GH-89237: fix hang in proactor `subprocess.wait_closed()` (#98572)

Lib/asyncio/proactor_events.py

@@ -60,6 +60,7 @@ class _ProactorBasePipeTransport(transports._FlowControlMixin,
         self._pending_write = 0
         self._conn_lost = 0
         self._closing = False  # Set when close() called.
+        self._called_connection_lost = False
         self._eof_written = False
         if self._server is not None:
             self._server._attach()
@@ -136,7 +137,7 @@ class _ProactorBasePipeTransport(transports._FlowControlMixin,
                 self._empty_waiter.set_result(None)
             else:
                 self._empty_waiter.set_exception(exc)
-        if self._closing:
+        if self._closing and self._called_connection_lost:
             return
         self._closing = True
         self._conn_lost += 1
@@ -166,6 +167,7 @@ class _ProactorBasePipeTransport(transports._FlowControlMixin,
             if server is not None:
                 server._detach()
                 self._server = None
+            self._called_connection_lost = True
 
     def get_write_buffer_size(self):
         size = self._pending_write

Lib/test/test_asyncio/test_proactor_events.py

@@ -290,7 +290,12 @@ class ProactorSocketTransportTests(test_utils.TestCase):
         tr._closing = True
         tr._force_close(None)
         test_utils.run_briefly(self.loop)
-        self.assertFalse(self.protocol.connection_lost.called)
+        # See https://github.com/python/cpython/issues/89237
+        # `protocol.connection_lost` should be called even if
+        # the transport was closed forcefully otherwise
+        # the resources held by protocol will never be freed
+        # and waiters will never be notified leading to hang.
+        self.assertTrue(self.protocol.connection_lost.called)
 
     def test_fatal_error_2(self):
         tr = self.socket_transport()

Misc/NEWS.d/next/Library/2022-10-23-18-30-39.gh-issue-89237.kBui30.rst

@@ -0,0 +1 @@
+Fix hang on Windows in ``subprocess.wait_closed()`` in :mod:`asyncio` with :class:`~asyncio.ProactorEventLoop`. Patch by Kumar Aditya.