uvicorn.run with reload or workers in ProcessPoolExecutor fails with Bad file descriptor error

Greetings!

I have run into an issue when trying to spawn several local uvicorns (obviously bound to different ports) from within Python. Instead of launching each one manually in a separate terminal, I have set up a function that spawns the desired servers in a ProcessPoolExecutor (imported from concurrent.futures). However, if the servers are configured with the workers or reload options, I get the following error:

    Process SpawnProcess-1:1:
    Traceback (most recent call last):
      File "/usr/lib/python3.8/multiprocessing/process.py", line 315, in _bootstrap
        self.run()
      File "/usr/lib/python3.8/multiprocessing/process.py", line 108, in run
        self._target(*self._args, **self._kwargs)
      File "/home/cantor/code/envs/tabula/lib/python3.8/site-packages/uvicorn/subprocess.py", line 55, in subprocess_started
        sys.stdin = os.fdopen(stdin_fileno)
      File "/usr/lib/python3.8/os.py", line 1023, in fdopen
        return io.open(fd, *args, **kwargs)
    OSError: [Errno 9] Bad file descriptor

Stating the obvious, there is an issue with reopening stdin in the child process.

Simple workarounds resulting in happy server spawning:

• Commenting out lines 54 and 55 in subprocess.py

    # if stdin_fileno is not None:
        # sys.stdin = os.fdopen(stdin_fileno)

• Setting 'stdin_fileno': None on line 35 in subprocess.py

To reproduce

pip install fastapi uvicorn

(Assuming the code is located in launch.py)

With workers:

    import uvicorn
    from fastapi import FastAPI
    from concurrent.futures import ProcessPoolExecutor

    app = FastAPI()

    if __name__ == '__main__':

        with ProcessPoolExecutor() as executor:

            executor.submit(uvicorn.run, 'launch:app', workers = 3)

Error:

    Process SpawnProcess-1:3:
    Traceback (most recent call last):
    File "/usr/lib/python3.8/multiprocessing/process.py", line 315, in _bootstrap
        self.run()
    File "/usr/lib/python3.8/multiprocessing/process.py", line 108, in run
        self._target(*self._args, **self._kwargs)
    File "/usr/lib/python3.8/site-packages/uvicorn/subprocess.py", line 55, in subprocess_started
        sys.stdin = os.fdopen(stdin_fileno)
    File "/usr/lib/python3.8/os.py", line 1023, in fdopen
        return io.open(fd, *args, **kwargs)
    OSError: [Errno 9] Bad file descriptor
    Process SpawnProcess-1:1:
    Traceback (most recent call last):
    File "/usr/lib/python3.8/multiprocessing/process.py", line 315, in _bootstrap
        self.run()
    File "/usr/lib/python3.8/multiprocessing/process.py", line 108, in run
        self._target(*self._args, **self._kwargs)
    File "/usr/lib/python3.8/site-packages/uvicorn/subprocess.py", line 55, in subprocess_started
        sys.stdin = os.fdopen(stdin_fileno)
    File "/usr/lib/python3.8/os.py", line 1023, in fdopen
        return io.open(fd, *args, **kwargs)
    OSError: [Errno 9] Bad file descriptor
    Process SpawnProcess-1:2:
    Traceback (most recent call last):
    File "/usr/lib/python3.8/multiprocessing/process.py", line 315, in _bootstrap
        self.run()
    File "/usr/lib/python3.8/multiprocessing/process.py", line 108, in run
        self._target(*self._args, **self._kwargs)
    File "/usr/lib/python3.8/site-packages/uvicorn/subprocess.py", line 55, in subprocess_started
        sys.stdin = os.fdopen(stdin_fileno)
    File "/usr/lib/python3.8/os.py", line 1023, in fdopen
        return io.open(fd, *args, **kwargs)
    OSError: [Errno 9] Bad file descriptor

With reload:

    import uvicorn
    from fastapi import FastAPI
    from concurrent.futures import ProcessPoolExecutor

    app = FastAPI()

    if __name__ == '__main__':

        with ProcessPoolExecutor() as executor:

            executor.submit(uvicorn.run, 'launch:app', reload = True)

Error:

    Process SpawnProcess-1:1:
    Traceback (most recent call last):
    File "/usr/lib/python3.8/multiprocessing/process.py", line 315, in _bootstrap
        self.run()
    File "/usr/lib/python3.8/multiprocessing/process.py", line 108, in run
        self._target(*self._args, **self._kwargs)
    File "/usr/lib/python3.8/site-packages/uvicorn/subprocess.py", line 55, in subprocess_started
        sys.stdin = os.fdopen(stdin_fileno)
    File "/usr/lib/python3.8/os.py", line 1023, in fdopen
        return io.open(fd, *args, **kwargs)
    OSError: [Errno 9] Bad file descriptor

With 'stdin_fileno': None on line 35 in subprocess.py:

INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     Started reloader process [705451] using statreload
INFO:     Started server process [705462]
INFO:     Waiting for application startup.
INFO:     Application startup complete.

Expected behavior

uvicorn server starting normally with a reloader or one or more workers.

Actual behavior

Server runs, but no reloader or worker subprocesses.

Environment

• OS: Ubuntu 20.04
• Python: CPython 3.8.6
• Uvicorn version: 0.12.3

[euri10]

In summary, I’d say that a try/except is perfectly acceptable here (although I’d also argue that reopening stdin is not even necessary, but I’d be happy to stand corrected on that). I would gladly create a PR for this, but before that someone with a higher level of authority should decide what the solution should look like - a try/except, open('/dev/stdin') (probably inside a try/except) or just remove lines 26 -- 29, 35,54 and 55 altogether if those parts do not work as originally intended.

ping @blueyed, maybe you have insights on this !

the original commit seems specifically tied to pdb, which I never use and have very little experience on

on top of that it was written before we changed the default multiprocessing context to be the spawn context which is not the linux default, so maybe the interaction is different than with a plain forked process,

I’d say @likeanaxon send a PR on this the way you think it should work, then we’ll advise !

[euri10]

the use case wont likely be supported, reload uses multiprocess spawn under the hood so you’re essentially multiprocessing^2