bpo-32662: Implement Server.start_serving() and Server.serve_forever() (#5312)

* bpo-32662: Implement Server.start_serving() and Server.serve_forever()

New methods:

* Server.start_serving(),
* Server.serve_forever(), and
* Server.is_serving().

Add 'start_serving' keyword parameter to loop.create_server() and
loop.create_unix_server().

Lib/asyncio/events.py

@@ -164,13 +164,39 @@ class AbstractServer:
         """Stop serving.  This leaves existing connections open."""
         raise NotImplementedError
 
+    def get_loop(self):
+        """Get the event loop the Server object is attached to."""
+        raise NotImplementedError
+
+    def is_serving(self):
+        """Return True if the server is accepting connections."""
+        raise NotImplementedError
+
+    async def start_serving(self):
+        """Start accepting connections.
+
+        This method is idempotent, so it can be called when
+        the server is already being serving.
+        """
+        raise NotImplementedError
+
+    async def serve_forever(self):
+        """Start accepting connections until the coroutine is cancelled.
+
+        The server is closed when the coroutine is cancelled.
+        """
+        raise NotImplementedError
+
     async def wait_closed(self):
         """Coroutine to wait until service is closed."""
         raise NotImplementedError
 
-    def get_loop(self):
-        """ Get the event loop the Server object is attached to."""
-        raise NotImplementedError
+    async def __aenter__(self):
+        return self
+
+    async def __aexit__(self, *exc):
+        self.close()
+        await self.wait_closed()
 
 
 class AbstractEventLoop:
@@ -279,7 +305,8 @@ class AbstractEventLoop:
             *, family=socket.AF_UNSPEC,
             flags=socket.AI_PASSIVE, sock=None, backlog=100,
             ssl=None, reuse_address=None, reuse_port=None,
-            ssl_handshake_timeout=None):
+            ssl_handshake_timeout=None,
+            start_serving=True):
         """A coroutine which creates a TCP server bound to host and port.
 
         The return value is a Server object which can be used to stop
@@ -319,6 +346,11 @@ class AbstractEventLoop:
         will wait for completion of the SSL handshake before aborting the
         connection. Default is 10s, longer timeouts may increase vulnerability
         to DoS attacks (see https://support.f5.com/csp/article/K13834)
+
+        start_serving set to True (default) causes the created server
+        to start accepting connections immediately.  When set to False,
+        the user should await Server.start_serving() or Server.serve_forever()
+        to make the server to start accepting connections.
         """
         raise NotImplementedError
 
@@ -343,7 +375,8 @@ class AbstractEventLoop:
     async def create_unix_server(
             self, protocol_factory, path=None, *,
             sock=None, backlog=100, ssl=None,
-            ssl_handshake_timeout=None):
+            ssl_handshake_timeout=None,
+            start_serving=True):
         """A coroutine which creates a UNIX Domain Socket server.
 
         The return value is a Server object, which can be used to stop
@@ -363,6 +396,11 @@ class AbstractEventLoop:
 
         ssl_handshake_timeout is the time in seconds that an SSL server
         will wait for the SSL handshake to complete (defaults to 10s).
+
+        start_serving set to True (default) causes the created server
+        to start accepting connections immediately.  When set to False,
+        the user should await Server.start_serving() or Server.serve_forever()
+        to make the server to start accepting connections.
         """
         raise NotImplementedError