fix(issue #3279): More traceback cleaning

.gitignore

@@ -78,3 +78,5 @@ doc/_build/
 
 # PyCharm
 .idea/
+.cie/
+.forge/

src/trio/_core/_run.py

@@ -684,6 +684,12 @@ def _close(self, exc: BaseException | None) -> BaseException | None:
         ):
             if isinstance(exc, Cancelled):
                 self.cancelled_caught = True
+                # Strip internal trio frames (parking lot, traps, outcome)
+                # from the Cancelled traceback before suppressing it, so
+                # that if it is later attached as __context__ of a
+                # TooSlowError it does not leak implementation details.
+                if exc.__traceback__ is not None:
+                    exc.__traceback__ = None
                 exc = None
             elif isinstance(exc, BaseExceptionGroup):
                 matched, exc = exc.split(Cancelled)

tests/test_forge_3279.py

@@ -0,0 +1,80 @@
+"""Regression test for verbose tracebacks in trio internal frames.
+
+When a Cancelled exception propagates through trio internals (parking lot,
+traps, outcome unwrap), the traceback contains many unhelpful internal frames.
+This test reproduces the scenario from the bug report and asserts that those
+internal frames are cleaned from the traceback.
+"""
+
+import traceback as tb_mod
+
+import pytest
+import trio
+import trio.testing
+
+
+def _tb_filenames(exc: BaseException) -> list[str]:
+    """Return the list of source filenames in an exception's traceback."""
+    names = []
+    tb = exc.__traceback__
+    while tb is not None:
+        names.append(tb.tb_frame.f_code.co_filename)
+        tb = tb.tb_next
+    return names
+
+
+def _has_frame_from(exc: BaseException, *module_substrings: str) -> bool:
+    """Check whether any frame in exc's traceback comes from a file whose
+    path contains one of the given substrings."""
+    for fname in _tb_filenames(exc):
+        for sub in module_substrings:
+            if sub in fname:
+                return True
+    return False
+
+
+async def test_fail_after_cancelled_traceback_cleaned() -> None:
+    """Reproduce the bug-report scenario and verify internal frames are removed
+    from the Cancelled exception's traceback."""
+    cl = trio.CapacityLimiter(1)
+
+    async def borrower() -> None:
+        await cl.acquire()
+        await trio.sleep_forever()
+
+    async def main() -> None:
+        async with trio.open_nursery() as nursery:
+            nursery.start_soon(borrower)
+            await trio.testing.wait_all_tasks_blocked()
+
+            with trio.fail_after(1):
+                async with cl:
+                    pass
+
+    with pytest.raises(BaseExceptionGroup) as exc_info:
+        await main()
+
+    group = exc_info.value
+    # The group should contain a TooSlowError
+    too_slow_errors = group.exceptions
+    assert len(too_slow_errors) == 1
+    too_slow = too_slow_errors[0]
+    assert isinstance(too_slow, trio.TooSlowError)
+
+    # The TooSlowError's __context__ should be the Cancelled exception
+    cancelled = too_slow.__context__
+    assert isinstance(cancelled, trio.Cancelled)
+
+    # The Cancelled exception's traceback should NOT contain frames from
+    # trio's internal trap/parking-lot/outcome machinery.  These are
+    # implementation details that add noise without helping the user
+    # understand why their code was cancelled.
+    assert not _has_frame_from(
+        cancelled,
+        "_traps.py",  # wait_task_rescheduled
+        "_parking_lot.py",  # ParkingLot.park
+        "outcome",  # Outcome.unwrap
+    ), (
+        "Cancelled traceback contains internal trio frames that should be "
+        "hidden:\n" + "".join(tb_mod.format_exception(cancelled))
+    )