ERROR Unable to acquire lock gh-ost.3726786.lock

Hi. When i run gh-ost on the master without the --execute using the command below:

```
./gh-ost --host=192.168.24.22 --user=ghost --password=ghostPass --database=test --table=tb1 --alter="ADD COLUMN x INT NOT NULL DEFAULT '0'" --chunk-size=2000 --max-load=Threads_connected=20 --allow-on-master`
```

I get the following error:

```
# throttle-additional-flag-file: /tmp/gh-ost.throttle [225/1843]
# Serving on unix socket: /tmp/gh-ost.test.tb1.sock
Copy: 0/0 100.0%; Applied: 0; Backlog: 0/1000; Time: 0s(total), 0s(copy); streamer: mysqld-bin.000006:12839070; State: migrating; ETA: due
CREATE TABLE `_tb1_gho` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`x` int(11) NOT NULL DEFAULT '0',
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
2018/04/16 10:53:10 binlogsyncer.go:107: [info] syncer is closing...
2018/04/16 10:53:10 binlogstreamer.go:47: [error] close sync with err: sync is been closing...
2018/04/16 10:53:10 binlogsyncer.go:122: [info] syncer is closed
Copy: 0/6 0.0%; Applied: 0; Backlog: 0/1000; Time: 0s(total), 0s(copy); streamer: mysqld-bin.000006:12839070; State: migrating; ETA: N/A
# Done
```

And when i run with the --execute option, i get the following error:

```
Copy: 8/8 100.0%; Applied: 0; Backlog: 0/1000; Time: 1m0s(total), 1s(copy); streamer: mysqld-bin.000006:13108629; State: migrating; ETA: due
2018-04-16 10:57:12 ERROR Unable to acquire lock gh-ost.3726786.lock
2018-04-16 10:57:12 ERROR Unable to acquire lock gh-ost.3726786.lock
2018-04-16 10:57:12 FATAL Unable to acquire lock gh-ost.3726786.lock
```

Either way, the command does not seem to be running as expected. I have also tried running on the slave with the initial commands you provided and still no luck. The tool seems to be promising as an alternative to Percona's pt-online-schema-change, but sad cannot get it to run.