Fixed a crash in Local.__del__ due to buggy WeakSet impl (#176)

asgiref/local.py

@@ -87,11 +87,16 @@ def _get_storage(self):
         return getattr(context_obj, self._attr_name)
 
     def __del__(self):
-        for context_obj in self._context_refs:
-            try:
-                delattr(context_obj, self._attr_name)
-            except AttributeError:
-                pass
+        try:
+            for context_obj in self._context_refs:
+                try:
+                    delattr(context_obj, self._attr_name)
+                except AttributeError:
+                    pass
+        except TypeError:
+            # WeakSet.__iter__ can crash when interpreter is shutting down due
+            # to _IterationGuard being None.
+            pass
 
     def __getattr__(self, key):
         with self._thread_lock:

tests/test_local.py

@@ -298,3 +298,20 @@ def run(self):
 
     await sync_to_async(sync_function)(5)
     assert test_local.counter == 6
+
+
+def test_local_del_swallows_type_error(monkeypatch):
+    test_local = Local()
+
+    blow_up_calls = 0
+
+    def blow_up(self):
+        nonlocal blow_up_calls
+        blow_up_calls += 1
+        raise TypeError()
+
+    monkeypatch.setattr("weakref.WeakSet.__iter__", blow_up)
+
+    test_local.__del__()
+
+    assert blow_up_calls == 1