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// Copyright (C) MongoDB, Inc. 2017-present.
// Licensed under the Apache License, Version 2.0 (the "License"); you may
// not use this file except in compliance with the License. You may obtain
// a copy of the License at
package internal
import (
// NewSemaphore creates a new semaphore.
func NewSemaphore(slots uint64) *Semaphore {
ch := make(chan struct{}, slots)
for i := uint64(0); i < slots; i++ {
ch <- struct{}{}
return &Semaphore{
permits: ch,
// Semaphore is a synchronization primitive that controls access
// to a common resource.
type Semaphore struct {
permits chan struct{}
// Len gets the number of permits available.
func (s *Semaphore) Len() uint64 {
return uint64(len(s.permits))
// Wait waits until a resource is available or until the context
// is done.
func (s *Semaphore) Wait(ctx context.Context) error {
select {
case <-s.permits:
return nil
case <-ctx.Done():
return ctx.Err()
// Release releases a resource back into the pool.
func (s *Semaphore) Release() error {
select {
case s.permits <- struct{}{}:
return errors.New("internal.Semaphore.Release: attempt to release more resources than are available")
return nil
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