Bugfix: Fix quart threadedrunner stop graceful shutdown#3824
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ThreadedRunner.stop() only had a graceful shutdown branch for FastAPI (keyed on `_uvicorn_server`). A Quart app fell through to the kill-based branch, which injects an async SystemExit via thread.kill() and then calls join() with no timeout. The server thread is parked in a blocking syscall (IOCP on Windows, epoll on POSIX), so the SystemExit is not delivered promptly and join() can hang indefinitely -- on Windows and Linux alike. Add a Quart branch that signals the backend's existing cooperative shutdown switch (backend._ws_shutdown_event) thread-safely on the server's own loop via call_soon_threadsafe, then joins bounded by stop_timeout. Flask/other backends keep the kill path unchanged.
Add a dash.testing regression test that starts a Quart app on ThreadedRunner and asserts stop() returns bounded by stop_timeout (run under a watchdog so a regression fails fast instead of wedging the suite). Verified it fails against the unpatched stop() and passes with the fix.
joschrag
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KoolADE85,
T4rk1n,
camdecoster and
ndrezn
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Thanks for the PR! Our team will take a look and follow up. |
T4rk1n
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Jun 26, 2026
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| quart_shutdown_event = getattr( | ||
| getattr(self._app, "backend", None), "_ws_shutdown_event", None | ||
| ) | ||
| # For FastAPI apps with uvicorn, use graceful shutdown | ||
| if self._app and hasattr(self._app, "_uvicorn_server"): | ||
| server = self._app._uvicorn_server # pylint: disable=protected-access | ||
| server = self._app._uvicorn_server | ||
| server.should_exit = True | ||
| self.thread.join(timeout=self.stop_timeout) # type: ignore[reportOptionalMemberAccess] | ||
| # For Quart apps, signal hypercorn's cooperative shutdown event. Only the | ||
| # main-thread signal handler sets it, but in tests the server runs in a | ||
| # worker thread, so we set it ourselves -- thread-safely, on the server's | ||
| # own loop (the event binds its loop on first await) -- then join bounded. | ||
| elif quart_shutdown_event is not None: |
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Could change these checks to be more clear and not dependent on private variables checks, we have the server_type attribute
server_type = getattr(self._app.backend, "server_type", "flask")
if server_type == "fastapi":
...
elif server_type == "quart":
...
else:
...
| from dash.testing.application_runners import ThreadedRunner | ||
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| def test_quart_threaded_runner_stop_is_graceful_and_bounded(): |
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Think we could make this test generic and parametrize with the three backends?
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dash.testing'sThreadedRunner.stop()only has a graceful-shutdown branch for FastAPI — it keys offhasattr(self._app, "_uvicorn_server"). A Quart app has no such attribute, so it falls through to theelsebranch:thread.kill()injects an asynchronousSystemExit, thenthread.join()is called with no timeout.the Quart backend already serves with
serve(shutdown_trigger=...)awaitingbackend._ws_shutdown_event; only the main-thread signal handler ever sets it, and in tests the server runs in a worker thread.stop()now sets that event itself — thread-safely, vialoop.call_soon_threadsafe(event.set)on the server's own loop — then joins bounded bystop_timeout.Closes #3823
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