959: Generic, executor-agnostic queue implementation r=ivmarkov a=ivmarkov
Hopefully relatively well documented.
Implementation relies on a fixed-size `SortedLinkedList` from `heapless`. (By default, for up to 128 timer schedules, but we can lower this number to - say - 64.)
As discussed earlier, on queue overflow, the `WakerRegistration` approach is utilized, whereas the waker that is ordered first in the queue is awoken to make room for the incoming one (which might be the waker that would be awoken after all!). Wakers are compared with `Waker::will_wake`, so the queue should actually not fill up that easily, if at all.
I've left provisions for the user to manually instantiate the queue using a dedicated macro - `generic_queue!` so that users willing to adjust the queue size, or users (like me) who have to use the queue in a complex "on-top-of-RTOS-but-the-timer-driver-calling-back-from-ISR" scenario can customize the mutex that protects the queue.
The one thing I'm not completely happy with is the need to call `{ embassy_time::queue::initialize() }` early on before any futures using embassy-time are polled, which is currently on the shoulders of the user. I'm open to any ideas where we can get rid of this and do it on the first call to `_embassy_time_schedule_wake`, without introducing very complex combinations of critical sections, atomics and whatnot.
Co-authored-by: ivmarkov <ivan.markov@gmail.com>
Co-authored-by: Dario Nieuwenhuis <dirbaio@dirbaio.net>
