Add no subscribers fallback option to GoChannel Pub/Sub

[annismckenzie]

I needed a way to listen to any message sent via the GoChannel Pub/Sub that didn't have a subscriber yet. This was mainly used for debugging but it could be useful for other purposes as well.

I made sure to add a test and left the rest of the tests untouched – the test suite is passing.

Would you be open to accepting this contribution? If so, I'd take another pass over the current implementation's comments, some would need updating (for example, That means if you send a message to a topic to which no subscriber is subscribed, that message will be discarded. – update: this is done ☑️).

[roblaszczak]

what do you think about adding configuration for the topic name instead? so we could have two options in Config

EnableFallback
NoSubscribersFallbackTopic

what do you think?

[annismckenzie]

I added both EnableNoSubscribersFallback (as per your review comment below) and NoSubscribersFallbackTopic to the configuration. I did want to leave the default topic (*) as a const in case the user doesn't set a custom fallback topic. I also added another test case for this.

[roblaszczak]

maybe it would be a bit longer, IMO it could be a bit more explicit, like EnableNoSubscribersFallback

[annismckenzie]

Yup, I agree that the longer version is more expressive and changed it.

[roblaszczak]

also, food for thought, but I wonder if this fallback couldn't be a closure 🤔

like NoSubscribersFallback func(msg message.Message, pubsub *GoChannel) error

It could be much more flexible 🤔 WDYT?

[annismckenzie]

That sounds worthwhile to investigate.

[annismckenzie]

I investigated this with a private project of mine where I'm using the fork and the closure is awkward to use because the function you want to call has to be known and available when instantiating the GoChannel. However, I'm passing this GoChannel object around for subscribing and publishing and then just use

noSubscribersMessages, err := pubSub.Subscribe(ctx, gochannel.NoSubscribersFallbackDefaultTopic)
if err != nil {
	return fmt.Errorf("subscribing to fallback topic: %w", err)
}
go func(ctx context.Context, messages <-chan *message.Message) {
	for {
		select {
		case msg := <-messages:
			// handle message

			// lastly:
			msg.Ack()
		case <-ctx.Done():
			return
		}
	}
}(ctx, noSubscribersMessages)

which works well and is easy to use. That is to say, I'd leave this as is.

[annismckenzie]

Just saw your review – thank you! I’ll take a closer look next week.

[annismckenzie]

Short update: I rebased and addressed your comments, see above. I also updated a couple comments as promised in the pull request description in 8808fd7 and this would be ready for another look.