Don't bother responding to client requests that have already disconnected (#8465)

This PR ports the quick fix from https://github.com/matrix-org/synapse/pull/2796 to further methods which handle media, URL preview and `/key/v2/server` requests. This prevents a harmless `ERROR` that comes up in the logs when we were unable to respond to a client request when the client had already disconnected. In this case we simply bail out if the client has already done so.

This is the 'simple fix' as suggested by https://github.com/matrix-org/synapse/issues/5304#issuecomment-574740003.

Fixes https://github.com/matrix-org/synapse/issues/6700
Fixes https://github.com/matrix-org/synapse/issues/5304

changelog.d/8465.bugfix

@@ -0,0 +1 @@
+Don't attempt to respond to some requests if the client has already disconnected.

synapse/http/server.py

@@ -651,6 +651,11 @@ def respond_with_json_bytes(
     Returns:
         twisted.web.server.NOT_DONE_YET if the request is still active.
     """
+    if request._disconnected:
+        logger.warning(
+            "Not sending response to request %s, already disconnected.", request
+        )
+        return
 
     request.setResponseCode(code)
     request.setHeader(b"Content-Type", b"application/json")

synapse/rest/media/v1/_base.py

@@ -213,6 +213,12 @@ async def respond_with_responder(
         file_size (int|None): Size in bytes of the media. If not known it should be None
         upload_name (str|None): The name of the requested file, if any.
     """
+    if request._disconnected:
+        logger.warning(
+            "Not sending response to request %s, already disconnected.", request
+        )
+        return
+
     if not responder:
         respond_404(request)
         return