Race condition in T5-pubsub?

[Jochen Berger <fo...@googlemail.com>]

Hi list,

I think I've discovered a race condition in the pubsub code which is 
kind of funny as I didn't even know these could occur in JavaScript.
The problem seems to be related to the purging of the publisher cache. 
The situation I experience the error in is fairly complex (with multiple 
listeners subscribing, unsubscribing and signaling, elements being 
replaced and the like. I haven't yet been able to reproduce the issue 
with a simpler use case and I'm not entirely sure what exactly happens 
here. Maybe someone with a better understanding of JavaScript could help 
here?
What seems to happen is that one thread (for want of a better word) 
calls the createPublisher function. Shortly afterward, another thread 
calls the subscribe function on the same topic, leading to the cache for 
that topic being cleared.
Now when the first "thread" arrives in line 167, where it says

for (var i = 0; i < publisher.listeners.length; i++) {

publisher.listeners is undefined as the reference was deleted in between.
I managed to fix the issues I encounter by creating a clone of the 
publisher.listeners array and using that for the loop.
I would have created a JIRA but I'm quite at a loss as to explaining 
what exactly is going on here.

Regards,
Jochen



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Re: Race condition in T5-pubsub?

[Jochen Berger <fo...@googlemail.com>]

Hi,

Am 16.12.2011 17:22, schrieb Howard Lewis Ship:
> Or t5-pub-sub should do a copy-on-write, rather than copying it for
> each iteration.

Hm, I haven't spent much thought on this idea yet, but I don't see how 
that could be done in JavaScript.
Also, this still leaves the problem of having to decide whether the 
following listeners should be notified for the current run.

Jochen

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Re: Race condition in T5-pubsub?

[Howard Lewis Ship <hl...@gmail.com>]

Or t5-pub-sub should do a copy-on-write, rather than copying it for
each iteration. I miss Clojure collections!

On Thu, Dec 15, 2011 at 3:37 AM, Jochen Berger <fo...@googlemail.com> wrote:
> Hi again,
>
> I think I was wrong with the term "race condition". After some investigation
> I found out what causes the problem. I created a simple demo application
> that I can upload I you want me to.
> Basically, the problem occurs, when there are multiple subscribers for the
> same topic and one of them changes the subscriptions (e.g. by unsubscribing)
> for the topic while the publisher function runs. That will lead to the
> listener cache being cleared and publisher.listeners being undefined upon
> the next iteration of the for loop.
> I think this could be fixed by iterating over a clone of publisher.listeners
> instead of publisher.listeners itself. Of course that would mean that if one
> listener unsubscribes other listeners while running, those would be notified
> for the current run anyway.
> Any thoughts on that?
>
>
> Jochen
>
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Re: Race condition in T5-pubsub?

[Jochen Berger <fo...@googlemail.com>]

Hi again,

I think I was wrong with the term "race condition". After some 
investigation I found out what causes the problem. I created a simple 
demo application that I can upload I you want me to.
Basically, the problem occurs, when there are multiple subscribers for 
the same topic and one of them changes the subscriptions (e.g. by 
unsubscribing) for the topic while the publisher function runs. That 
will lead to the listener cache being cleared and publisher.listeners 
being undefined upon the next iteration of the for loop.
I think this could be fixed by iterating over a clone of 
publisher.listeners instead of publisher.listeners itself. Of course 
that would mean that if one listener unsubscribes other listeners while 
running, those would be notified for the current run anyway.
Any thoughts on that?

Jochen

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