Allow acquiring n permits from PollSemaphore

[duarten]

Adds PollSemaphore::poll_acquire_many, to allow acquiring n permits
from it.

Motivation

Feature parity with Semaphore.

Solution

Introduce the more general poll_acquire_many function, to which the poll_acquire one now delegates.

[Darksonn]

This implementation is wrong. You're only handling the case where the permits are immediately available.

Regardless, I'm not sure this should be part of the poll method. Perhaps we should instead provide a method that sets the number of permits to acquire per acquire?

[duarten]

This implementation is wrong. You're only handling the case where the permits are immediately available.

Oops, fixed.

Regardless, I'm not sure this should be part of the poll method. Perhaps we should instead provide a method that sets the number of permits to acquire per acquire?

That seems much less ergonomic, especially since the number of permits to acquire may change over time (when using a semaphore to limit requests with different weights or when bounding memory usage).

[Darksonn]

You have to reset the future every time you wish to change the number of permits you wish to acquire. Otherwise the method will return a different number of permits than what you're asking for, if the previous permits had a different count.

You would need to add a test for this kind of thing.

[Darksonn]

Thanks.