Multiprocessing Flask OSError: [Errno 48] Address already in use / OSError: [Errno 38] Socket oper-CodePudding

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.

CodePudding user response：

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

CodePudding user response：

The error is telling us:

OSError: [Errno 48] Address already in use

This is probably because something else is using the same port, or even an instance of this same program.