Define web.Application.on_startup() handler

Changes:

    - Define `web.Application.on_startup()` signal handler.
    - Run `app.on_startup()` along with the request handler within an event
loop in `web.run_app()` and in `worker.GunicornWebWorker()`.

Problems:
    - Tests in `test_run_app.py` don't pass because loop is mocked
with `wrap` parameter and `loop.create_server()` returns mock.
    - Tests in `test_worker.py` don't pass on the `master` branch
and complain on `pytest.skip()`.

aiohttp/web.py

@@ -171,6 +171,7 @@ def __init__(self, *, logger=web_logger, loop=None,
         self._on_pre_signal = PreSignal()
         self._on_post_signal = PostSignal()
         self._on_response_prepare = Signal(self)
+        self._on_startup = Signal(self)
         self._on_shutdown = Signal(self)
         self._on_cleanup = Signal(self)
 
@@ -190,6 +191,10 @@ def on_pre_signal(self):
     def on_post_signal(self):
         return self._on_post_signal
 
+    @property
+    def on_startup(self):
+        return self._on_startup
+
     @property
     def on_shutdown(self):
         return self._on_shutdown
@@ -214,6 +219,14 @@ def make_handler(self, **kwargs):
         return self._handler_factory(
             self, self.router, loop=self.loop, **kwargs)
 
+    @asyncio.coroutine
+    def startup(self):
+        """Causes on_startup signal
+
+        Should be called in the event loop along with the request handler.
+        """
+        yield from self.on_startup.send(self)
+
     @asyncio.coroutine
     def shutdown(self):
         """Causes on_shutdown signal
@@ -267,9 +280,11 @@ def run_app(app, *, host='0.0.0.0', port=None,
     loop = app.loop
 
     handler = app.make_handler()
-    srv = loop.run_until_complete(loop.create_server(handler, host, port,
-                                                     ssl=ssl_context,
-                                                     backlog=backlog))
+    server = loop.create_server(handler, host, port, ssl=ssl_context,
+                                backlog=backlog)
+    srv, startup_res = loop.run_until_complete(asyncio.gather(server,
+                                                              app.startup(),
+                                                              loop=loop))
 
     scheme = 'https' if ssl_context else 'http'
     print("======== Running on {scheme}://{host}:{port}/ ========\n"

aiohttp/worker.py

@@ -34,7 +34,9 @@ def run(self):
         self._runner = ensure_future(self._run(), loop=self.loop)
 
         try:
-            self.loop.run_until_complete(self._runner)
+            self.loop.run_until_complete(asyncio.gather(self._runner,
+                                                        self.wsgi.startup(),
+                                                        loop=self.loop))
         finally:
             self.loop.close()