AsyncSemaphore

AsyncSemaphore.swift

class AsyncSemaphore {
    
    struct waiter {
        var continuation: CheckedContinuation<Void,Never>
        var next: UnsafeMutablePointer<waiter>?
    }

    var value : Int
    var waitList : UnsafeMutablePointer<waiter>?
    var lock = os_unfair_lock()
    
    init(value: Int) {
        self.value = value
    }
    
    func wait() async {
        os_unfair_lock_lock(&self.lock)
        if self.value > 0 {
            self.value -= 1
            os_unfair_lock_unlock(&self.lock)
        } else {
            let w = UnsafeMutablePointer<waiter>.allocate(capacity: 1)
            let _ : Void = await withCheckedContinuation({ k in
                w.initialize(to: waiter(continuation: k, next: self.waitList))
                self.waitList = w                
                os_unfair_lock_unlock(&self.lock)
            })
            w.deallocate()
        }
    }
    
    func signal() {
        os_unfair_lock_lock(&self.lock)
        if let w = self.waitList {
            self.waitList = w.pointee.next
            w.pointee.continuation.resume()
        } else {
            self.value += 1
        }
        os_unfair_lock_unlock(&self.lock)
    }
}