Incorrect current user count after user stops in distributed mode

[CharleneHu-42]

Prerequisites

• I am using the latest version of Locust
• I am reporting a bug, not asking a question

Description

I have a test scenario where the custom load shape will generate random number of new users at each tick, and each user will stop itself after receiving the response of a HTTP GET request.

We observed that if we use distributed load generation by setting --process parameter while starting locust, sometimes when users are stopped between two ticks, the get_current_user_count() method is not able to to return the correct user number, some stopped users still counts in get_current_user_count() in subsequent ticks. This issue does not occur in local runner (w/o setting --process).

See below logs:

[2024-09-18 16:35:56,625] host/DEBUG/test_load_shape: Current users: 4, New users: 0, Target users: 4
[2024-09-18 16:35:57,075] host/DEBUG/urllib3.connectionpool: http://host-ip:80 "GET /sleep/5 HTTP/11" 200 None
[2024-09-18 16:35:57,075] host/INFO/root: Stopping current user: <test_load_shape.MyUser object at 0x7f362c904310>
[2024-09-18 16:35:57,626] host/DEBUG/test_load_shape: Current users: 4, New users: 0, Target users: 4
[2024-09-18 16:35:58,628] host/DEBUG/test_load_shape: Current users: 4, New users: 2, Target users: 6

One user has received the response and stopped, the correct user number should be 3, but in the subsequent ticks, the current user returned from get_current_user_count() still remains at 4.

Command line

locust -f test_load_shape.py -H http:///sleep/5 --headless --process 1 --loglevel DEBUG -s 120

Locustfile contents

import numpy as np
import logging
from locust import HttpUser, task, LoadTestShape

logger = logging.getLogger(__name__)

class PoissonLoadShape(LoadTestShape):
    use_common_options = True

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.arrival_rate = 0.4

    def tick(self):

        new_users = np.random.poisson(lam=self.arrival_rate)
        current_users=self.get_current_user_count()
        user_count = current_users + new_users
        logger.debug(
            "Current users: {current_users}, New users: {new_users}, Target users: {target_users}".format(
                current_users=current_users, new_users=new_users, target_users=user_count
            )
        )
        # Avoid illegal spawn_rate value of 0
        spawn_rate = max(0.01, new_users)
        return (user_count, spawn_rate)

class MyUser(HttpUser):
    
    @task
    def my_task(self):
        self.client.get("")
        logging.info("Stopping current user: {user_id}".format(user_id=self))
        self.stop(force=True)

Python version

3.11.9

Locust version

2.31.4

Operating system

Ubuntu 22.04.1

[cyberw]

Hi!

Users stopping themselves is not really a supported scenario. I think it should be, but right now it isn't.

I'm not sure if this is an easy or hard fix, but either way I don't have time to fix it myself so someone would have to volunteer.