Comment by pjc50
9 hours ago
I like how C# handles this. You're not forced to support cancellation, but it's strongly encouraged. The APIs all take a CancellationToken, which is driven by a CancellationTokenSource from the ultimate caller. This can then either be manually checked, or when you call a library API it will notice and throw an OperationCancelledException.
Edit: note that there is a "wrong" way to do this as well. The Java thread library provides a stop() function. But since that's exogenous, it doesn't necessarily get cleaned up properly. We had to have an effort to purge it from our codebase after discovering that stopping a thread while GRPC was in progress broke all future GRPC calls from all threads, presumably due to some shared data structure being left inconsistent. "Cooperative" (as opposed to preemptive) cancel is much cleaner.
I am surprised that you had to go out of your way to remove Thread.stop from existing Java code. It's been deprecated since 1998, and the javadoc page explains pretty clearly why it's inherently unsafe.
It's hard to miss all the warnings unless you're literally just looking at the method name and nothing else.
I was certainly surprised to see it when I found it.
Not to mention that I feel like it's pretty unusual to be creating and managing threads yourself in Java these days, instead of using a thread pool/executor.
That’s barely-junior interview question indeed.
One of Java's the ecosystem fundamental platforms is that it's multi-threading. It's gone through too many models.
And since Java has a metric ton of blog posts from the 2000s and 2010s, a lot of search engines lead you to older models.java itself has gone from green threads to OS threads and back to green threads now.
AbortSignal is same thing on the Web. It's unfortunate TC39 failed to ever bring a CancelToken to the language to standardize the pattern outside browsers.
Browsers have said that they are unwilling to ship any new cancelation mechanisms given that AbortSignal already exists, so we can't ship a different CancelToken. But I think there's a path to standardizing a subset of the existing AbortSignal machinery [1].
(I am on TC39 and while this isn't my highest priority I did bring the topic for discussion at the last meeting [2], and there was support from the rest of committee.)
[1] https://github.com/tc39/proposal-concurrency-control/issues/...
[2] https://github.com/bakkot/structured-concurrency-for-js
TC39 seems to be failing at many things for the past 10 years.
Hard disagree, TC39 has done great work over the last 10 years. To name a few: - Async/await - Rest/spread - Async iterators - WeakRefs - Explicit Resource Management - Temporal
It's decisions are much more well thought out than WHATWG standards. AbortSignal extending from EventTarget was a terrible call.
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I don't like it - you're forced to pass around this token, constantly manage the lifecycle of cancellation sources - and incredibly bug prone thing in async context, and it quickly gets very confusing when you have multiple tokens/sources.
I understand why they did it - a promise essentially is just some code, and a callback that will be triggered by someone at some point in time - you obviously get no quality of service promises on what happens if you cancel a promise, unless you as a dev take care to offer some.
It's also obvious that some operations are not necessarily designed to be cancellable - imagine a 'delete user' request - you cancelled it, now do you still have a user? Maybe, maybe you have some cruft lying around.
But still, other than the obvious wrong solution - C# had a Thread.Abort() similar to the stop() function that you mentioned, that was basically excommunicated from .NET more then a decade ago, I'm still not happy with the right one.
Very rare unless you are spawning your own.
Usually, you are passing through a runtime provided token (e.g. ASP.NET).
Not that rare in my experience, I constantly had to write software like this. Not every day, but it certainly did come up quite often in my code and others'
Oh and oone more thing - the very (developer-managed) complexity makes it that people constantly got it wrong, usually just enough (as often with the case of threading) that it worked fine 90% of the time, and was very hard to make a case to management why we should invest effort into fixing it.
Not rare in the slightest. C# is used in a lot of places that aren't the web and don't have extra frameworks piled on.
If you're writing a bare C# library for desktop deployment, you're managing your own cancellation sources.
Cancelling a token doesn't immediately abort the underlying Task. It is up to the implementation of that task to poll the token and actively decide when to abort.
In your example, you'd design your delete task such that if you want it to be cancelable, it can only be canceled before data is modified. You simply don't abort in the middle of a database transaction.
Moreover, because of the way cancellation tokens work, you can't abort blocking function calls unless you also pass the token along. There just isn't a mechanism that can interrupt a long IO operation or whatever unless you explicitly go to the effort to make that happen.
A cancellation token is more of a "pretty please stop what you're doing when you feel like it" concept than Thread.Abort().
C# has very good support for this.
You can even link cancellation tokens together and have different cancellation "roots".
> "Cooperative" (as opposed to preemptive) cancel is much cleaner.
Which what Thread.interrupt does.