This PR is for fixing the same problem what #104 have, but with another solution.

Problem

Deadlock occurred when we use stdout.readline to listen to the subprocess that doesn't end with EOF.

I found that my Allzpark will freeze when closing the application that it just launched, IF Allzpark was launched from a Rez resolved context. And the freeze was because the stdout.readline keep sending empty line b"" due to no EOF signal.

So why there has no EOF emitted from subprocess when Allzpark is launched from a resolved context ? I haven't figure it out. :(
It seems like cmd.exe in subprocess is not able to send EOF, not sure.

Here's a simple reproducible :

>>> import subprocess
>>> popen = subprocess.Popen(["start", "cmd"], shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
>>> for line in iter(popen.stdout.readline, ""):
...     print(line)
...     # now close the terminal window
...
b''
b''
b''
b''
# endless empty line

Nevertheless, I came up a solution that seems solid. 🤔

I have changed the stderr listening function to act like Popen.communicate(), since I think stderr doesn't need to be streamed.

buffer = []
buffer.append(stderr.read())
# subprocess ended
stderr.close()

(Referenced from Python's subprocess.py)

With that we can know when the subprocess is ended, and send our own EOF flag to the stdout listening thread to make it stop . Problem solved.