Poll-style APIs for sync primitives

[NeoLegends]

Is your feature request related to a problem? Please describe.

As far as I'm aware the sync primitives (Mutex, Semaphore) do not expose a poll-style API, making them harder to use in hand-implemented futures or, more importantly, Service::poll_ready.

Describe the solution you'd like

Expose a .poll() interface on those primitives that can be used w/o acquiring a future that must be polled first.

Describe alternatives you've considered

Currently we get the future, box it up to get nameable types (since they use async fn) and then poll those, but that's hacky and less performant.

Additional context

Implementing a tower::Service that needs access to an async Semaphore do perform it's work. Came across this issue while implementing poll_ready.

[NeoLegends]

A workaround to achieve poll-based functionality now (at the cost of synchronization overhead) is to spawn a task whose only responsibility is to acquire semaphore / mutex permits and send them into a channel. That can then be polled via the regular stream mechanisms.

[Darksonn]

We are working on this for mpsc in #3105. The poll_* functions can in some cases not be added directly on the primitive as the operation requires a !Unpin piece that is stored in the future. It is however possible build a wrapper that includes this piece and has &mut self poll methods rather than the usual &self methods.

[NeoLegends]

For future readers: there is a wrapper for Semaphores with optimized allocation here: https://docs.rs/tokio-util/0.6.4/tokio_util/sync/struct.PollSemaphore.html

[Darksonn]

Yes. Beyond that, there is also PollSender and the things in tokio_stream::wrappers. Can you update the original issue to mention the things that are missing? E.g. mutex is still missing.

[matklad]

PollSemaphore is missing API to acquire n permits at once. Context: I want to throttle AsyncRead to read at most n bytes per second. The way I want to do that is by creating a semaphore with n permits, which is refilled every second, and I need to use that semaphore from my struct that implements AsyncRead.

[Darksonn]

That should be relatively easy to add. If you want to write a PR for it, then go ahead.

[Darksonn]

Closed by #5137.