Don't bother responding to client requests that have already disconnected

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