MDEV-4485: Master did not allow slave to connect from the very start (empty GTID pos) if GTIDs from other multi_source master was present

The idea in the code was to protect the user that tries to connect a slave
to a master with completely different domains than what was intended. If
none of the domains in the start position are present at all in the master
binlog, we gave an error.

However, this is a stupid idea. Because when a slave connects to a master
to start replication from the very start of binlogs - such as when setting
up new master->slave servers from scratch - there will be just this
situation, the requested slave position is empty for all the domains in the
master's binlog.

So the code that gives this error is wrong, and the solution is simply to
remove it.

mysql-test/suite/rpl/r/rpl_gtid_mdev4485.result

@@ -0,0 +1,15 @@
+include/rpl_init.inc [topology=1->3]
+*** MDEV-4485. Master did not allow slave to connect from the very start (empty GTID pos) if GTIDs from other multi_source master was present ***
+create table t1 (i int);
+create table t2 (i int);
+set default_master_connection = 'm2';
+change master to master_host='127.0.0.1', master_port=SERVER_MYPORT_2, master_user='root', master_use_gtid=slave_pos;
+include/start_slave.inc
+insert into t2 values (1);
+drop table t1;
+drop table t2;
+set default_master_connection = 'm2';
+include/stop_slave.inc
+RESET SLAVE ALL;
+set default_master_connection = '';
+include/rpl_end.inc

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

@@ -0,0 +1,18 @@
+!include suite/rpl/rpl_1slave_base.cnf
+!include include/default_client.cnf
+
+[mysqld.1]
+log-slave-updates
+gtid-domain-id=1
+
+[mysqld.2]
+log-slave-updates
+gtid-domain-id=2
+
+[mysqld.3]
+log-slave-updates
+gtid-domain-id=3
+
+[ENV]
+SERVER_MYPORT_3=                @mysqld.3.port
+SERVER_MYSOCK_3=                @mysqld.3.socket

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

@@ -0,0 +1,40 @@
+--let $rpl_topology= 1->3
+--source include/rpl_init.inc
+
+--echo *** MDEV-4485. Master did not allow slave to connect from the very start (empty GTID pos) if GTIDs from other multi_source master was present ***
+
+--connection server_1
+create table t1 (i int);
+
+--connection server_2
+create table t2 (i int);
+
+--connection server_3
+
+set default_master_connection = 'm2';
+--replace_result $SERVER_MYPORT_2 SERVER_MYPORT_2
+eval change master to master_host='127.0.0.1', master_port=$SERVER_MYPORT_2, master_user='root', master_use_gtid=slave_pos;
+--source include/start_slave.inc
+
+--connection server_2
+insert into t2 values (1);
+--save_master_pos
+
+--connection server_3
+--sync_with_master 0,'m2'
+
+--connection server_1
+drop table t1;
+
+--connection server_2
+drop table t2;
+
+--connection server_3
+--sync_with_master 0,'m2'
+set default_master_connection = 'm2';
+--source include/stop_slave.inc
+RESET SLAVE ALL;
+set default_master_connection = '';
+
+--connection server_1
+--source include/rpl_end.inc

sql/sql_repl.cc

@@ -901,8 +901,6 @@ check_slave_start_position(THD *thd, slave_connection_state *st,
   rpl_gtid **delete_list= NULL;
   uint32 delete_idx= 0;
   bool slave_state_loaded= false;
-  uint32 missing_domains= 0;
-  rpl_gtid missing_domain_gtid;
 
   for (i= 0; i < st->hash.records; ++i)
   {
@@ -943,14 +941,7 @@ check_slave_start_position(THD *thd, slave_connection_state *st,
           We do not have anything in this domain, neither in the binlog nor
           in the slave state. So we are probably one master in a multi-master
           setup, and this domain is served by a different master.
-
-          This is not an error, however if we are missing _all_ domains
-          requested by the slave, then we still give error (below, after
-          the loop).
         */
-        if (!missing_domains)
-          missing_domain_gtid= *slave_gtid;
-        ++missing_domains;
         continue;
       }
 
@@ -1043,14 +1034,6 @@ check_slave_start_position(THD *thd, slave_connection_state *st,
     }
   }
 
-  if (missing_domains == st->hash.records && missing_domains > 0)
-  {
-    *errormsg= "Requested slave GTID state not found in binlog";
-    *error_gtid= missing_domain_gtid;
-    err= ER_GTID_POSITION_NOT_FOUND_IN_BINLOG;
-    goto end;
-  }
-
   /* Do any delayed deletes from the hash. */
   if (delete_list)
   {