Allow pickling Cancelled (#3250)

* Allow pickling `Cancelled`

* Add newsfragment

* Use `collections.abc.Callable` not `typing.Callable`

* Fix coverage

* Generalize __reduce__

* Address PR review

Co-authored-by: jakkdl <h6+github@pm.me>

* Add a test for coverage reasons

* Revert generalization

It's too much complexity to do it correctly.

* Ensure that we get any extra cases that need pickling

* Remove now-unnecessary new `__new__`

---------

Co-authored-by: jakkdl <h6+github@pm.me>

Author: A5rocks

newsfragments/3248.bugfix.rst

@@ -0,0 +1 @@
+Allow pickling `trio.Cancelled`, as they can show up when you want to pickle something else. This does not rule out pickling other ``NoPublicConstructor`` objects -- create an issue if necessary.

src/trio/_core/_exceptions.py

@@ -1,5 +1,12 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
 from trio._util import NoPublicConstructor, final
 
+if TYPE_CHECKING:
+    from collections.abc import Callable
+
 
 class TrioInternalError(Exception):
     """Raised by :func:`run` if we encounter a bug in Trio, or (possibly) a
@@ -63,6 +70,9 @@ class Cancelled(BaseException, metaclass=NoPublicConstructor):
     def __str__(self) -> str:
         return "Cancelled"
 
+    def __reduce__(self) -> tuple[Callable[[], Cancelled], tuple[()]]:
+        return (Cancelled._create, ())
+
 
 class BusyResourceError(Exception):
     """Raised when a task attempts to use a resource that some other task is

src/trio/_core/_tests/test_run.py

@@ -3,6 +3,7 @@
 import contextvars
 import functools
 import gc
+import pickle
 import sys
 import threading
 import time
@@ -2235,6 +2236,13 @@ def test_Cancelled_subclass() -> None:
         type("Subclass", (_core.Cancelled,), {})
 
 
+# https://github.com/python-trio/trio/issues/3248
+def test_Cancelled_pickle() -> None:
+    cancelled = _core.Cancelled._create()
+    cancelled = pickle.loads(pickle.dumps(cancelled))
+    assert isinstance(cancelled, _core.Cancelled)
+
+
 def test_CancelScope_subclass() -> None:
     with pytest.raises(TypeError):
         type("Subclass", (_core.CancelScope,), {})