Multiprocessing + Flask OSError: [Errno 48] Address already in use / OSError: [Errno 38] Socket operation on non-socket

I am trying to build a Flask app that runs a function that eventually utilizes the multiprocessing library.

import time
from flask import Flask, request

import multiprocessing as mp

app = Flask(__name__)

def proc():
    time.sleep(10)

@app.route('/greeting')
def greeting():
    p = mp.Process(target=proc, args=())
    p.start()
    return ('Hello there')

app.run(port=8000)

The problem occurs when I go to http://localhost:8000/greeting and get the following error trying to call p.start().

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/local/Cellar/[email protected]/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/spawn.py", line 116, in spawn_main
    exitcode = _main(fd, parent_sentinel)
  File "/usr/local/Cellar/[email protected]/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/spawn.py", line 125, in _main
    prepare(preparation_data)
  File "/usr/local/Cellar/[email protected]/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/spawn.py", line 236, in prepare
    _fixup_main_from_path(data['init_main_from_path'])
  File "/usr/local/Cellar/[email protected]/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/spawn.py", line 287, in _fixup_main_from_path
    main_content = runpy.run_path(main_path,
  File "/usr/local/Cellar/[email protected]/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/runpy.py", line 268, in run_path
    return _run_module_code(code, init_globals, run_name,
  File "/usr/local/Cellar/[email protected]/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/runpy.py", line 97, in _run_module_code
    _run_code(code, mod_globals, init_globals,
  File "/usr/local/Cellar/[email protected]/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/Users/michaelequi/Github/drlm/apps/mp_flask.py", line 18, in <module>
    app.run(port=8000)
  File "/Users/michaelequi/Github/drlm/venv/drlm/lib/python3.9/site-packages/flask/app.py", line 920, in run
    run_simple(t.cast(str, host), port, self, **options)
  File "/Users/michaelequi/Github/drlm/venv/drlm/lib/python3.9/site-packages/werkzeug/serving.py", line 1010, in run_simple
    inner()
  File "/Users/michaelequi/Github/drlm/venv/drlm/lib/python3.9/site-packages/werkzeug/serving.py", line 950, in inner
    srv = make_server(
  File "/Users/michaelequi/Github/drlm/venv/drlm/lib/python3.9/site-packages/werkzeug/serving.py", line 782, in make_server
    return ThreadedWSGIServer(
  File "/Users/michaelequi/Github/drlm/venv/drlm/lib/python3.9/site-packages/werkzeug/serving.py", line 688, in __init__
    super().__init__(server_address, handler)  # type: ignore
  File "/usr/local/Cellar/[email protected]/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/socketserver.py", line 452, in __init__
    self.server_bind()
  File "/usr/local/Cellar/[email protected]/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/http/server.py", line 138, in server_bind
    socketserver.TCPServer.server_bind(self)
  File "/usr/local/Cellar/[email protected]/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/socketserver.py", line 466, in server_bind
    self.socket.bind(self.server_address)
OSError: [Errno 48] Address already in use

If I change the code such that debug=True (app.run(port=8000, debug=True)) I get a different error!

Traceback (most recent call last):
  File "/usr/local/Cellar/[email protected]/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/threading.py", line 973, in _bootstrap_inner
    self.run()
  File "/usr/local/Cellar/[email protected]/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/threading.py", line 910, in run
    self._target(*self._args, **self._kwargs)
  File "/Users/michaelequi/Github/drlm/venv/drlm/lib/python3.9/site-packages/werkzeug/serving.py", line 950, in inner
    srv = make_server(
  File "/Users/michaelequi/Github/drlm/venv/drlm/lib/python3.9/site-packages/werkzeug/serving.py", line 782, in make_server
    return ThreadedWSGIServer(
  File "/Users/michaelequi/Github/drlm/venv/drlm/lib/python3.9/site-packages/werkzeug/serving.py", line 676, in __init__
    real_sock = socket.fromfd(fd, self.address_family, socket.SOCK_STREAM)
  File "/usr/local/Cellar/[email protected]/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/socket.py", line 545, in fromfd
    return socket(family, type, proto, nfd)
  File "/usr/local/Cellar/[email protected]/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/socket.py", line 232, in __init__
    _socket.socket.__init__(self, family, type, proto, fileno)
OSError: [Errno 38] Socket operation on non-socket

This occurs no matter what port I run the flask server on. Anyone know what may be causing this and how to fix it? I am using Python 3.9.9 on MacOS Big Sur.

Replace

app.run(port=8000)

with

if __name__ == '__main__':
    app.run(port=8000)

When using multiprocessing, anything that is "top level" gets run in every thread. So both your main thread and the thread you're creating try to open port 8000