Creating a ThreadPool in parent process (like in the uwsgi prefork mode) fails later

App-Fixed.wsgi

from flask import Flask
from multiprocessing.pool import ThreadPool
from time import sleep

app = Flask(__name__)

@app.before_first_request
def initialize():
    app.pool = ThreadPool(10)

@app.route("/")
def index():
    app.pool.apply_async(hello)
    return 'ok'

def hello():
    for i in range(5):
        print i
        sleep(1)

app.debug = True
application = app

App.wsgi

from flask import Flask
from multiprocessing.pool import ThreadPool
from time import sleep

app = Flask(__name__)
pool = ThreadPool(10)

@app.route("/")
def index():
    pool.apply_async(hello)
    return 'ok'

def hello():
    for i in range(5):
        print i
        sleep(1)

app.debug = True
if __name__ == "__main__":
    app.run(host='0.0.0.0')
else:
    application = app

uwsgi.ini

[uwsgi]
master       =  true
plugins      =  python
enable-threads = true
project_dir  =  %d

chdir        =  %(project_dir)
wsgi-file    =  %(project_dir)/App.wsgi
close-on-exec = true

http-socket  = :5000

The problem is that the pool is created on file import (in the master) and when the worker is forked, it doesn't inherit the threads.

the fix is to create the pool in the worker:

@app.before_first_request
def initialize():
    app.pool = ThreadPool(10)

and then use app.pool instead of the global pool

Also this will fail too:

pool = ThreadPool()
pid = os.fork()
if pid == 0: # child
    pool.apply(...)

will not work. Some discussion here: http://bugs.python.org/issue6923