[3.12] gh-79033: Try to fix asyncio.Server.wait_closed() again (GH-111336) (#111424)

gh-79033: Try to fix asyncio.Server.wait_closed() again (GH-111336)

* Try to fix asyncio.Server.wait_closed() again

I identified the condition that `wait_closed()` is intended
to wait for: the server is closed *and* there are no more
active connections.

When this condition first becomes true, `_wakeup()` is called
(either from `close()` or from `_detach()`) and it sets `_waiters`
to `None`. So we just check for `self._waiters is None`; if it's
not `None`, we know we have to wait, and do so.

A problem was that the new test introduced in 3.12 explicitly
tested that `wait_closed()` returns immediately when the server
is *not* closed but there are currently no active connections.
This was a mistake (probably a misunderstanding of the intended
semantics). I've fixed the test, and added a separate test that
checks exactly for this scenario.

I also fixed an oddity where in `_wakeup()` the result of the
waiter was set to the waiter itself. This result is not used
anywhere and I changed this to `None`, to avoid a GC cycle.

* Update Lib/asyncio/base_events.py

---------

(cherry picked from commit 2655369559)

Co-authored-by: Guido van Rossum <guido@python.org>
Co-authored-by: Carol Willing <carolcode@willingconsulting.com>

Lib/asyncio/base_events.py

@@ -305,7 +305,7 @@ class Server(events.AbstractServer):
         self._waiters = None
         for waiter in waiters:
             if not waiter.done():
-                waiter.set_result(waiter)
+                waiter.set_result(None)
 
     def _start_serving(self):
         if self._serving:
@@ -377,7 +377,27 @@ class Server(events.AbstractServer):
             self._serving_forever_fut = None
 
     async def wait_closed(self):
-        if self._waiters is None or self._active_count == 0:
+        """Wait until server is closed and all connections are dropped.
+
+        - If the server is not closed, wait.
+        - If it is closed, but there are still active connections, wait.
+
+        Anyone waiting here will be unblocked once both conditions
+        (server is closed and all connections have been dropped)
+        have become true, in either order.
+
+        Historical note: In 3.11 and before, this was broken, returning
+        immediately if the server was already closed, even if there
+        were still active connections. An attempted fix in 3.12.0 was
+        still broken, returning immediately if the server was still
+        open and there were no active connections. Hopefully in 3.12.1
+        we have it right.
+        """
+        # Waiters are unblocked by self._wakeup(), which is called
+        # from two places: self.close() and self._detach(), but only
+        # when both conditions have become true. To signal that this
+        # has happened, self._wakeup() sets self._waiters to None.
+        if self._waiters is None:
             return
         waiter = self._loop.create_future()
         self._waiters.append(waiter)