Fix for #712 - don't skip next sub if previous sub unsubscribes itself

[lukewestby]

The Problem

It's possible that someone can unsubscribe from a port in JavaScript in the same tick that the port's subscription callback is called. Since the array that stores these subscription callbacks is shared and mutated in place by subscribe and unsubscribe that unsubscribing in the same tick as a callback can cause the following subscription to be skipped. This is all demonstrated and explained in #712.

Prior art

See https://github.com/reactjs/redux/blob/master/src/createStore.js#L63-L67

The fix

This fix solves that issue by making the subs array immutable with respect to unsubscribing and iterating. When a subscription callback drops itself the subs array will be cloned before the callback is spliced out. Then, during iteration when a message comes in a separate reference is made to the subs array so that changes to that array are only picked up once iteration moves on to the next message.

Expected behavior after merging

The result is that when a subscription callback drops itself, we wait for the next message before pulling in changes made to the subs array while processing the previous message. If there are multiple messages waiting for the port, the subscription that was removed will only process the ones that came in before it unsubscribed itself.

Alternatives

I think most alternatives to this would just be variations on the underlying idea of cloning and re-referencing at a particular point. I think this one has the lowest memory and performance cost as opposed to cloning before processing messages, for example.

[process-bot]

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[process-bot]

Thanks for the pull request! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[evancz]

This approach makes sense to me. It seems like the simplest nicest way to do this. Nice work!

Also, this PR is great :D

[evancz]

This approach makes sense to me. It seems like the simplest nicest way to do this. Nice work!

Also, this PR is great :D

[lukewestby]

🤘 thanks evan!

[lukewestby]

🤘 thanks evan!