Update server-side websocket close FAQ

Due to improvements done in aio-libs#754, websocket `close()`
can now be called from a separate task.

docs/faq.rst

@@ -162,49 +162,43 @@ For example we have an application with two endpoints:
    2. ``/logout_user`` that when invoked needs to close all open
       websockets for that user.
 
-Keep in mind that you can only ``.close()`` a websocket from inside
-the handler task, and since the handler task is busy reading from the
-websocket, it can't react to other events.
-
-One simple solution is keeping a shared registry of websocket handler
-tasks for a user in the :class:`aiohttp.web.Application` instance and
-``cancel()`` them in ``/logout_user`` handler::
+One simple solution is keeping a shared registry of websocket responses
+for a user in the :class:`aiohttp.web.Application` instance and
+call :meth:`aiohttp.web.WebSocketResponse.close` on all of them in ``/logout_user`` handler::
 
     async def echo_handler(request):
 
-        ws = web.WebSocketResponse()
+        ws = aiohttp.web.WebSocketResponse()
         user_id = authenticate_user(request)
         await ws.prepare(request)
-        request.app['websockets'][user_id].add(asyncio.Task.current_task())
 
+        request.app['websockets'][user_id].append(ws)
         try:
             async for msg in ws:
-                # handle incoming messages
-                ...
-
-        except asyncio.CancelledError:
-            print('websocket cancelled')
+                ws.send_str(msg.data)
         finally:
-            request.app['websockets'][user_id].remove(asyncio.Task.current_task())
-        await ws.close()
+            request.app['websockets'][user_id].remove(ws)
+
         return ws
 
+
     async def logout_handler(request):
 
         user_id = authenticate_user(request)
 
-        for task in request.app['websockets'][user_id]:
-            task.cancel()
+        ws_closers = [ws.close() for ws in request.app['websockets'][user_id] if not ws.closed]
 
-        # return response
-        ...
+        # Watch out, this will keep us from returing the response until all are closed
+        ws_closers and await asyncio.wait(ws_closers)
+
+        return aiohttp.web.Response(text='OK')
 
     def main():
         loop = asyncio.get_event_loop()
         app = aiohttp.web.Application(loop=loop)
         app.router.add_route('GET', '/echo', echo_handler)
         app.router.add_route('POST', '/logout', logout_handler)
-        app['websockets'] = defaultdict(set)
+        app['websockets'] = defaultdict(deque)
         aiohttp.web.run_app(app, host='localhost', port=8080)