Subprocess launched in Uvicorn worker always returns zero exit code

Hi everyone, I am facing one of those little bugs that almost seem impossible to happen (at least IMHO):

Imagine that I have the following “ASGI server app” (server.py), in which I want to make a call to a bash command using subprocess.Popen (I know, there is no ASGI app, but this is just an example):

import subprocess

# The command mkdir without any other
# arguments would normally return nonzero 
# exit code (it returns 1):
proc = subprocess.Popen(["mkdir"], stdout=subprocess.PIPE,
                         stderr=subprocess.PIPE)

output, error = (str(s.decode()).strip() for s in proc.communicate())

print("Output: ", output)
print("Error: ", error)

# However, inside a Uvicorn worker, it returns 0 exit code:
print("Exit code: ", proc.returncode)

And we run our “app” with:

gunicorn -k uvicorn.workers.UvicornWorker server.py

The exit code result of subprocess.Popen is always 0, even though it shouldn’t. The output is:

Output:
Error: mkdir: missing operand
Try 'mkdir --help' for more information.
Exit code: 0

However, if I just use Gunicorn’s default sync workers:

gunicorn server.py

The return code is the right one:

Output:
Error: mkdir: missing operand
Try 'mkdir --help' for more information.
Exit code: 1

Is there any reason why Uvicorn’s Gunicorn worker always return zero exit code from subprocess.Popen?

I know it is a weird thing to do, but I actually need a bash call inside the Uvicorn workers.

Thank you!

[julioasotodv]

It turns out that quite a bunch of libraries perform that call by using the autover library: https://github.com/pyviz-dev/autover

If Popen does not work correctly, uvicorn turns out to be incompatible with such libraries, for instance.

[euri10]

See also https://github.com/encode/uvicorn/pull/895