[undertow-dev] Unexpected RequestLimitingHandler behavior after queue gets full

[Piotr Betkier]

Hello,

I wanted to introduce a constraint on the number of requests my server
tries to handle at a time and played around with RequestLimitingHandler a
little bit. However, I observed unexpected behavior once RequestLimit's
queue gets full.

I've created a RequestLimitingHandler with both maximumConcurrentRequests
and queueSize set to 1 and nextHandler that responds after a 3 second
sleep. This is what I observed:

When I sent 2 concurrent requests with a curl, I got the first response
after 3 seconds and the second one after 6 - just as expected, the second
request got queued.

When I sent 3 concurrent requests, I got two correct responses and one 513
- just as expected, my third request couldn't fit into the queue so a
failure handler got called.

Now it got tricky - this made RequestLimit enter an invalid state, where
its "state" field got decremented below the value that represents 0 current
requests. Now "state & MASK_CURRENT" gives 2147483647 instead of 0, so the
next time I make a single request, RequestLimit thinks there are 2147483647
requests in the system currently and queues my request instead of handling
it. Then I make yet another request and now the queue is full, so 513 error
is generated instantly which in turn makes RequestLimit poll my first
pending request from the queue and dispatch it for handling. RequestLimit
got corrupted and is unusable from now on.

I hope I explained my observations clearly. Is there something wrong in my
setup, my understanding of Undertow or it's a bug? This is my code snippet,
tested with undertow 1.1.0.Final and 1.2.0.Beta5:

import io.undertow.Undertow;
import io.undertow.server.HttpHandler;
import io.undertow.server.HttpServerExchange;
import io.undertow.server.handlers.RequestLimitingHandler;
import io.undertow.util.Headers;

public class Main {

    public static void main(String[] args) {
        Undertow server = Undertow.builder()
                .addHttpListener(8080, "localhost")
                .setHandler(new RequestLimitingHandler(1, 1, handler()))
                .build();

        server.start();
    }

    private static HttpHandler handler() {
        return new HttpHandler() {
            @Override
            public void handleRequest(final HttpServerExchange exchange)
throws Exception {
                Thread.sleep(3000);
                exchange.getResponseHeaders().put(Headers.CONTENT_TYPE,
"text/plain");
                exchange.getResponseSender().send("Hello World");
            }
        };
    }

}


Cheers,
Piotr
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