip_counter=1;
+ERROR HY000: When using GTID, @@sql_slave_skip_counter can not be used. Instead, setting @@gtid_slave_pos explicitly can be used to skip to after a given GTID position.
+CHANGE MASTER TO master_host = '127.0.0.1', master_port = MASTER_PORT,
 MASTER_USE_GTID=CURRENT_POS;
+SET GLOBAL sql_slave_skip_counter=10;
+ERROR HY000: When using GTID, @@sql_slave_skip_counter can not be used. Instead, setting @@gtid_slave_pos explicitly can be used to skip to after a given GTID position.
 include/start_slave.inc
 SELECT * FROM t1 ORDER BY a;
 a	b

mysql-test/suite/rpl/t/rpl_gtid_basic.test

@@ -68,8 +68,16 @@ save_master_pos;
 
 connection server_4;
 --replace_result $MASTER_MYPORT MASTER_PORT
+eval CHANGE MASTER TO master_host = '127.0.0.1', master_port = $MASTER_MYPORT,
+     MASTER_USE_GTID=SLAVE_POS;
+# Test that sql_slave_skip_counter is prevented in GTID mode.
+--error ER_SLAVE_SKIP_NOT_IN_GTID
+SET GLOBAL sql_slave_skip_counter=1;
+--replace_result $MASTER_MYPORT MASTER_PORT
 eval CHANGE MASTER TO master_host = '127.0.0.1', master_port = $MASTER_MYPORT,
      MASTER_USE_GTID=CURRENT_POS;
+--error ER_SLAVE_SKIP_NOT_IN_GTID
+SET GLOBAL sql_slave_skip_counter=10;
 --source include/start_slave.inc
 sync_with_master;
 SELECT * FROM t1 ORDER BY a;

mysql-test/suite/rpl/t/rpl_gtid_mdev4485.test

@@ -28,6 +28,7 @@ drop table t1;
 
 --connection server_2
 drop table t2;
+--save_master_pos
 
 --connection server_3
 --sync_with_master 0,'m2'

sql/log_event.cc

@@ -6112,6 +6112,16 @@ void Binlog_checkpoint_log_event::pack_info(THD *thd, Protocol *protocol)
 {
   protocol->store(binlog_file_name, binlog_file_len, &my_charset_bin);
 }
+
+
+Log_event::enum_skip_reason
+Binlog_checkpoint_log_event::do_shall_skip(rpl_group_info *rgi)
+{
+  enum_skip_reason reason= Log_event::do_shall_skip(rgi);
+  if (reason == EVENT_SKIP_COUNT)
+    reason= EVENT_SKIP_NOT;
+  return reason;
+}
 #endif
 
 
@@ -6668,6 +6678,16 @@ Gtid_list_log_event::do_apply_event(rpl_group_info *rgi)
 }
 
 
+Log_event::enum_skip_reason
+Gtid_list_log_event::do_shall_skip(rpl_group_info *rgi)
+{
+  enum_skip_reason reason= Log_event::do_shall_skip(rgi);
+  if (reason == EVENT_SKIP_COUNT)
+    reason= EVENT_SKIP_NOT;
+  return reason;
+}
+
+
 void
 Gtid_list_log_event::pack_info(THD *thd, Protocol *protocol)
 {

sql/log_event.h

@@ -3057,6 +3057,7 @@ class Binlog_checkpoint_log_event: public Log_event
   bool is_valid() const { return binlog_file_name != 0; }
 #ifdef MYSQL_SERVER
   bool write(IO_CACHE* file);
+  enum_skip_reason do_shall_skip(rpl_group_info *rgi);
 #endif
 };
 
@@ -3274,6 +3275,7 @@ class Gtid_list_log_event: public Log_event
   bool to_packet(String *packet);
   bool write(IO_CACHE *file);
   virtual int do_apply_event(rpl_group_info *rgi);
+  enum_skip_reason do_shall_skip(rpl_group_info *rgi);
 #endif
   static bool peek(const char *event_start, uint32 event_len,
                    uint8 checksum_alg,

sql/share/errmsg-utf8.txt

@@ -6555,3 +6555,5 @@ ER_CHANGE_SLAVE_PARALLEL_THREADS_ACTIVE
 	eng "Cannot change @@slave_parallel_threads while another change is in progress"
 ER_PRIOR_COMMIT_FAILED
 	eng "Commit failed due to failure of an earlier commit on which this one depends"
+ER_SLAVE_SKIP_NOT_IN_GTID
+	eng "When using GTID, @@sql_slave_skip_counter can not be used. Instead, setting @@gtid_slave_pos explicitly can be used to skip to after a given GTID position."

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;
 }