gh-135056: Add a --cors CLI argument to http.server

Add a --cors command line argument to the stdlib http.server module, which will
add an `Access-Control-Allow-Origin: *` header to all responses. As part of this
implementation, add a `response_headers` argument to SimpleHTTPRequestHandler
and HttpServer, which allows callers to add addition headers to the response.

Author: aisipos

Doc/library/http.server.rst

@@ -362,7 +362,7 @@ instantiation, of which this module provides three different variants:
          delays, it now always returns the IP address.
 
 
-.. class:: SimpleHTTPRequestHandler(request, client_address, server, directory=None)
+.. class:: SimpleHTTPRequestHandler(request, client_address, server, directory=None, response_headers=None)
 
    This class serves files from the directory *directory* and below,
    or the current directory if *directory* is not provided, directly
@@ -374,6 +374,10 @@ instantiation, of which this module provides three different variants:
    .. versionchanged:: 3.9
       The *directory* parameter accepts a :term:`path-like object`.
 
+   .. versionchanged:: 3.15
+      The *response_headers* parameter accepts an optional dictionary of
+      additional HTTP headers to add to each response.
+
    A lot of the work, such as parsing the request, is done by the base class
    :class:`BaseHTTPRequestHandler`.  This class implements the :func:`do_GET`
    and :func:`do_HEAD` functions.
@@ -428,6 +432,9 @@ instantiation, of which this module provides three different variants:
       followed by a ``'Content-Length:'`` header with the file's size and a
       ``'Last-Modified:'`` header with the file's modification time.
 
+      The headers specified in the dictionary instance argument
+      ``response_headers`` are each individually sent in the response.
+
       Then follows a blank line signifying the end of the headers, and then the
       contents of the file are output.
 
@@ -437,6 +444,9 @@ instantiation, of which this module provides three different variants:
       .. versionchanged:: 3.7
          Support of the ``'If-Modified-Since'`` header.
 
+      .. versionchanged:: 3.15
+         Support ``response_headers`` as an instance argument.
+
 The :class:`SimpleHTTPRequestHandler` class can be used in the following
 manner in order to create a very basic webserver serving files relative to
 the current directory::
@@ -543,6 +553,14 @@ The following options are accepted:
 
    .. versionadded:: 3.14
 
+.. option:: --cors
+
+   Adds an additional CORS (Cross-Origin Resource sharing) header to each response::
+
+     Access-Control-Allow-Origin: *
+
+   .. versionadded:: 3.15
+
 
 .. _http.server-security:
 

Lib/http/server.py

@@ -117,13 +117,22 @@ class HTTPServer(socketserver.TCPServer):
     allow_reuse_address = True    # Seems to make sense in testing environment
     allow_reuse_port = True
 
+    def __init__(self, *args, response_headers=None, **kwargs):
+        self.response_headers = response_headers if response_headers is not None else {}
+        super().__init__(*args, **kwargs)
+
     def server_bind(self):
         """Override server_bind to store the server name."""
         socketserver.TCPServer.server_bind(self)
         host, port = self.server_address[:2]
         self.server_name = socket.getfqdn(host)
         self.server_port = port
 
+    def finish_request(self, request, client_address):
+        """Finish one request by instantiating RequestHandlerClass."""
+        args = (request, client_address, self)
+        kwargs = dict(response_headers=self.response_headers) if self.response_headers else dict()
+        self.RequestHandlerClass(*args, **kwargs)
 
 class ThreadingHTTPServer(socketserver.ThreadingMixIn, HTTPServer):
     daemon_threads = True
@@ -132,7 +141,7 @@ class ThreadingHTTPServer(socketserver.ThreadingMixIn, HTTPServer):
 class HTTPSServer(HTTPServer):
     def __init__(self, server_address, RequestHandlerClass,
                  bind_and_activate=True, *, certfile, keyfile=None,
-                 password=None, alpn_protocols=None):
+                 password=None, alpn_protocols=None, response_headers=None):
         try:
             import ssl
         except ImportError:
@@ -150,7 +159,8 @@ def __init__(self, server_address, RequestHandlerClass,
 
         super().__init__(server_address,
                          RequestHandlerClass,
-                         bind_and_activate)
+                         bind_and_activate,
+                         response_headers=response_headers)
 
     def server_activate(self):
         """Wrap the socket in SSLSocket."""
@@ -692,10 +702,11 @@ class SimpleHTTPRequestHandler(BaseHTTPRequestHandler):
         '.xz': 'application/x-xz',
     }
 
-    def __init__(self, *args, directory=None, **kwargs):
+    def __init__(self, *args, directory=None, response_headers=None, **kwargs):
         if directory is None:
             directory = os.getcwd()
         self.directory = os.fspath(directory)
+        self.response_headers = response_headers or {}
         super().__init__(*args, **kwargs)
 
     def do_GET(self):
@@ -736,6 +747,10 @@ def send_head(self):
                 new_url = urllib.parse.urlunsplit(new_parts)
                 self.send_header("Location", new_url)
                 self.send_header("Content-Length", "0")
+                # User specified response_headers
+                if self.response_headers is not None:
+                    for header, value in self.response_headers.items():
+                        self.send_header(header, value)
                 self.end_headers()
                 return None
             for index in self.index_pages:
@@ -795,6 +810,9 @@ def send_head(self):
             self.send_header("Content-Length", str(fs[6]))
             self.send_header("Last-Modified",
                 self.date_time_string(fs.st_mtime))
+            if self.response_headers is not None:
+                for header, value in self.response_headers.items():
+                    self.send_header(header, value)
             self.end_headers()
             return f
         except:
@@ -970,7 +988,7 @@ def _get_best_family(*address):
 def test(HandlerClass=BaseHTTPRequestHandler,
          ServerClass=ThreadingHTTPServer,
          protocol="HTTP/1.0", port=8000, bind=None,
-         tls_cert=None, tls_key=None, tls_password=None):
+         tls_cert=None, tls_key=None, tls_password=None, response_headers=None):
     """Test the HTTP request handler class.
 
     This runs an HTTP server on port 8000 (or the port argument).
@@ -981,9 +999,10 @@ def test(HandlerClass=BaseHTTPRequestHandler,
 
     if tls_cert:
         server = ServerClass(addr, HandlerClass, certfile=tls_cert,
-                             keyfile=tls_key, password=tls_password)
+                             keyfile=tls_key, password=tls_password,
+                             response_headers=response_headers)
     else:
-        server = ServerClass(addr, HandlerClass)
+        server = ServerClass(addr, HandlerClass, response_headers=response_headers)
 
     with server as httpd:
         host, port = httpd.socket.getsockname()[:2]
@@ -1024,6 +1043,8 @@ def _main(args=None):
     parser.add_argument('port', default=8000, type=int, nargs='?',
                         help='bind to this port '
                              '(default: %(default)s)')
+    parser.add_argument('--cors', action='store_true',
+                        help='Enable Access-Control-Allow-Origin: * header')
     args = parser.parse_args(args)
 
     if not args.tls_cert and args.tls_key:
@@ -1051,15 +1072,19 @@ def server_bind(self):
             return super().server_bind()
 
         def finish_request(self, request, client_address):
-            self.RequestHandlerClass(request, client_address, self,
-                                     directory=args.directory)
+            handler_args = (request, client_address, self)
+            handler_kwargs = dict(directory=args.directory)
+            if self.response_headers:
+                handler_kwargs['response_headers'] = self.response_headers
+            self.RequestHandlerClass(*handler_args, **handler_kwargs)
 
     class HTTPDualStackServer(DualStackServerMixin, ThreadingHTTPServer):
         pass
     class HTTPSDualStackServer(DualStackServerMixin, ThreadingHTTPSServer):
         pass
 
     ServerClass = HTTPSDualStackServer if args.tls_cert else HTTPDualStackServer
+    response_headers = {'Access-Control-Allow-Origin': '*'} if args.cors else None
 
     test(
         HandlerClass=SimpleHTTPRequestHandler,
@@ -1070,6 +1095,7 @@ class HTTPSDualStackServer(DualStackServerMixin, ThreadingHTTPSServer):
         tls_cert=args.tls_cert,
         tls_key=args.tls_key,
         tls_password=tls_key_password,
+        response_headers=response_headers
     )
 
 

Lib/test/test_httpservers.py

@@ -81,11 +81,12 @@ def test_https_server_raises_runtime_error(self):
 
 
 class TestServerThread(threading.Thread):
-    def __init__(self, test_object, request_handler, tls=None):
+    def __init__(self, test_object, request_handler, tls=None, server_kwargs=None):
         threading.Thread.__init__(self)
         self.request_handler = request_handler
         self.test_object = test_object
         self.tls = tls
+        self.server_kwargs = server_kwargs or {}
 
     def run(self):
         if self.tls:
@@ -95,7 +96,8 @@ def run(self):
                 request_handler=self.request_handler,
             )
         else:
-            self.server = HTTPServer(('localhost', 0), self.request_handler)
+            self.server = HTTPServer(('localhost', 0), self.request_handler,
+                                     **self.server_kwargs)
         self.test_object.HOST, self.test_object.PORT = self.server.socket.getsockname()
         self.test_object.server_started.set()
         self.test_object = None
@@ -113,12 +115,14 @@ class BaseTestCase(unittest.TestCase):
 
     # Optional tuple (certfile, keyfile, password) to use for HTTPS servers.
     tls = None
+    server_kwargs = None
 
     def setUp(self):
         self._threads = threading_helper.threading_setup()
         os.environ = os_helper.EnvironmentVarGuard()
         self.server_started = threading.Event()
-        self.thread = TestServerThread(self, self.request_handler, self.tls)
+        self.thread = TestServerThread(self, self.request_handler, self.tls,
+                                       self.server_kwargs)
         self.thread.start()
         self.server_started.wait()
 
@@ -824,6 +828,16 @@ def test_path_without_leading_slash(self):
                          self.tempdir_name + "/?hi=1")
 
 
+class CorsHTTPServerTestCase(SimpleHTTPServerTestCase):
+    server_kwargs = dict(
+        response_headers = {'Access-Control-Allow-Origin': '*'}
+    )
+    def test_cors(self):
+        response = self.request(self.base_url + '/test')
+        self.check_status_and_reason(response, HTTPStatus.OK)
+        self.assertEqual(response.getheader('Access-Control-Allow-Origin'), '*')
+
+
 class SocketlessRequestHandler(SimpleHTTPRequestHandler):
     def __init__(self, directory=None):
         request = mock.Mock()
@@ -1306,6 +1320,7 @@ class CommandLineTestCase(unittest.TestCase):
         'tls_cert': None,
         'tls_key': None,
         'tls_password': None,
+        'response_headers': None,
     }
 
     def setUp(self):
@@ -1371,6 +1386,17 @@ def test_protocol_flag(self, mock_func):
                     mock_func.assert_called_once_with(**call_args)
                     mock_func.reset_mock()
 
+    @mock.patch('http.server.test')
+    def test_cors_flag(self, mock_func):
+        self.invoke_httpd('--cors')
+        call_args = self.args | dict(
+            response_headers={
+                'Access-Control-Allow-Origin': '*'
+            }
+        )
+        mock_func.assert_called_once_with(**call_args)
+        mock_func.reset_mock()
+
     @unittest.skipIf(ssl is None, "requires ssl")
     @mock.patch('http.server.test')
     def test_tls_cert_and_key_flags(self, mock_func):