Fix: Make URL cache clearing thread-safe in _django_set_urlconf

[pankaj-bind]

Problem

When running tests in parallel with pytest-xdist, the @pytest.mark.urls fixture can cause a race condition. Multiple test workers may try to modify the global ROOT_URLCONF and clear the URL cache at the same time, leading to unpredictable behavior and intermittent test failures.

Solution

This change introduces a threading.Lock() around the URL configuration and cache-clearing logic in the _django_set_urlconf fixture. This ensures that these operations are atomic, preventing multiple threads from interfering with each other and making the URL cache clearing thread-safe.

[kingbuzzman]

If your test is anything to go off of, this does not show a problem currently in the code as it stand. Can you elaborate more?

[kingbuzzman]

Not used

Suggested change

empty="urlpatterns = []",

[kingbuzzman]

I copied this test over to my main branch, and ran this on py3.13

count=0
while true; do
    count=$((count + 1))
    if ! pytest tests/test_urls.py -vv > /dev/null 2>&1; then
        echo "Test failed on iteration $count"
        break
    fi
    printf "."
done

it has not failed -- keeps going, after 100+ iterations (keep in mind, i only copied the test, not the "fix")

[kingbuzzman]

PS. I also ran this with pytest -n auto (same loop as above) -- no difference, i dont see the race condition

[bluetech]

pytest-xdist runs workers in separate processes, not threads. So it's unclear what this is intended to fix.

[kingbuzzman]

@pankaj-bind I don't want to discourage you, what is the issue you're trying to solve? we can help you with it.