When starting a new host process via start-host, it can sometimes take a long time to start the CRD host (maybe 3 minutes). In this situation, the session can be terminated even when the host reports a successful start.

Python sets up a parent/child process pair, connected via a pipe.
When the CRD host is ready, it sends SIGUSR1 to the child. The child handles this and sends "READY" to the parent via the pipe.
However, if the parent process has exited (perhaps due to start-host timeout), the child fails to write data to the pipe, with an uncaught IOError exception.

I think we should trap the IOError exception at that point. Even if the parent process has gone away, it's still better to allow the child to continue running.

Example log of a failed session, which took 3 minutes to start successfully but was then killed because of the unhandled exception:
[0310/105554.051208:INFO:signaling_connector.cc(73)] Signaling connected. New JID: [...]
[0310/105554.334353:ERROR:heartbeat_sender.cc(271)] Received error: Host ID not found
[0310/105604.477453:ERROR:heartbeat_sender.cc(271)] Received error: Host ID not found
[0310/105854.833419:INFO:remoting_me2me_host.cc(935)] Host ready to receive connections.
2018-03-10 10:58:54,833:INFO:Host ready to receive connections.
Traceback (most recent call last):
  File "/usr/lib/python2.7/logging/__init__.py", line 885, in emit
    self.flush()
  File "/usr/lib/python2.7/logging/__init__.py", line 845, in flush
    self.stream.flush()
IOError: [Errno 32] Broken pipe
Logged from file chrome-remote-desktop, line 741
Traceback (most recent call last):
  File "/opt/google/chrome-remote-desktop/chrome-remote-desktop", line 1772, in <module>
    sys.exit(main())
  File "/opt/google/chrome-remote-desktop/chrome-remote-desktop", line 1696, in main
    pid, status = waitpid_handle_exceptions(-1, deadline)
  File "/opt/google/chrome-remote-desktop/chrome-remote-desktop", line 1305, in waitpid_handle_exceptions
    pid_result, status = os.waitpid(pid, 0)
  File "/opt/google/chrome-remote-desktop/chrome-remote-desktop", line 743, in sigusr1_handler
    ParentProcessLogger.release_parent_if_connected(True)
  File "/opt/google/chrome-remote-desktop/chrome-remote-desktop", line 1041, in release_parent_if_connected
    instance._release_parent(success)
  File "/opt/google/chrome-remote-desktop/chrome-remote-desktop", line 1014, in _release_parent
    self._write_file.flush()
IOError: [Errno 32] Broken pipe
2018-03-10 10:58:54,862:INFO:Cleanup.
2018-03-10 10:58:54,863:INFO:Terminating X server
Gdk-Message: chrome-remote-desktop-host: Fatal IO error 11 (Resource temporarily unavailable) on X server :20.

2018-03-10 10:58:54,872:INFO:Terminating session
2018-03-10 10:58:54,902:INFO:Terminating host
[0310/105854.911995:WARNING:remoting_user_session.cc(464)] Child exited with status 1