[Bugfix] Fix spawn_new_process_for_each_test silently swallowing test failures

[dzhengAP]

Problem

spawn_new_process_for_each_test was broken — it always passed regardless
of what the test function did, causing silent test coverage failures.

The previous implementation ran python -m <module_name> in the child
process, which re-executed the module's __main__ block instead of
actually calling the test function. check_returncode() always saw
exit 0, so any exception raised inside the test was silently swallowed.

Repro from the issue:

@create_new_process_for_each_test("spawn")
def test_failing():
    raise ValueError  # always passed before this fix

Fix

Serialize the test function and its arguments with cloudpickle and
pass them to a minimal child script via stdin. The child writes its
full traceback to a temp file on failure; the parent reads it and
raises RuntimeError with the traceback included, making CI failures
actionable.

This matches the robustness of fork_new_process_for_each_test which
already handled exception propagation correctly.

Verification

Tested locally with the exact repro from #41415:

Testing failure propagation...
FIXED: failure correctly propagated
Test subprocess 'test_failing' failed (exit code 1):
Traceback (most recent call last):
  File "<string>", line 4, in <module>
    f(*args, **kwargs)
  File "/tmp/test_fix3.py", line 45, in test_failing
    raise ValueError('should propagate')
ValueError: should propagate

Testing success case...
FIXED: success case passes cleanly

• Failing tests now correctly raise RuntimeError with the full traceback from the child process
• Passing tests continue to work normally

Fixes #41415

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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[gemini-code-assist]

Code Review

This pull request updates tests/utils.py to reimplement spawn_new_process_for_each_test using subprocess and cloudpickle, ensuring that test failures are correctly propagated to the parent process. However, the current implementation contains a critical syntax error because the old function body was left dangling at the module level after the header was removed, which will cause an IndentationError. Additionally, the new logic for handling exceptions in the child process needs to account for pytest.skip to prevent skipped tests from being incorrectly flagged as failures.

[ProExpertProg]

Please combine the two approaches, and add tests that check that this works, 1 passing test and 1 failing test (with pytest.mark.xfail) to make sure the failures are caught

[dzhengAP]

@ProExpertProg

Done! My implementation already addressed the root cause — switching from python -m (which only imports the module and never calls f) to python -c with an inline runner that explicitly calls f(*args, **kwargs). This incorporates the same fix as @sriharshamudumba's approach, plus adds proper error propagation via a traceback file and Skipped exception handling.
Added three tests as requested:

test_spawn_decorator_passing — verifies a passing function completes normally
test_spawn_decorator_failure_is_caught (xfail, strict=True) — verifies failures are never silently swallowed
test_spawn_decorator_parametrized — verifies args/kwargs are forwarded correctly to the subprocess

All tests pass locally:
collected 3 items
tests/test_spawn_decorator.py::test_spawn_decorator_passing PASSED [ 33%]
tests/test_spawn_decorator.py::test_spawn_decorator_failure_is_caught XFAIL [ 66%]
tests/test_spawn_decorator.py::test_spawn_decorator_parametrized PASSED [100%]
========================= 2 passed, 1 xfailed in 0.20s =========================

[mergify]

Hi @dzhengAP, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

Tip

Is mypy failing?

mypy is run differently in CI. If the failure is related to this check, please use the following command to run it locally:

# For mypy (substitute "3.10" with the failing version if needed)
pre-commit run --hook-stage manual mypy-3.10

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @dzhengAP.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[dzhengAP]

Updated the test file to address both review comments: importing the actual spawn_new_process_for_each_test from tests.utils and decorating at module level. Also fixed a line-length lint error in tests/utils.py. @ProExpertProg @tjtanaa please let me know if anything else is needed!

[ProExpertProg]

@claude review once

[ProExpertProg]

Also test the skipping logic please. And what happens if the decorator is above the parametrize marks?

[mergify]

Hi @dzhengAP, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

Tip

Is mypy failing?

mypy is run differently in CI. If the failure is related to this check, please use the following command to run it locally:

# For mypy (substitute "3.10" with the failing version if needed)
pre-commit run --hook-stage manual mypy-3.10

[dzhengAP]

@ProExpertProg Hi Luka, fixed and all CI tests passed.

[ProExpertProg]

@njhill @Isotr0py could you guys take a look at the logits processor test?

[ProExpertProg]

@dzhengAP You need to manually add tests/test_spawn_decorator.py to a test. I think the easiest way is to add it to tests/utils_ folder

[dzhengAP]

@njhill @Isotr0py could you guys take a look at the logits processor test?

Background is the logits processor test was broken for the same reason as the cudagraph test: @create_new_process_for_each_test uses spawn + cloudpickle to run the test in a subprocess, and anything unpicklable in the function's arguments causes a PicklingError. The client fixture was an AsyncOpenAI object containing a threading.RLock — not picklable across process boundaries.

The fixed logic there is test_custom_logitsprocs has been refactored to create the async client inside the test body (srv.get_async_client()) rather than receiving it as a fixture kwarg, making it compatible with @create_new_process_for_each_test("spawn"). test_invalid_custom_logitsproc_arg does not use the decorator so the fixture-based client is fine there.

@njhill @Isotr0py @ProExpertProg all CI tests passed.

[dzhengAP]

Follow-up fix and relevant discussion for XPU/ROCm compatibility submitted in #41895 — restores mp.set_start_method("spawn").The failure-propagation fix remains intact.

Please do not merge that autog-enerated revert#41887. I've submitted a targeted fix in #41895 that restores mp.set_start_method("spawn") for XPU/ROCm compatibility without reverting the entire #41423 fix.

The 3 CI failures in build #64792 are pre-existing bugs that were silently swallowed by the old decorator — not regressions. Reverting #41423 would bring back the silent failure swallowing behavior, which is worse.

Additionally, the reimplementation in this revert PR is non-functional — it doesn't pass args/kwargs to the subprocess, so tests would appear to pass without actually running.

Please close this revert and let #41895 land instead. Happy to follow up with fixes for the 3 exposed pre-existing failures separately.

cc @jikunshang @ProExpertProg