avoid error when coro.cr_frame is None

Author: Zac-HD

newsfragments/3337.bugfix.rst

@@ -0,0 +1,2 @@
+Fixed a rare TOC/TOU bug in `trio.lowlevel.Task.iter_await_frames` where
+the underlying coroutine has already been closed.

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:  # TOCTOU-safe check for None
+                    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:  # TOCTOU-safe check for None
+                    yield gi_frame, gi_frame.f_lineno
                 coro = coro.gi_yieldfrom
             elif coro.__class__.__name__ in [
                 "async_generator_athrow",

src/trio/_core/_tests/test_run.py

@@ -3005,3 +3005,27 @@ async def main() -> None:
             await _core.wait_task_rescheduled(inner_abort)
 
     _core.run(main)
+
+
+async def test_closed_task_iter_await_frames() -> None:
+    """Test that Task.__iter__ handles cr_frame/gi_frame becoming None.
+
+    When a coroutine completes, its cr_frame/gi_frame attributes become None.
+    The iterator should handle this gracefully rather than trying to access
+    attributes on None.
+    """
+    completed_task = None
+
+    async with _core.open_nursery() as nursery:
+
+        async def capture_task() -> None:
+            nonlocal completed_task
+            completed_task = _core.current_task()
+            await _core.checkpoint()
+
+        nursery.start_soon(capture_task)
+
+    # Task has completed, so coro.cr_frame should be None, thus no frames
+    assert completed_task is not None
+    assert completed_task.coro.cr_frame is None
+    assert list(completed_task.iter_await_frames()) == []