asyncio: Add server_hostname as create_connection() argument, with secure default.

Lib/asyncio/base_events.py

@@ -275,8 +275,27 @@ class BaseEventLoop(events.AbstractEventLoop):
     @tasks.coroutine
     def create_connection(self, protocol_factory, host=None, port=None, *,
                           ssl=None, family=0, proto=0, flags=0, sock=None,
-                          local_addr=None):
+                          local_addr=None, server_hostname=None):
         """XXX"""
+        if server_hostname is not None and not ssl:
+            raise ValueError('server_hostname is only meaningful with ssl')
+
+        if server_hostname is None and ssl:
+            # Use host as default for server_hostname.  It is an error
+            # if host is empty or not set, e.g. when an
+            # already-connected socket was passed or when only a port
+            # is given.  To avoid this error, you can pass
+            # server_hostname='' -- this will bypass the hostname
+            # check.  (This also means that if host is a numeric
+            # IP/IPv6 address, we will attempt to verify that exact
+            # address; this will probably fail, but it is possible to
+            # create a certificate for a specific IP address, so we
+            # don't judge it here.)
+            if not host:
+                raise ValueError('You must set server_hostname '
+                                 'when using ssl without a host')
+            server_hostname = host
+            
         if host is not None or port is not None:
             if sock is not None:
                 raise ValueError(
@@ -357,7 +376,7 @@ class BaseEventLoop(events.AbstractEventLoop):
             sslcontext = None if isinstance(ssl, bool) else ssl
             transport = self._make_ssl_transport(
                 sock, protocol, sslcontext, waiter,
-                server_side=False, server_hostname=host)
+                server_side=False, server_hostname=server_hostname)
         else:
             transport = self._make_socket_transport(sock, protocol, waiter)
 

Lib/asyncio/events.py

@@ -172,7 +172,7 @@ class AbstractEventLoop:
 
     def create_connection(self, protocol_factory, host=None, port=None, *,
                           ssl=None, family=0, proto=0, flags=0, sock=None,
-                          local_addr=None):
+                          local_addr=None, server_hostname=None):
         raise NotImplementedError
 
     def create_server(self, protocol_factory, host=None, port=None, *,

Lib/asyncio/selector_events.py

@@ -573,7 +573,7 @@ class _SelectorSslTransport(_SelectorTransport):
             'server_side': server_side,
             'do_handshake_on_connect': False,
         }
-        if server_hostname is not None and not server_side and ssl.HAS_SNI:
+        if server_hostname and not server_side and ssl.HAS_SNI:
             wrap_kwargs['server_hostname'] = server_hostname
         sslsock = sslcontext.wrap_socket(rawsock, **wrap_kwargs)
 
@@ -619,7 +619,7 @@ class _SelectorSslTransport(_SelectorTransport):
 
         # Verify hostname if requested.
         peercert = self._sock.getpeercert()
-        if (self._server_hostname is not None and
+        if (self._server_hostname and
             self._sslcontext.verify_mode != ssl.CERT_NONE):
             try:
                 ssl.match_hostname(peercert, self._server_hostname)

Lib/test/test_asyncio/test_base_events.py

@@ -444,6 +444,60 @@ class BaseEventLoopWithSelectorTests(unittest.TestCase):
         self.assertRaises(
             OSError, self.loop.run_until_complete, coro)
 
+    def test_create_connection_server_hostname_default(self):
+        self.loop.getaddrinfo = unittest.mock.Mock()
+        def mock_getaddrinfo(*args, **kwds):
+            f = futures.Future(loop=self.loop)
+            f.set_result([(socket.AF_INET, socket.SOCK_STREAM,
+                           socket.SOL_TCP, '', ('1.2.3.4', 80))])
+            return f
+        self.loop.getaddrinfo.side_effect = mock_getaddrinfo
+        self.loop.sock_connect = unittest.mock.Mock()
+        self.loop.sock_connect.return_value = ()
+        self.loop._make_ssl_transport = unittest.mock.Mock()
+        def mock_make_ssl_transport(sock, protocol, sslcontext, waiter, **kwds):
+            waiter.set_result(None)
+        self.loop._make_ssl_transport.side_effect = mock_make_ssl_transport
+        ANY = unittest.mock.ANY
+        # First try the default server_hostname.
+        self.loop._make_ssl_transport.reset_mock()
+        coro = self.loop.create_connection(MyProto, 'python.org', 80, ssl=True)
+        self.loop.run_until_complete(coro)
+        self.loop._make_ssl_transport.assert_called_with(ANY, ANY, ANY, ANY,
+                                                         server_side=False,
+                                                         server_hostname='python.org')
+        # Next try an explicit server_hostname.
+        self.loop._make_ssl_transport.reset_mock()
+        coro = self.loop.create_connection(MyProto, 'python.org', 80, ssl=True,
+                                           server_hostname='perl.com')
+        self.loop.run_until_complete(coro)
+        self.loop._make_ssl_transport.assert_called_with(ANY, ANY, ANY, ANY,
+                                                         server_side=False,
+                                                         server_hostname='perl.com')
+        # Finally try an explicit empty server_hostname.
+        self.loop._make_ssl_transport.reset_mock()
+        coro = self.loop.create_connection(MyProto, 'python.org', 80, ssl=True,
+                                           server_hostname='')
+        self.loop.run_until_complete(coro)
+        self.loop._make_ssl_transport.assert_called_with(ANY, ANY, ANY, ANY,
+                                                         server_side=False,
+                                                         server_hostname='')
+
+    def test_create_connection_server_hostname_errors(self):
+        # When not using ssl, server_hostname must be None (but '' is OK).
+        coro = self.loop.create_connection(MyProto, 'python.org', 80, server_hostname='')
+        self.assertRaises(ValueError, self.loop.run_until_complete, coro)
+        coro = self.loop.create_connection(MyProto, 'python.org', 80, server_hostname='python.org')
+        self.assertRaises(ValueError, self.loop.run_until_complete, coro)
+
+        # When using ssl, server_hostname may be None if host is non-empty.
+        coro = self.loop.create_connection(MyProto, '', 80, ssl=True)
+        self.assertRaises(ValueError, self.loop.run_until_complete, coro)
+        coro = self.loop.create_connection(MyProto, None, 80, ssl=True)
+        self.assertRaises(ValueError, self.loop.run_until_complete, coro)
+        coro = self.loop.create_connection(MyProto, None, None, ssl=True, sock=socket.socket())
+        self.assertRaises(ValueError, self.loop.run_until_complete, coro)
+
     def test_create_server_empty_host(self):
         # if host is empty string use None instead
         host = object()