[undertow-dev] dispatch and isDispatched do not agree

[Stuart Douglas]

On Wed, Nov 23, 2016 at 7:56 AM, Oliver Dain <oliver at analyticspot.com> wrote:
> Wow! Thanks for the super-fast reply.
>
> I think I'm a little confused about the difference between being in an IO
> thread and being dispatched. Is it the case that if the server thinks I'm
> "not dispatched" (that is, if the server will send a response on my behalf
> when my method returns) that I'm always on an IO thread? I often do some
> work on my own thread pools and those threads often interact with the
> HttpServerExchange object. I know that some interactions with the exchange
> object (e.g. recieveFullBytes) will take an exchange that is in a dispatched
> state and put it in a non-dispatched state. So I want to just keep checking
> that I'm dispatched and, if I'm not, dispatch again. So my code might look
> something like:
>
> void handle(HttpServerExchange exchange) {
>     exchange.dispatch(myOwnThreadPoolExecutor, () -> {
>         log.info("I am not not on an IO thread and I am dispatched.");
>         doSomethingWithExchange(exchange);
>         assert exchange.isDispatched();
>         assert !exchange.isInIoThread();
>         // how do I check to see if I'm still in a dispatched state?
>     });
> }

So the basic idea behind the 'dispatch' is that it is a way of
guaranteeing thread safety without needing to synchronize the exchange
object. Effectively it allows you to pass an exchange from one thread
to another in a way that guarantees that the first thread is 'done'
with the exchange before the second thread starts.

>
> (note: the above is a bad, super-simplified example and in the actual code
> there's a lot of asynchronous stuff happening that bounces the execution
> from one thread pool to another).
>
> The reason for all of this is that I have some edge cases which appear to be
> race conditions in which I know for sure that I call exchange.dispatch() and
> then I start doing some work but the exchange gets ended before I've
> finished my computations and have a response to send leaving the client with
> an empty response. I'm not entirely sure how that could happen, but my best
> guess right now is that I interact with the exchange in some way that causes
> it to no longer be in in the dispatched state and, as a result, the
> HttpServerExchange is ending the exchange for me before I'm ready.

I think I would need to look at a more specific example of what you
are doing, but it is possibly related to how you are handing stuff in
other threads? The exchange itself is not thread safe, if you start
doing work on the exchange in another thread without having dispatched
you can get a race condition.

Stuart

>
> Thanks again,
> Oliver
>
>
>
> On Tue, Nov 22, 2016 at 12:47 PM Stuart Douglas <sdouglas at redhat.com> wrote:
>>
>> After the call stack returns and the dispatch actually happens
>> isDispatched() will return false (allowing the exchange to be
>> dispatched again).
>>
>> In general you should not really need to check this anyway, if you
>> want to know if you are in the IO thread you should use isInIiThread()
>> instead.
>>
>> Stuart
>>
>> On Wed, Nov 23, 2016 at 7:44 AM, Oliver Dain <oliver at analyticspot.com>
>> wrote:
>> > I've got some pretty simple code that looks something like:
>> >
>> > if (!exhange.isDispatched()) {
>> >     exchange.dispatch(() -> {
>> >         assert exchange.isDispatched();
>> >          // more stuff....
>> >     });
>> > }
>> >
>> > but the assert fails. That is, when dispatch calls my runnable
>> > isDispatched
>> > is not true.
>> >
>> > I believe I have a bug somewhere with how/when dispatches are happening
>> > so
>> > I've littered my code with such asserts and they fail reliably even when
>> > I'm
>> > quite sure the exchange has been dispatched (e.g. when it is literally
>> > the
>> > first call in a Runnable passed to to the dispatch() method).
>> >
>> > What am I doing wrong?
>> > --
>> > CTO, Analytic Spot
>> > 44 West Broadway #222
>> > Eugene, OR 97401
>> > analyticspot.com • 425-296-6556
>> > www.linkedin.com/in/oliverdain
>> >
>> > _______________________________________________
>> > undertow-dev mailing list
>> > undertow-dev at lists.jboss.org
>> > https://lists.jboss.org/mailman/listinfo/undertow-dev
>
> --
> CTO, Analytic Spot
> 44 West Broadway #222
> Eugene, OR 97401
> analyticspot.com • 425-296-6556
> www.linkedin.com/in/oliverdain