test_start event blocks the UI

[denniseffing]

Describe the bug

When a user starts a load test and the test_start event is fired, the UI is basically doing nothing until the test_start event hooks have finished executing.

Expected behavior

UI should open up the statistics dashboard and change the status from READY to a new state called INITIALIZING.

Actual behavior

UI hangs on "Start new Locust run screen" until test_start execution has finished.

Steps to reproduce

1. Add a test_start listener that pauses the thread for 5 seconds.

@events.test_start.add_listener
def on_test_start(**kwargs):
    time.sleep(5)

2. Start a test

Environment

• OS: macOS 10.15.3
• Python version: 3.7.7
• Locust version: 1.0.3
• Locust command line that you ran: locust
• Locust file contents (anonymized if necessary):

from locust import events, HttpUser, task, between
import time


@events.test_start.add_listener
def on_test_start(**kwargs):
    time.sleep(5)


class TestUser(HttpUser):
    wait_time = between(5, 9)

    @task
    def index_page(self):
        self.client.get(url=self.host)

[camilojimenez]

I looked into this and I found that there is a relatively easy way to fix it (without changing too much the current ways). I put a PR for this (#1479). However, the displayed state is not 'fully' consistent (it says ready while in reality is preparing).

A better (but more involved) solution would be to add an extra state signaling that the test_start events are running (something like PREPARING).

Thoughts?

[denniseffing]

Well that was indeed an easy fix, thank you for the PR. However, I think that adding an additional state would be the most optimal solution.

[cyberw]

the solution feels good enough for me. Marking as low prio.