Fix exception cause/context lost before task_status.started()

[Fridayai700]

Summary

Fixes #3261.

When a task raises an exception before calling task_status.started(), Nursery.start() unwraps it from the internal ExceptionGroup using:

raise exc.exceptions[0] from None

The from None explicitly sets __cause__ to None and __suppress_context__ to True, destroying any cause or context the original exception carried.

Fix

Replace the bare raise ... from None with raise_saving_context(), which preserves both __cause__ and __context__. This is the same utility already used by raise_single_exception_from_group() for the same pattern.

Before

>>> excgroup.exceptions[0].__cause__   # should be SyntaxError, was None
>>> excgroup.exceptions[0].__context__ # should be TypeError, was the ExceptionGroup

After

>>> excgroup.exceptions[0].__cause__   # SyntaxError("bar")
>>> excgroup.exceptions[0].__context__ # TypeError("baz")

Test plan

• New regression test test_start_exception_preserves_cause_and_context
• All 28 start-related tests pass
• All 154 test_run.py tests pass

[A5rocks]

Sure whatever go go gadget Claude.

(As a note to humans I'd rather PRs from people familiar with the code base (even using a coding agent!) rather than this kinda thing. But this is simple enough + is literally just what @jakkdl proposed so this seems fine.)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (6deaf2a) to head (48aadaf).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3394   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             128          128           
  Lines           19415        19424    +9     
  Branches         1318         1318           
===============================================
+ Hits            19415        19424    +9     

Files with missing lines	Coverage Δ
src/trio/_core/_run.py	100.00000% <100.00000%> (ø)
src/trio/_core/_tests/test_run.py	100.00000% <100.00000%> (ø)

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[jakkdl]

Suggested change

	with pytest.raises(BaseExceptionGroup) as exc_info:
	async with _core.open_nursery() as nursery:
	await nursery.start(task)
	assert len(exc_info.value.exceptions) == 1
	exc = exc_info.value.exceptions[0]
	assert isinstance(exc, ValueError)
	assert isinstance(exc.__cause__, SyntaxError)
	assert isinstance(exc.__context__, TypeError)
	with pytest.RaisesGroup(
	pytest.RaisesExc(ValueError,
	check=lambda exc: isinstance(exc.__cause__, SyntaxError) and isinstance(exc.__context__, TypeError)
	)
	) as exc_info:
	async with _core.open_nursery() as nursery:
	await nursery.start(task)

not 100% sure that putting the exc.__cause__ and exc.__context__ checks inside RaisesExc is better than outside, but it should at least use RaisesGroup.

[jakkdl]

this is making me think there should be a lint rule that checks for pytest.raises([Base]ExceptionGroup) - created python-trio/flake8-async#430

[Fridayai700]

Updated to use pytest.RaisesGroup with pytest.RaisesExc as suggested. Much cleaner — thanks for the review!

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