The asyncio.subprocess.Process class never closes the subprocess transport, whereas this transport is private.

I propose to close the transport as soon as possible: when transport.get_returncode() is called and its result is not None.

I'm not sure that it's the most efficient way to close the transport. It may be better to close the transport in the connection_lost() method of the protocol.

The patch also checks in Process.communicate() that the process is alive, and is clears the reference to the transport.