avoid error when coro.cr_frame is None

Author: Zac-HD

newsfragments/3337.bugfix.rst

@@ -0,0 +1,3 @@
+`trio.lowlevel.Task.iter_await_frames` now works on completed tasks, by
+returning an empty list of frames if the underlying coroutine has been closed.
+Previously, it raised an internal error.

src/trio/_core/_run.py

@@ -1588,11 +1588,13 @@ def print_stack_for_task(task):
         while coro is not None:
             if hasattr(coro, "cr_frame"):
                 # A real coroutine
-                yield coro.cr_frame, coro.cr_frame.f_lineno
+                if cr_frame := coro.cr_frame:  # None if the task has finished
+                    yield cr_frame, cr_frame.f_lineno
                 coro = coro.cr_await
             elif hasattr(coro, "gi_frame"):
                 # A generator decorated with @types.coroutine
-                yield coro.gi_frame, coro.gi_frame.f_lineno
+                if gi_frame := coro.gi_frame:  # pragma: no branch
+                    yield gi_frame, gi_frame.f_lineno  # pragma: no cover
                 coro = coro.gi_yieldfrom
             elif coro.__class__.__name__ in [
                 "async_generator_athrow",

src/trio/_tests/test_tracing.py

@@ -69,3 +69,20 @@ async def test_task_iter_await_frames_async_gen() -> None:
         ]
 
         nursery.cancel_scope.cancel()
+
+
+async def test_closed_task_iter_await_frames() -> None:
+    async with trio.open_nursery() as nursery:
+        task = object()
+
+        async def capture_task() -> None:
+            nonlocal task
+            task = trio.lowlevel.current_task()
+            await trio.lowlevel.checkpoint()
+
+        nursery.start_soon(capture_task)
+
+    # Task has completed, so coro.cr_frame should be None, thus no frames
+    assert isinstance(task, trio.lowlevel.Task)  # Ran `capture_task`
+    assert task.coro.cr_frame is None  # and the task was over, but
+    assert list(task.iter_await_frames()) == []  # look, no crash!