1 files changed, 160 insertions, 0 deletions
diff --git a/vendor/golang.org/x/sync/semaphore/semaphore.go b/vendor/golang.org/x/sync/semaphore/semaphore.go
new file mode 100644
index 0000000..b618162
--- /dev/null
+++ b/vendor/golang.org/x/sync/semaphore/semaphore.go
@@ -0,0 +1,160 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package semaphore provides a weighted semaphore implementation.
+package semaphore // import "golang.org/x/sync/semaphore"
+
+import (
+	"container/list"
+	"context"
+	"sync"
+)
+
+type waiter struct {
+	n     int64
+	ready chan<- struct{} // Closed when semaphore acquired.
+}
+
+// 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
+	cur     int64
+	mu      sync.Mutex
+	waiters list.List
+}
+
+// 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.
+func (s *Weighted) Acquire(ctx context.Context, n int64) error {
+	done := ctx.Done()
+
+	s.mu.Lock()
+	select {
+	case <-done:
+		// ctx becoming done has "happened before" acquiring the semaphore,
+		// whether it became done before the call began or while we were
+		// waiting for the mutex. We prefer to fail even if we could acquire
+		// the mutex without blocking.
+		s.mu.Unlock()
+		return ctx.Err()
+	default:
+	}
+	if s.size-s.cur >= n && s.waiters.Len() == 0 {
+		// Since we hold s.mu and haven't synchronized since checking done, if
+		// ctx becomes done before we return here, it becoming done must have
+		// "happened concurrently" with this call - it cannot "happen before"
+		// we return in this branch. So, we're ok to always acquire here.
+		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()
+		<-done
+		return ctx.Err()
+	}
+
+	ready := make(chan struct{})
+	w := waiter{n: n, ready: ready}
+	elem := s.waiters.PushBack(w)
+	s.mu.Unlock()
+
+	select {
+	case <-done:
+		s.mu.Lock()
+		select {
+		case <-ready:
+			// Acquired the semaphore after we were canceled.
+			// Pretend we didn't and put the tokens back.
+			s.cur -= n
+			s.notifyWaiters()
+		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 ctx.Err()
+
+	case <-ready:
+		// Acquired the semaphore. Check that ctx isn't already done.
+		// We check the done channel instead of calling ctx.Err because we
+		// already have the channel, and ctx.Err is O(n) with the nesting
+		// depth of ctx.
+		select {
+		case <-done:
+			s.Release(n)
+			return ctx.Err()
+		default:
+		}
+		return nil
+	}
+}
+
+// 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
+}
+
+// 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()
+}
+
+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)
+	}
+}