resolved threads+gc problem, though not sure if in a good way

Author: jakkdl

src/trio/_core/_run.py

@@ -1640,25 +1640,7 @@ def _attempt_delivery_of_any_pending_cancel(self) -> None:
         if not self._cancel_status.effectively_cancelled:
             return
 
-        # FIXME: I haven't quite figured out how to get the cancel reason to stay
-        # alive for passing into other threads, but also not cause any cyclic garbage.
-        if (
-            self._cancel_status is None
-            or self._cancel_status._scope._cancel_reason is None
-        ):
-            # _cancel_status._cancel_reason is None when misnesting
-            cancelled = Cancelled._create(source="unknown", reason="misnesting")
-        else:
-            cancelled = Cancelled._create(
-                source=self._cancel_status._scope._cancel_reason.source,
-                reason=self._cancel_status._scope._cancel_reason.reason,
-                source_task=self._cancel_status._scope._cancel_reason.source_task,
-            )
-
-        def raise_cancel() -> NoReturn:
-            raise cancelled
-
-        self._attempt_abort(raise_cancel)
+        self._attempt_abort(RaiseCancel(self._cancel_status._scope._cancel_reason))
 
     def _attempt_delivery_of_pending_ki(self) -> None:
         assert self._runner.ki_pending
@@ -1672,6 +1654,24 @@ def raise_cancel() -> NoReturn:
         self._attempt_abort(raise_cancel)
 
 
+class RaiseCancel:
+    def __init__(self, reason: CancelReason | None) -> None:
+        if reason is None:
+            self.cancelled = Cancelled._create(source="unknown", reason="misnesting")
+        else:
+            self.cancelled = Cancelled._create(
+                source=reason.source,
+                reason=reason.reason,
+                source_task=reason.source_task,
+            )
+
+    def __call__(self) -> NoReturn:
+        try:
+            raise self.cancelled
+        finally:
+            del self.cancelled
+
+
 ################################################################
 # The central Runner object
 ################################################################

src/trio/_threads.py

@@ -430,7 +430,9 @@ def deliver_worker_fn_result(result: outcome.Outcome[RetT]) -> None:
 
         def abort(raise_cancel: RaiseCancelT) -> trio.lowlevel.Abort:
             # fill so from_thread_check_cancelled can raise
-            cancel_register[0] = raise_cancel
+            import copy
+
+            cancel_register[0] = copy.copy(raise_cancel)
             if abandon_bool:
                 # empty so report_back_in_trio_thread_fn cannot reschedule
                 task_register[0] = None