Eagerly import the ASGI app in the parent process#2919
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Splits Config.load() into a small load_app() (imports the app, sets loaded_app) plus the rest, and calls config.load_app() in uvicorn.run() before the supervisor or Server is constructed. This: - Fails fast in the parent on a bad app path; no more infinite worker restart loop with --workers > 1. - Imports the app synchronously before any event loop starts, so apps that do `app = asyncio.run(...)` at module load time no longer crash with "loop already running". - Primes sys.modules in the parent, so future thread workers (#2900) see a cache hit in load_app() instead of racing on the importlib global lock. The spawn child must still re-import (fresh interpreter, possibly non-picklable loaded_app) - _subprocess.get_subprocess() shallow-copies the Config and clears loaded/loaded_app on the copy before pickling. A TODO points at the deeper fix: splitting Config (settings) from the runtime state Server actually needs. Config.load() is now idempotent (early-return when self.loaded), so defensive `if not config.loaded: config.load()` guards in Server, LifespanOn, and the websockets sans-io protocol stay correct. Closes #941 Closes #2440 Supersedes #942 Supersedes #2444
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Codex review caught a real bug: storing loaded_app on the parent's Config breaks spawn pickling for non-picklable apps (DB pools, locks, etc.) because Multiprocess passes server.run as a bound method, and that bound method captures the same Config object. Stripping loaded_app from the kwargs Config copy doesn't help - the bound method still drags the original. Fix: in uvicorn.run(), import the app for fail-fast validation and discard the result. Workers re-import freshly via Server.run -> config.load(). The parent never carries loaded_app, so the bound-method pickle is safe. Removes the _subprocess.py shallow-copy hack (no longer needed) and the test that asserted on it. The #941 regression test moved from test_config.py to test_main.py, where it now verifies via Server.run patching that the import lands in sys.modules before Server.run is called.
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Uvicorn 0.47.0 eagerly imports the ASGI app in the parent process before forking workers (Kludex/uvicorn#2919). This appears to delay worker readiness on CI runners, causing the 10s `/health` poll in tests/webhooks/test_e2e.py and tests/tracking/test_rest_tracking.py to time out. Pin to verify. Co-Authored-By: Claude <noreply@anthropic.com> Signed-off-by: harupy <17039389+harupy@users.noreply.github.com>
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uvicorn 0.47 adds a serialized parent-process import of the ASGI app to the multi-worker startup path (Kludex/uvicorn#2919), pushing `mlflow server` cold-start past the prior 10-attempt (~10-30s) budget on CI. 30 attempts gives a 30-90s ceiling, which absorbs the new latency while still failing fast on a genuinely broken server. Co-Authored-By: Claude <noreply@anthropic.com> Signed-off-by: harupy <17039389+harupy@users.noreply.github.com>
This was referenced Jun 10, 2026
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Summary
Splits
Config.load()so the app import is reusable in isolation, and calls it eagerly fromuvicorn.run()before the supervisor orServeris constructed.Config.load_app()imports the app viaimport_from_stringand returns it. No state mutation.Config.load()callsself.loaded_app = self.load_app()for the existing back-compat behavior.uvicorn.run()callsconfig.load_app()and discards the return value: enough to fail fast on a bad import string, without leaving anything on the parent'sConfigfor spawn workers to pick up.This:
--workers > 1.app = asyncio.run(...)at module load time no longer crash withRuntimeError: this event loop is already running.sys.modulesin the parent, so future thread workers (Add free-threaded thread worker class #2900) see a cache hit when each thread runs its ownConfig.load()instead of racing on the importlib global lock for the initial import.Spawn workers are unaffected:
uvicorn.run()doesn't store the imported app onConfig, so the boundserver.runmethod thatMultiprocessships across pickle never carries it. Workers re-import inServer.run -> Config.load.Closes / supersedes
Config#loadout ofServer#serveto enable Eager Asynchronous Construction of App #942Test plan
uv run pytestpasses locally (tests/test_config.py::test_socket_bindexcluded due to a port collision unrelated to this change).uv run mypy uvicornclean.uvicorn.run()is reverted:tests/test_main.py::test_run_fails_fast_in_parent_on_bad_app_path(Reduce config load and increase priority #2440)tests/test_main.py::test_run_imports_app_before_starting_event_loop(Eager Asynchronous Construction of App Interferes with Running Event Loop #941)AI Disclaimer
This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.