Closing connections (disconnect) is not thread-safe

Calling disconnect or disconnect_all in a multi-threaded environment is unsafe. Consider the following code:

import threading


import time
from mongoengine import *

connect(host="mongodb://127.0.0.1:9800/test_db_1")

class User(Document):
    email = StringField(required=True)
    first_name = StringField(max_length=50)
    last_name = StringField(max_length=50)

def thread_1_task():
    for user in User.objects:
        print(user.email)
    disconnect()

def thread_2_task():
    time.sleep(5)
    for user in User.objects:   # <----- This query raises a mongoengine.connection.ConnectionFailure
        print(user.email)
    disconnect()


def main():
    t1 = threading.Thread(target=thread_1_task)
    t2 = threading.Thread(target=thread_2_task)

    t1.start()
    t2.start()

    t1.join()
    t2.join()

if __name__ == "__main__":
    main()

After taking a look at connection.py, I found that the problem was that connections/db references were being stored in a process level dict. The solution is to probably use either threading.local or contextvars to store these references so that each thread / co-routine would get its own connection object thereby making it safe to execute disconnect in concurrent environments.

If agreed upon, I can submit a PR which I've been working on to fix this.

Start by reading connection.py and reproduce the disconnect/disconnect_all race using the threaded example in the issue. Trace how connection and database references are stored and determine the concurrency-safe ownership needed for threads or coroutines. Done means concurrent queries remain usable when another worker disconnects, with the existing connection behavior preserved.