celery: wait for exported spans instead of result.ready() in tests

[aniketwaghh]

Description

The celery tests wait on result.ready() before asserting on spans, but Celery stores the task result before it dispatches task_postrun, and CeleryInstrumentor ends the run span from its task_postrun receiver. So the result can be ready while the run span has not been ended and exported yet, and the assertion runs against an incomplete exporter. That is the 1 != 2 in the issue.

These now wait for the expected number of exported spans instead. test_task_clears_start_time_cache also waits for the start-time cache to drain, because that pop happens a few statements after the span is ended in the same receiver. test_baggage and test_uninstrument keep result.ready(), which is the right thing for them to wait on.

Test-only change, no instrumentation code touched.

Two things worth flagging:

• The old loops break on timeout and fall through into the assertion, so a genuine hang showed up as a confusing span-count mismatch. These raise with the reason instead.
• test_metrics.py waits the same way and has the same race. I left it alone to keep this to one logical change, but it is the same fix if you want it. It already carries a skipif(PyPy, reason="Fails randomly in pypy"), which looks like this race showing up there.

Prior analysis of this race in the docker suite is in #4964 by @Pissinatti-py, for #653. Different file and different issue, so no overlap with this change, but they described the cause first and it is worth reading alongside.

Fixes #5009

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Ordering measured on macOS arm64 against f1b9368aa, showing the result landing before the span ends:

   34.304 ms  task_prerun
   35.232 ms  backend.mark_as_failure ENTER
   35.302 ms  task_failure
   35.446 ms  task_postrun          <- run span ends here
   36.029 ms  result.ready() == True

AT result.ready(): ['apply_async/celery_test_tasks.task_raises']
ONE SECOND LATER : ['apply_async/...', 'run/celery_test_tasks.task_raises']

To make it deterministic rather than waiting for a loaded runner, I connected a 0.5s task_postrun receiver ahead of the instrumentor's, which widens the real window without touching any source:

from celery import signals
signals.task_postrun.connect(lambda *a, **k: time.sleep(0.5), weak=False)

With that loaded, on main:

8 failed, 22 passed
FAILED test_tasks.py::TestCeleryInstrumentation::test_task
FAILED test_tasks.py::TestCeleryInstrumentation::test_task_clears_start_time_cache
FAILED test_tasks.py::TestCeleryInstrumentation::test_task_not_instrumented_does_not_raise
FAILED test_tasks.py::TestCeleryInstrumentation::test_task_raises
FAILED test_tasks.py::TestCeleryInstrumentation::test_task_use_span_links
FAILED test_tasks.py::TestCelerySignatureTask::test_hidden_task
FAILED test_metrics.py::TestMetrics::test_basic_metric
FAILED test_metrics.py::TestMetrics::test_metric_uninstrument

test_task_raises fails with AssertionError: 1 != 2 at test_tasks.py:123, the error and line from the CI traceback in the issue.

With this change, same widened window:

2 failed, 28 passed

The six in test_tasks.py are gone. The two remaining are test_metrics.py, which this PR does not touch.

Unmodified suite:

uvx --from tox --with tox-uv tox -e py312-test-instrumentation-celery   ->  30 passed
uvx --from tox --with tox-uv tox -e lint-instrumentation-celery         ->  10.00/10
ruff check / ruff format --check                                        ->  clean

• Reproduced the race on main and confirmed the fix removes it
• Full celery suite green on py3.12 and py3.14

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated - test-only, so I believe this needs the Skip Changelog label rather than a fragment. Happy to add one if you would rather have it.
• Unit tests have been added - existing tests made deterministic
• Documentation has been updated - not applicable

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: aniketwaghh / name: aniketwaghh (810703d)

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