[3.11] [3.12] GH-110894: Call loop exception handler for exceptions in client_connected_cb (GH-111601) (GH-111632) (#111634)

* [3.12] GH-110894: Call loop exception handler for exceptions in client_connected_cb (GH-111601) (GH-111632)
(cherry picked from commit 9aa88290d8)

Co-authored-by: Kumar Aditya <kumaraditya@python.org>
Call loop exception handler for exceptions in `client_connected_cb` of `asyncio.start_server` so that applications can handle it..
(cherry picked from commit 229f44d353)

* gh-111644: Fix asyncio test_unhandled_exceptions() (#111713)

Fix test_unhandled_exceptions() of test_asyncio.test_streams: break
explicitly a reference cycle.

Fix also StreamTests.tearDown(): the loop must not be closed
explicitly, but using set_event_loop() which takes care of shutting
down the executor with executor.shutdown(wait=True).
BaseEventLoop.close() calls executor.shutdown(wait=False).

(cherry picked from commit ac01e2243a)

---------

Co-authored-by: Kumar Aditya <kumaraditya@python.org>
Co-authored-by: Victor Stinner <vstinner@python.org>

Lib/asyncio/streams.py

@@ -245,7 +245,19 @@ class StreamReaderProtocol(FlowControlMixin, protocols.Protocol):
             res = self._client_connected_cb(reader,
                                             self._stream_writer)
             if coroutines.iscoroutine(res):
+                def callback(task):
+                    exc = task.exception()
+                    if exc is not None:
+                        self._loop.call_exception_handler({
+                            'message': 'Unhandled exception in client_connected_cb',
+                            'exception': exc,
+                            'transport': transport,
+                        })
+                        transport.close()
+
                 self._task = self._loop.create_task(res)
+                self._task.add_done_callback(callback)
+
             self._strong_reader = None
 
     def connection_lost(self, exc):