> If the Rust async/await support is indeed so flexible that with some work you could tie it together with a pre-emptive runtime of your own custom creation
I think that's exactly what it is. AIUI it's basically an API for async/await (and maybe a default runtime?) that different back-ends can be plugged into. At least that's what I've come away with over the months of reading random bits about it here.
As described in the blog post, the rust compiler generates a state machine for every async function and generates the appropriate poll methods for the state transition. This is fundamentally at odds with the preemption, which would then have to indroduce new intermediate states into the diagram which it won't be able to do at runtime.
But if Rust is the operating system/kernel, whenever it decides to to schedule something is preemption for anything downstream, right?
I mean, you don't actually use preemption in the kernel right? Don't you have to handle all that yourself, since there's nothing higher level than you to handle it for you? In that respect, doesn't plugging in a Futures runtime that looks for device flags and stuff as appropriate and flags tasks to wake up/finish accomplish the same thing? (those are actual questions, not leading statements)
If you would write a basic scheduler, at some point you'd have to await the userspace code but you wouldn't have any way to force it to stop running. If the userspace code would enter an infinite loop it would hold the kernel thread forever. Within a constrained environment, eg. the kernel itself (and even that's sufficiently complex with loadable drivers that you might end up with bad interactions) I could see some use for async await, but you'd still need to be able to preempt untrusted code.
As far as I understand the scheduler and userspace processes are going to be completely orthogonal features to this. In general would it be complex[1] in this case to integrate both preemptive and cooperative multitasking?
I think that's exactly what it is. AIUI it's basically an API for async/await (and maybe a default runtime?) that different back-ends can be plugged into. At least that's what I've come away with over the months of reading random bits about it here.