semaphore: make semaphore resizable

[sherifabdlnaby]

Semaphores are often used to bound concurrency (e.g worker-pool) and it is sometimes preferred to be able to resize the semaphore/workerpool to bound concurrency dynamically in respond to load.

This implementation is more performant than channel-based semaphore (channel based semaphore can't be resized as channel buffer is immutable),There is another implementations of a non-channel-based semaphores that supports resizing, but I found many bugs/deadlocks using them and they're all less performant than x/sync/semaphore.

The current implementation of x/sync/semaphore can be easily extended to be resizable, without affecting performance by any means.

Github Issue: https://github.com/golang/go/issues/29721

Edit:

I think I made a small mistake, I already posted the CR on Gerrit, I submitted this PR just for visibility and assumed from the repo's other PR(s) that the bot won't import it to Gerrit. The code is identical, so I think abandoning any of the duplicate changes is fine. Original Gerrit Review: https://go-review.googlesource.com/c/sync/+/157718

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[gopherbot]

This PR (HEAD: d5e48378160bf49067979cb4d8f840f39df6f033) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/sync/+/157680 to see it.

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[sherifabdlnaby]

Pull Request Updated and Synced with upstream 👍🏻

Fixes

• golang/go#29721

Changes

Upstream changes introduced a change to the API Behavior in case of an acquire() that is bigger than the size of the semaphore.

• Before: It was added to the queue (even though it was doomed to never get acquired)
• Now: It returns an error.

In the case of introducing Resize(), The previous API behavior to block instead of returning an error makes sense, because an acquire() is no longer 100% doomed to never get acquired. As in the case of a resize that increases the semaphore's size enough the acquire() call can get unblocked.

I reverted the API to this previous behavior to block and wait if the size of acquire() is > size of semaphore.

[gopherbot]

This PR (HEAD: 7c9f24f79d4fc66180ba1fd165c299cba13f547f) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/sync/+/157680 to see it.

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[thespags]

What's the status of getting this PR merged into master?

[ianlancetaylor]

The proposal (#29721) was declined.