fail() in RoutingContextImpl constructor causes "Unhandled exception in router" even if error was handled

[Darker]

Version

Discovered in 4.3.8, but I can see it in master too.

Context

When a request path is empty or does not start with /, RoutingContext fails in its constructor:

    if (path == null || path.length() == 0) {
      // HTTP paths must start with a '/'
      fail(400);
    } else if (path.charAt(0) != '/') {
      // For compatiblity we return `Not Found` when a path does not start with `/`
      fail(404);
    }

This will trigger failure handler assigned to this route immediately. However this constructor is called in RouterImpl like this:

new RoutingContextImpl(null, this, request, state.getRoutes()).next();

What this means that the order of actions is such:

1. Routing context constructor
   1. Call fail() and failed() now returns true
   2. doFail() causes iterateNext
   3. Error handler is found
   4. Error handler is executed
2. Constructor was completed
3. Back in RouterImpl, next() is called on constructed RoutingContextImpl
4. Because routing context iterated already, it cannot find the error handler
5. "Unhandled exception in router" appears in the log

Do you have a reproducer?

Try this:

final Router router = Router.router(vertx);

router.get("/*")
        .handler(routingContext -> {
            routingContext.response().setStatusCode(200);
            routingContext.end("Hello world!");
        })
        .failureHandler(routingContext->{
            int statusCode = routingContext.statusCode();
            final var failure = routingContext.failure();
            final var request = routingContext.request();
            if(failure != null) {
                if(failure instanceof HttpException) {
                    statusCode = ((HttpException)failure).getStatusCode();
                    LOGGER.info("Handling HttpException at path {} code {}", request.path(), statusCode);
                }
                else {
                    statusCode = 500;
                    LOGGER.warn("Handling unexpected exception at path {}", request.path(), statusCode);
                }
            }
            else if(statusCode == -1) {
                statusCode = 500;
                LOGGER.warn("status code was -1 at path {}", request.path(), statusCode);
            }
            else {
                LOGGER.info("Handling error code without exception at path {} code {}", request.path(), statusCode);
            }
            try {
                routingContext.end("error message here")
                        .onFailure((e)->{
                            LOGGER.error("cannot send error for path {}", request.path());
                            e.printStackTrace();
                        })
                        .onSuccess((unused)->{
                            LOGGER.info("sent error for path {}", request.path());
                        });
            }
            catch(Exception justSoYouKnowItDoesNotThrow) {
                LOGGER.error("cannot call end()", justSoYouKnowItDoesNotThrow);
            }
        });
return vertx
    .createHttpServer(new HttpServerOptions().setHandle100ContinueAutomatically(true).setTcpKeepAlive(true))
    .requestHandler(router)
    .listen(port, host)
    .onSuccess(res -> LOGGER.info("Listening on {}:{}", host, port))
    .onFailure(
        cause
        -> LOGGER.warn("Cannot listen on {}:{} error: {}", host, port, cause.toString()));

I can see this in my log if I make a request:

INFO  HttpServer Handling error code without exception at path blablabla code 404
ERROR RoutingContext Unhandled exception in router
INFO  HttpServer sent error for path blablabla

To make the request, I just used raw python socket and sent this:

    tcp_conn = socket.create_connection(("127.0.0.1", 8080))
    request_str = (f"GET blablabla HTTP/1.1\r\n"
                + f"Host: 127.0.0.1\r\n"
                + f"Content-Length: 0\r\n"
                + "Connection: close\r\n"
                + "\r\n")
    request_data = request_str.encode()
    tcp_conn.send(request_data)