make Sanic.create_server return an asyncio.Server

- adding 2 new parameters to Sanic.create_server:
   * return_asyncio_server=False - defines whether there's
     a need to return an asyncio.Server or run it right away
   * asyncio_server_kwargs=None - for python 3.7 uvloop doesn't
     support all necessary features like "start_serving",
     so, in order to make sanic work well with asyncio from 3.7
     there's a need to introduce generic way for passing
     kwargs for "loop.create_server"

Closes: #1469

sanic/app.py

@@ -1121,6 +1121,8 @@ class Sanic:
         backlog: int = 100,
         stop_event: Any = None,
         access_log: Optional[bool] = None,
+        return_asyncio_server=False,
+        asyncio_server_kwargs=None,
     ) -> None:
         """
         Asynchronous version of :func:`run`.
@@ -1154,6 +1156,13 @@ class Sanic:
         :type stop_event: None
         :param access_log: Enables writing access logs (slows server)
         :type access_log: bool
+        :param return_asyncio_server: flag that defines whether there's a need
+                                      to return asyncio.Server or
+                                      start it serving right away
+        :type return_asyncio_server: bool
+        :param asyncio_server_kwargs: key-value arguments for
+                                      asyncio/uvloop create_server method
+        :type asyncio_server_kwargs: dict
         :return: Nothing
         """
 
@@ -1184,7 +1193,7 @@ class Sanic:
             loop=get_event_loop(),
             protocol=protocol,
             backlog=backlog,
-            run_async=True,
+            run_async=return_asyncio_server,
         )
 
         # Trigger before_start events
@@ -1193,7 +1202,10 @@ class Sanic:
             server_settings.get("loop"),
         )
 
-        return await serve(**server_settings)
+        return await serve(
+            asyncio_server_kwargs=asyncio_server_kwargs,
+            **server_settings
+        )
 
     async def trigger_events(self, events, loop):
         """Trigger events (functions or async)

sanic/server.py

@@ -656,6 +656,7 @@ def serve(
     websocket_write_limit=2 ** 16,
     state=None,
     graceful_shutdown_timeout=15.0,
+    asyncio_server_kwargs=None,
 ):
     """Start asynchronous HTTP Server on an individual process.
 
@@ -700,6 +701,8 @@ def serve(
     :param router: Router object
     :param graceful_shutdown_timeout: How long take to Force close non-idle
                                       connection
+    :param asyncio_server_kwargs: key-value args for asyncio/uvloop
+                                  create_server method
     :return: Nothing
     """
     if not run_async:
@@ -734,7 +737,9 @@ def serve(
         state=state,
         debug=debug,
     )
-
+    asyncio_server_kwargs = (
+        asyncio_server_kwargs if asyncio_server_kwargs else {}
+    )
     server_coroutine = loop.create_server(
         server,
         host,
@@ -743,6 +748,7 @@ def serve(
         reuse_port=reuse_port,
         sock=sock,
         backlog=backlog,
+        **asyncio_server_kwargs
     )
 
     # Instead of pulling time at the end of every request,

tests/test_app.py

@@ -17,6 +17,11 @@ def test_app_loop_running(app):
     assert response.text == "pass"
 
 
+def test_create_asyncio_server(app):
+    asyncio_srv = app.create_server(return_asyncio_server=True)
+    assert isinstance(asyncio_srv, asyncio.AbstractServer)
+
+
 def test_app_loop_not_running(app):
     with pytest.raises(SanicException) as excinfo:
         app.loop