Set the response code to 499 when a client disconnects during an upload

The response code isn't actually sent to the client, because the
connection has already closed by this point. But it causes the status
code to appear as 499 in the logs instead of 0.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

registry/handlers/helpers.go

@@ -40,6 +40,12 @@ func copyFullPayload(responseWriter http.ResponseWriter, r *http.Request, destWr
 		// error to keep the logs cleaner.
 		select {
 		case <-clientClosed:
+			// Set the response code to "499 Client Closed Request"
+			// Even though the connection has already been closed,
+			// this causes the logger to pick up a 499 error
+			// instead of showing 0 for the HTTP status.
+			responseWriter.WriteHeader(499)
+
 			ctxu.GetLogger(context).Error("client disconnected during " + action)
 			return errors.New("client disconnected")
 		default: