Work on

ef1e21cfb1e7b737abf9be61b51d9dd2eaee560a
commit ef1e21cfb1e7b737abf9be61b51d9dd2eaee560a
diff --git a/Lib/asyncio/unix_events.py b/Lib/asyncio/unix_events.py
@@ -1236,16 +1236,22 @@ def is_active(self):
         return True
 
     def close(self):
-        threads = list(self._threads.values())
-        for thread in threads:
-            thread.join()
+        pass
 
     def __enter__(self):
         return self
 
     def __exit__(self, exc_type, exc_val, exc_tb):
         pass
 
+    def __del__(self, _warn=warnings.warn):
+        threads = [thread for thread in list(self._threads.values())
+                   if thread.is_alive()]
+        if threads:
+            _warn(f"{self.__class__} has registered but not finished child processes",
+                  ResourceWarning,
+                  source=self)
+
     def add_child_handler(self, pid, callback, *args):
         loop = events.get_running_loop()
         thread = threading.Thread(target=self._do_waitpid,
@@ -1289,6 +1295,8 @@ def _do_waitpid(self, loop, expected_pid, callback, args):
             loop.call_soon_threadsafe(callback, pid, returncode, *args)
 
         self._threads.pop(expected_pid)
+        import time
+        time.sleep(0.5)
 
 
 class _UnixDefaultEventLoopPolicy(events.BaseDefaultEventLoopPolicy):

diff --git a/Lib/test/test_asyncio/utils.py b/Lib/test/test_asyncio/utils.py
@@ -1,5 +1,6 @@
 """Utilities shared by tests."""
 
+import asyncio
 import collections
 import contextlib
 import io
@@ -512,6 +513,18 @@ def close_loop(loop):
         if executor is not None:
             executor.shutdown(wait=True)
         loop.close()
+        policy = support.maybe_get_event_loop_policy()
+        if policy is not None:
+            try:
+                watcher = policy.get_child_watcher()
+            except NotImplementedError:
+                # watcher is not implemented by EventLoopPolicy, e.g. Windows
+                pass
+            else:
+                if isinstance(watcher, asyncio.ThreadedChildWatcher):
+                    threads = list(watcher._threads.values())
+                    for thread in threads:
+                        thread.join()
 
     def set_event_loop(self, loop, *, cleanup=True):
         assert loop is not None