gh-149123: Avoid InvalidStateError in Windows Proactor polling

Lib/asyncio/windows_events.py

@@ -804,11 +804,13 @@ def _poll(self, timeout=None):
                 try:
                     value = callback(transferred, key, ov)
                 except OSError as e:
-                    f.set_exception(e)
-                    self._results.append(f)
+                    if not f.done():
+                        f.set_exception(e)
+                        self._results.append(f)
                 else:
-                    f.set_result(value)
-                    self._results.append(f)
+                    if not f.done():
+                        f.set_result(value)
+                        self._results.append(f)
                 finally:
                     f = None
 

Lib/test/test_asyncio/test_windows_events.py

@@ -220,6 +220,38 @@ def test_wait_for_handle_cancel(self):
         fut.cancel()
         fut.cancel()
 
+    def _poll_with_future_done_during_callback(self, callback_exception=None):
+        proactor = self.loop._proactor
+        ov = _overlapped.Overlapped(_winapi.NULL)
+        fut = windows_events._OverlappedFuture(ov, loop=self.loop)
+
+        class Obj:
+            pass
+
+        def callback(transferred, key, ov):
+            self.assertFalse(fut.done())
+            fut.cancel()
+            if callback_exception is not None:
+                raise callback_exception
+            return object()
+
+        proactor._cache[ov.address] = (fut, ov, Obj(), callback)
+        _overlapped.PostQueuedCompletionStatus(proactor._iocp, 0, 0,
+                                               ov.address)
+
+        proactor._poll(0)
+
+        self.assertTrue(fut.cancelled())
+        self.assertEqual(proactor._results, [])
+
+    def test_poll_skips_exception_if_future_done_during_callback(self):
+        exc = ConnectionResetError(_overlapped.ERROR_OPERATION_ABORTED,
+                                   "operation aborted")
+        self._poll_with_future_done_during_callback(exc)
+
+    def test_poll_skips_result_if_future_done_during_callback(self):
+        self._poll_with_future_done_during_callback()
+
     def test_read_self_pipe_restart(self):
         # Regression test for https://bugs.python.org/issue39010
         # Previously, restarting a proactor event loop in certain states

Misc/NEWS.d/next/Library/2026-07-02-01-55-00.gh-issue-149123.GN8o4M.rst

@@ -0,0 +1,2 @@
+Avoid raising :exc:`asyncio.InvalidStateError` from the Windows proactor event
+loop when an overlapped future becomes done while handling an I/O completion.