avoid refcycles in trio.run, especially if an Exception is raised

demo.py

@@ -0,0 +1,72 @@
+import trio
+
+
+async def main():
+    err = None
+    with trio.CancelScope() as scope:
+        scope.cancel()
+        try:
+            await trio.sleep_forever()
+        except BaseException as e:
+            err = e
+            raise
+    breakpoint()
+
+
+# trio.run(main)
+
+import gc
+
+import objgraph
+from anyio import CancelScope, get_cancelled_exc_class
+
+
+async def test_exception_refcycles_propagate_cancellation_error() -> None:
+    """Test that TaskGroup deletes cancelled_exc"""
+    exc = None
+
+    with CancelScope() as cs:
+        cs.cancel()
+        try:
+            await trio.sleep_forever()
+        except get_cancelled_exc_class() as e:
+            exc = e
+            raise
+
+    assert isinstance(exc, get_cancelled_exc_class())
+    gc.collect()
+    objgraph.show_chain(
+        objgraph.find_backref_chain(
+            gc.get_referrers(exc)[0],
+            objgraph.is_proper_module,
+        ),
+    )
+
+
+# trio.run(test_exception_refcycles_propagate_cancellation_error)
+
+
+class MyException(Exception):
+    pass
+
+
+async def main():
+    raise MyException
+
+
+def inner():
+    try:
+        trio.run(main)
+    except MyException:
+        pass
+
+
+import refcycle
+
+gc.disable()
+gc.collect()
+inner()
+garbage = refcycle.garbage()
+for i, component in enumerate(garbage.source_components()):
+    component.export_image(f"{i}_example.svg")
+garbage.export_image("example.svg")

src/trio/_core/_run.py

@@ -1706,6 +1706,7 @@ def close(self) -> None:
         self.asyncgens.close()
         if "after_run" in self.instruments:
             self.instruments.call("after_run")
+        self.system_nursery: Nursery | None = None
         # This is where KI protection gets disabled, so we do it last
         self.ki_manager.close()
 
@@ -1920,6 +1921,7 @@ def task_exited(self, task: Task, outcome: Outcome[Any]) -> None:
         task._activate_cancel_status(None)
         self.tasks.remove(task)
         if task is self.init_task:
+            self.init_task = None
             # If the init task crashed, then something is very wrong and we
             # let the error propagate. (It'll eventually be wrapped in a
             # TrioInternalError.)
@@ -1930,6 +1932,7 @@ def task_exited(self, task: Task, outcome: Outcome[Any]) -> None:
                 raise TrioInternalError
         else:
             if task is self.main_task:
+                self.main_task = None
                 self.main_task_outcome = outcome
                 outcome = Value(None)
             assert task._parent_nursery is not None, task
@@ -2394,12 +2397,15 @@ def run(
         sniffio_library.name = prev_library
     # Inlined copy of runner.main_task_outcome.unwrap() to avoid
     # cluttering every single Trio traceback with an extra frame.
-    if isinstance(runner.main_task_outcome, Value):
-        return cast(RetT, runner.main_task_outcome.value)
-    elif isinstance(runner.main_task_outcome, Error):
-        raise runner.main_task_outcome.error
-    else:  # pragma: no cover
-        raise AssertionError(runner.main_task_outcome)
+    try:
+        if isinstance(runner.main_task_outcome, Value):
+            return cast(RetT, runner.main_task_outcome.value)
+        elif isinstance(runner.main_task_outcome, Error):
+            raise runner.main_task_outcome.error
+        else:  # pragma: no cover
+            raise AssertionError(runner.main_task_outcome)
+    finally:
+        del runner
 
 
 def start_guest_run(
@@ -2808,6 +2814,7 @@ def unrolled_run(
             if isinstance(runner.main_task_outcome, Error):
                 ki.__context__ = runner.main_task_outcome.error
             runner.main_task_outcome = Error(ki)
+        del runner
 
 
 ################################################################