(CLJS) alts! sporadically failing to unblock despite channel activity (Safari 7)


Hi all, bit of a tricky bug to report here... We're seeing some problems with using core.async in ClojureScript on Safari 7. Our application is built around a large event loop that blocks on a message from one of many channels that correspond to user activity or API calls. The problem seems to lie within this event loop - we are using alts! to pull a message out of any available channel, but sometimes logging shows that we reach alts! and never unblock. However, with a little more logging, I can see that there are subsequent writes to one of the channels in the list of channels passed to alts!, so I'm not really sure what's going on.

That's the high level overview, now on to some code.

Our main event loop is as follows:

process-message here is our a function internal to our application, but I don't think it's details are necessarily important. In the scenario where Safari gets stuck, the log looks like:

Note that we see a log entry for "alts! channel hashes", but we never seen "alts! unblocked". However, note the list of hashes passed to alts!. Channel 19 is mentioned, but subsequently we put! to channel 19... yet we still don't get unblocked. Something that also strikes me as suspicious, is that while we're blocked at alts!, two calls to put! have succeeded immediately, for a channel that is bounded to contain only one element at a time. Maybe I'm misunderstanding something, but I wouldn't expect the immediate-put callback to be invoked more than once. Interestingly that last put! doesn't invoke the callback.

Unfortunately, reproduction of this bug is reasonably difficult. I can somewhat reliably reproduce it by quitting Safari, re-opening it, and navigating to the dev server. About 1 in 15 attempts get stuck in this fashion. I wondered if it was something to do with Safari's MessageChannel implementation - you can see in the log entries where nexttick.js calls its callback, which seems to be how dispatch is working in my browser.

I'd be very happy to help provide any more information that's useful, but this problem is now outside my ability to debug it. While the code is proprietary, I'd be happy to temporarily add people to the Github project in order to try and get this fixed. We have development APIs servers that you can point at, so it should be just a case of running lein cljs.

I've attached our code for our Socket.io wrapper and our main event loop. Sadly I do not yet have a minimal test-case - I wouldn't really know where to begin.