Go信号量设计

type Mutex struct {
    state int32 
    sema  uint32
}

// defined in package runtime
// Semacquire waits until *s > 0 and then atomically decrements it.
// It is intended as a simple sleep primitive for use by the synchronization
// library and should not be used directly.
func runtime_Semacquire(s *uint32)
// Semrelease atomically increments *s and notifies a waiting goroutine
// if one is blocked in Semacquire.
// It is intended as a simple wakeup primitive for use by the synchronization
// library and should not be used directly.
// If handoff is true, pass count directly to the first waiter.
// skipframes is the number of frames to omit during tracing, counting from
// runtime_Semrelease's caller.
func runtime_Semrelease(s *uint32, handoff bool, skipframes int)

// NewWeighted creates a new weighted semaphore with the given
// maximum combined weight for concurrent access.
func NewWeighted(n int64) *Weighted {
	w := &Weighted{size: n}
	return w
}
// Weighted provides a way to bound concurrent access to a resource.
// The callers can request access with a given weight.
type Weighted struct {
	size    int64       // 最大资源数
	cur     int64       // 当前已被使用的资源
	mu      sync.Mutex
	waiters list.List   // 等待队列
}

// Acquire acquires the semaphore with a weight of n, blocking until resources
// are available or 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) Acquire(ctx context.Context, n int64) error {
	s.mu.Lock()
	if s.size-s.cur >= n && s.waiters.Len() == 0 {
		s.cur += n
		s.mu.Unlock()
		return nil
	}
	if n > s.size {
		// Don't make other Acquire calls block on one that's doomed to fail.
		s.mu.Unlock()
		<-ctx.Done()
		return ctx.Err()
	}
	ready := make(chan struct{})
	w := waiter{n: n, ready: ready}
	elem := s.waiters.PushBack(w)
	s.mu.Unlock()
	select {
	case <-ctx.Done():
		err := ctx.Err()
		s.mu.Lock()
		select {
		case <-ready:
			// Acquired the semaphore after we were canceled.  Rather than trying to
			// fix up the queue, just pretend we didn't notice the cancelation.
			err = nil
		default:
			isFront := s.waiters.Front() == elem
			s.waiters.Remove(elem)
			// If we're at the front and there're extra tokens left, notify other waiters.
			if isFront && s.size > s.cur {
				s.notifyWaiters()
			}
		}
		s.mu.Unlock()
		return err
	case <-ready:
		return nil
	}
}

s.mu.Lock()
if s.size-s.cur >= n && s.waiters.Len() == 0 {
        s.cur += n
        s.mu.Unlock()
        return nil
}

if n > s.size {
        // Don't make other Acquire calls block on one that's doomed to fail.
        s.mu.Unlock()
        <-ctx.Done()
        return ctx.Err()
}

ready := make(chan struct{})
w := waiter{n: n, ready: ready}
elem := s.waiters.PushBack(w)
s.mu.Unlock()

select {
    case <-ctx.Done():
            err := ctx.Err()
            s.mu.Lock()
            select {
            case <-ready:
                    // Acquired the semaphore after we were canceled.  Rather than trying to
                    // fix up the queue, just pretend we didn't notice the cancelation.
                    err = nil
            default:
                    isFront := s.waiters.Front() == elem
                    s.waiters.Remove(elem)
                    // If we're at the front and there're extra tokens left, notify other waiters.
                    if isFront && s.size > s.cur {
                            s.notifyWaiters()
                    }
            }
            s.mu.Unlock()
            return err
    case <-ready:
            return nil
    }

func (s *Weighted) notifyWaiters() {
	for {
		next := s.waiters.Front()
		if next == nil {
			break // No more waiters blocked.
		}
		w := next.Value.(waiter)
		if s.size-s.cur < w.n {
			// Not enough tokens for the next waiter.  We could keep going (to try to
			// find a waiter with a smaller request), but under load that could cause
			// starvation for large requests; instead, we leave all remaining waiters
			// blocked.
			//
			// Consider a semaphore used as a read-write lock, with N tokens, N
			// readers, and one writer.  Each reader can Acquire(1) to obtain a read
			// lock.  The writer can Acquire(N) to obtain a write lock, excluding all
			// of the readers.  If we allow the readers to jump ahead in the queue,
			// the writer will starve — there is always one token available for every
			// reader.
			break
		}
		s.cur += w.n
		s.waiters.Remove(next)
		close(w.ready)
	}
}

// Release releases the semaphore with a weight of n.
func (s *Weighted) Release(n int64) {
	s.mu.Lock()
	s.cur -= n
	if s.cur < 0 {
		s.mu.Unlock()
		panic("semaphore: released more than held")
	}
	s.notifyWaiters()
	s.mu.Unlock()
}

// TryAcquire acquires the semaphore with a weight of n without blocking.
// On success, returns true. On failure, returns false and leaves the semaphore unchanged.
func (s *Weighted) TryAcquire(n int64) bool {
	s.mu.Lock()
	success := s.size-s.cur >= n && s.waiters.Len() == 0
	if success {
		s.cur += n
	}
	s.mu.Unlock()
	return success
}