You can cancel socket operations using signals. You can eg have one or more background threads running timers which will interrupt the blocking IO if it doesn’t return in a timely manner. A lot of very important frameworks and services that are used in billions of transactions per day use this model.
Of course you can. It does mean that you need cooperation between the child and parent thread (to set up the signal handler so that resources are cleaned up) though. That's easy in a framework, kind of a pain in the ass if you're just trying to get some opaque client you were passed to do something in <10 seconds.
And that's just for IO. I mentioned elsewhere that you may want to cancel pure compute work.
You can see my point, I assume, that when your userspace program can cancel tasks natively it's much easier to work with?
Can you cancel a tight computing loop (i.e. without system calls and without yielding of any sort) with async? I wonder how? Also if you can inject a cleanup code in your async task what prevents you from doing it with threads? Such things existed long before async/await and system calls didn't change for async/await. Also, what's the difference between "framework" and async/await runtime, isn't the latter a kind of a framework?
Without any yielding? Seems hard. You could park the thread idk.
> what's the difference between "framework" and async/await runtime,
Sure, in that in both cases you have the threads managed for you. But there's a difference between spawning a raw pthread, which will have no signal handlers/ cleanup hooks, and one managed by a framework where it can add all of those things and more.
Interesting, in the Java world Thread.stop is deprecated too: https://docs.oracle.com/javase/7/docs/technotes/guides/concu... Which means there is no good way to actually stop a thread involuntary. Of course in most simple apps it's not a big deal, but I would not do it in long-running apps.
OTOH in Rust async model is based on polling. Which means that poll may never block, but instead has to set a wake callback if no data is available. So there is no way to interrupt a rogue task and all async functions should rely on callbacks to wake them (welcome to Windows 3.1, only inside out!). Thread model is much more lax in this sense, e.g. even though my web server (akka-http) is based on futures, nothing prevents me from blocking inside my future, in most cases I can get away with it. As I understand it's not possible in Rust async model, I can only use non-blocking async functions inside async function. So in reality you don't interrupt or clean up anything in Rust when a timeout happens, you simply abandon execution (i.e. stop polling). I wonder what happens with resources if there were allocated.
> As I understand it's not possible in Rust async model,
You can block, you're just going to block all other futures that are executed on that same underlying thread. But all sorts of things block, for loops block.
This is the same as Java, I believe. Akka also has special actors called IO Workers that are designed for blocking work - Rust has the same thing with `spawn_blocking`, which will place the work onto a dedicated threadpool.
> So in reality you don't interrupt or clean up anything in Rust when a timeout happens
You don't interrupt, you are yielded to. It's cooperative.
> I wonder what happens with resources if there were allocated.
When a Future is dropped its state is dropped as well, so they are all freed.
The last comment is actually pretty interesting and spot on. In the Java/JDK world - which you can assume as a „framework“ - you can cancel blocking IO via the Thread.interrupt() mechanism. And that works because it’s deeply integrated into the framework, similar like async Rust runtimes provide support for cancellation.
> Show me how to cancel a network requests using only threads, with no access to the underlying socket APIs?
It’s been a long time since I did this in Rust. But why do you not have access to the sockets or at least a set_timeout method? Is it a higher level lib that omits such crucial features?
In Go, the super common net.Conn interface has deadline methods. Not everyone knows their importance but generally you have something like it piped through to the higher layers.
EDIT: Oh I see you replied to my other comment. Please disregard.
Total rust newb here, but does that need the full async story, or is it a limitation of an API somewhere? From the point of view of the code using the request's response could you use a channel with recv_timeout? Is the problem there that the thread with the socket connection is still going and there's no way to stop it?
The ability to cancel an operation without talking to the operating system requires that your program has yield points. That yielding is what allows another part of the program to take control and say "OK, I'm done with you now, no need to finish".
Yes, the problem is that your thread would continue to perform work even if you stopped waiting on it.
Maybe I don't understand the complexity, but in good old Ruby I can easily stop a thread if I don't need result anymore. No async needed and no yield points necessary. Doesn't it apply to Rust too?
I assume that Ruby does in fact have yield points in some form, such as a global lock. Killing a thread is only possible (for a pthread) via the `pthread_cancel` API. That API is very dangerous and is generally not something you'd ever want to use manually - the thread will not clean up any memory or other resources, any shared memory is left in a tricky state.
To gracefully shut a thread down you need yielding of some kind.