add weighted semaphore to golang.org/x/sync?

[Jonathan Amsterdam]

Google has a nice implementation of a weighted semaphore that I'd like to put in golang.org/x/sync. Godoc is below.

Are folks OK with this? Does anyone want to review the CL?

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package semaphore

    Package semaphore provides semaphore implementations.

TYPES

type Weighted struct {

    // contains filtered or unexported fields

}

    Weighted provides a way to bound concurrent access to a resource. The

    callers can request access with a given weight.

func NewWeighted(n int64) *Weighted

    NewWeighted creates a new weighted semaphore with the given maximum

    combined weight for concurrent access.

func (s *Weighted) Acquire(ctx context.Context, n int64) error

    Acquire acquires the semaphore with a weight of n, blocking only until

    ctx is done. On success, returns nil. On failure, returns ctx.Err() and

    leaves the semaphore unchanged.

    If ctx is already done, Acquire may still succeed without blocking.

func (s *Weighted) Release(n int64)

    Release releases the semaphore with a weight of n.

func (s *Weighted) TryAcquire(n int64) bool

    TryAcquire acquires the semaphore with a weight of n without blocking.

    On success, returns true. On failure, returns false and leaves the

    semaphore unchanged.

[Tylor Arndt]

Are weighted semaphores == counting semaphores?

It would be really nice to have a AccquireTimeout like this package has: https://github.com/tarndt/sema