Fix spurious exceptions when client closes conncetion

If a HTTP handler throws an exception while processing a request we
automatically write a JSON error response. If the handler had already
started writing a response twisted throws an exception.

We should check for this case and simple abort the connection if there
was an error after the response had started being written.

synapse/http/server.py

@@ -84,6 +84,17 @@ def wrap_json_request_handler(h):
             logger.info(
                 "%s SynapseError: %s - %s", request, code, e.msg
             )
+
+            # Only respond with an error response if we haven't already started
+            # writing, otherwise lets just kill the connection
+            if request.startedWriting:
+                if request.transport:
+                    try:
+                        request.transport.abortConnection()
+                    except Exception:
+                        # abortConnection throws if the connection is already closed
+                        pass
+            else:
                 respond_with_json(
                     request, code, e.error_dict(), send_cors=True,
                     pretty_print=_request_user_agent_is_curl(request),
@@ -100,6 +111,16 @@ def wrap_json_request_handler(h):
                 request,
                 f.getTraceback().rstrip(),
             )
+            # Only respond with an error response if we haven't already started
+            # writing, otherwise lets just kill the connection
+            if request.startedWriting:
+                if request.transport:
+                    try:
+                        request.transport.abortConnection()
+                    except Exception:
+                        # abortConnection throws if the connection is already closed
+                        pass
+            else:
                 respond_with_json(
                     request,
                     500,