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