Pass AbortSignal to callbacks passed to defer(), switchMap() etc

[felixfbecker]

Description:
This is a small PoC for my proposal outlined in #3122 (comment)
Initially for defer() and switchMap().
AbortControllers only get instantiated if available in the environment so nothing breaks if they are not polyfilled.

Note that I am creating AbortControllers in the operator implementations - ideally those would be passed to the callback of the Observable constructor, but that was more effort to implement. My goal was just to create a PoC and keep the discussion going as this has been an ongoing pain point.

Related issue (if exists):
#3122 (comment)

[coveralls]

Coverage decreased (-0.03%) to 96.918% when pulling bd15359 on felixfbecker:abortsignal into 9ea0ec9 on ReactiveX:master.

[benlesh]

Looking at this, I'm not sure this is the proper approach right now. Considering right now AbortSignal isn't widely used or supported, I'd prefer just using Subscription in lieu of AbortSignal, and then figuring out a way to migrate Subscription to be an AbortController or an AbortController-shaped thing, given that Subscription is basically both an AbortController and an AbortSignal.

In the experimental branch, I'm pasing Subscription as a cancellation token of sorts to methods like forEach and toPromise. I hadn't thought about defer, etc.

Also, I'm keeping a close eye on this: tc39/proposal-cancellation#22

Which would resolve all of our problems, by allowing us to check a symbol and use what we get from that.

I really appreciate the work here, @felixfbecker, and we're definitely on the same page. I'm not ready to put this into the current stable release yet though, we'll definitely be trying some of this out in the next alpha.

[felixfbecker]

Considering right now AbortSignal isn't widely used or supported

Are you referring to browser support? AbortSignal is supported by all modern browsers:

And it's already used by fetch and the WebAuthentication API (w3c/webauthn#544)

It's not supported in NodeJS of course, but I would take the same stance as with Promises - you can polyfill if you want to use it.

For the future, Node has shipped DOM APIs before - e.g. WHATWG URL

given that Subscription is basically both an AbortController and an AbortSignal.

I think this ascpect makes Subscription basically unusable for a general-purpose cancellation signal. If I pass a signal to a function, that function should have the ability to fork the token and cancel sub-operations, but it should never be able to cancel the operations of the caller.

In the experimental branch, I'm pasing Subscription as a cancellation token of sorts to methods like forEach and toPromise. I hadn't thought about defer, etc.

For me, defer, switchMap etc. are actually the more important use cases, because I see a lot that people want to do single-result Promise operations for every event in an Observable chain (e.g., call the fetch API or an API wrapper that returns a single result in reaction to events). Only rarely do you need to use a (multi-value) Observable from within an async function.