Avoid calling closed? outside of synchronize block

[djmb]

closed? calls process on the connection which is not safe because we have not synchronised the connection pool. Another thread might concurrently checkout the connection and start sending commands as well.

[djmb]

When using SSHKit with a 100+ hosts, we sometimes get IOErrors like this:

[ERROR (IOError): Exception while executing on host foo: closed stream]

Debugging showed that a packet was being sent to the remote server twice, which was causing it to close the connection. This was caused by two threads using the connection concurrently - the eviction thread and one of the SSHKit parallel runner threads.

The duplicate packets came from here in net-ssh, where both threads call send before either calls output.consume!.

Looking at the eviction code, the call to closed?(first_conn) outside the synchronize block looked suspicious as it calls conn.process(0) which will send a test packet.

The check looks like a performance optimisation, so I think it should be safe to remove. We've not seen the errors again with this fix running.

[mattbrictson]

Thanks for the fix! Could you make the following change to address the rubocop failure in CI?

[djmb]

Thanks for the review @mattbrictson - I've committed your suggestion.