Implement handling of HEAD requests

changelog.d/7999.bugfix

@@ -0,0 +1 @@
+Fix a long standing bug where HEAD requests were not properly rendered.

synapse/http/server.py

@@ -216,6 +216,14 @@ def render(self, request):
         defer.ensureDeferred(self._async_render_wrapper(request))
         return NOT_DONE_YET
 
+    def _async_render_HEAD(self, request):
+        """
+        The default handling of HEAD method is to treat it as a GET request.
+
+        This is an async version of twisted.web.resource.Resource.render_HEAD.
+        """
+        return self._async_render_GET(request)
+
     @wrap_async_request_handler
     async def _async_render_wrapper(self, request: SynapseRequest):
         """This is a wrapper that delegates to `_async_render` and handles
@@ -579,7 +587,7 @@ def set_cors_headers(request: Request):
     """
     request.setHeader(b"Access-Control-Allow-Origin", b"*")
     request.setHeader(
-        b"Access-Control-Allow-Methods", b"GET, POST, PUT, DELETE, OPTIONS"
+        b"Access-Control-Allow-Methods", b"GET, HEAD, POST, PUT, DELETE, OPTIONS"
     )
     request.setHeader(
         b"Access-Control-Allow-Headers",

synapse/http/servlet.py

@@ -270,6 +270,7 @@ class attribute containing a pre-compiled regular expression. The automatic
     instance methods associated with the corresponding HTTP method:
 
       on_GET
+      on_HEAD
       on_PUT
       on_POST
       on_DELETE
@@ -284,13 +285,20 @@ def register(self, http_server):
         if hasattr(self, "PATTERNS"):
             patterns = self.PATTERNS
 
-            for method in ("GET", "PUT", "POST", "OPTIONS", "DELETE"):
+            for method in ("GET", "HEAD", "PUT", "POST", "OPTIONS", "DELETE"):
                 if hasattr(self, "on_%s" % (method,)):
                     servlet_classname = self.__class__.__name__
                     method_handler = getattr(self, "on_%s" % (method,))
                     http_server.register_paths(
                         method, patterns, method_handler, servlet_classname
                     )
 
+                    # If there's not HEAD handler, but there is a GET handler,
+                    # register it again.
+                    if method == "GET" and not hasattr(self, "on_HEAD"):
+                        http_server.register_paths(
+                            "HEAD", patterns, method_handler, servlet_classname
+                        )
+
         else:
             raise NotImplementedError("RestServlet must register something.")

tests/test_server.py

@@ -312,3 +312,19 @@ def callback(request, **kwargs):
         self.assertEqual(location_headers, [b"/no/over/there"])
         cookies_headers = [v for k, v in headers if k == b"Set-Cookie"]
         self.assertEqual(cookies_headers, [b"session=yespls"])
+
+    def test_head_request(self):
+        """A head request should work by being turned into a GET request."""
+
+        def callback(request):
+            request.write(b"response")
+            request.finish()
+
+        res = WrapHtmlRequestHandlerTests.TestResource()
+        res.callback = callback
+
+        request, channel = make_request(self.reactor, b"HEAD", b"/path")
+        render(request, res, self.reactor)
+
+        self.assertEqual(channel.result["code"], b"200")
+        self.assertNotIn("body", channel.result)