Races involving channels can result in dropped messages

I’m using channels extensively for private communication between sagas. I have a watch saga that races three channels: one for receiving requests, one for receiving responses, and one for handling details of the request itself.

It seems the race effect guarantees a single winner, even if more than one channel has a message. The messages from the losers are dropped.

Here’s a sagaMonitor screenshot which I hope demonstrates the problem:

In the code, I have:

const { request, handler, response } = yield race({
    request: take(channelRequest),
    handler: take(channelHandler),
    response: take(channelResponse),
});

In the case shown in the screenshot, only handler was set after that code. The other two variables were undefined. Next time response won the race, its message was NOT the one shown in the screenshot: it was a subsequent one.

[Andarist]

If I understand you correctly your code is fine, however for now you might lose some buffered items WHEN in the exact moment of yield race you have waiting items in both queues. In the same moment they wont be both returned to the saga, only the first one will be, and the second one will get dropped in the bakcground silently without you even knowing that.

Fix is already on the master, I’ll try to have it published asap as 0.12.1.

[Andarist]

It took some time, but the fix finally has landed in the newest version! v0.12.1 has been published.