| // Licensed to the Apache Software Foundation (ASF) under one |
| // or more contributor license agreements. See the NOTICE file |
| // distributed with this work for additional information |
| // regarding copyright ownership. The ASF licenses this file |
| // to you 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 |
| // |
| // http://www.apache.org/licenses/LICENSE-2.0 |
| // |
| // Unless required by applicable law or agreed to in writing, |
| // software distributed under the License is distributed on an |
| // "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY |
| // KIND, either express or implied. See the License for the |
| // specific language governing permissions and limitations |
| // under the License. |
| |
| package internal |
| |
| import ( |
| "context" |
| "sync/atomic" |
| |
| log "github.com/sirupsen/logrus" |
| ) |
| |
| type Semaphore interface { |
| // Acquire a permit, if one is available and returns immediately, |
| // reducing the number of available permits by one. |
| Acquire(ctx context.Context) bool |
| |
| // Try to acquire a permit. The method will return immediately |
| // with a `true` if it was possible to acquire a permit and |
| // `false` otherwise. |
| TryAcquire() bool |
| |
| // Release a permit, returning it to the semaphore. |
| // Release a permit, increasing the number of available permits by |
| // one. If any threads are trying to acquire a permit, then one is |
| // selected and given the permit that was just released. That thread |
| // is (re)enabled for thread scheduling purposes. |
| // There is no requirement that a thread that releases a permit must |
| // have acquired that permit by calling Acquire(). |
| // Correct usage of a semaphore is established by programming convention |
| // in the application. |
| Release() |
| } |
| |
| type semaphore struct { |
| maxPermits int32 |
| permits int32 |
| ch chan bool |
| } |
| |
| func NewSemaphore(maxPermits int32) Semaphore { |
| if maxPermits <= 0 { |
| log.Fatal("Max permits for semaphore needs to be > 0") |
| } |
| |
| return &semaphore{ |
| maxPermits: maxPermits, |
| permits: 0, |
| ch: make(chan bool), |
| } |
| } |
| |
| func (s *semaphore) Acquire(ctx context.Context) bool { |
| permits := atomic.AddInt32(&s.permits, 1) |
| if permits <= s.maxPermits { |
| return true |
| } |
| |
| // Block on the channel until a new permit is available |
| // or the context expires |
| select { |
| case <-s.ch: |
| return true |
| case <-ctx.Done(): |
| atomic.AddInt32(&s.permits, -1) |
| return false |
| } |
| } |
| |
| func (s *semaphore) TryAcquire() bool { |
| for { |
| currentPermits := atomic.LoadInt32(&s.permits) |
| if currentPermits >= s.maxPermits { |
| // All the permits are already exhausted |
| return false |
| } |
| |
| if atomic.CompareAndSwapInt32(&s.permits, currentPermits, currentPermits+1) { |
| // Successfully incremented counter |
| return true |
| } |
| } |
| } |
| |
| func (s *semaphore) Release() { |
| permits := atomic.AddInt32(&s.permits, -1) |
| if permits >= s.maxPermits { |
| // Unblock the next in line to acquire the semaphore |
| s.ch <- true |
| } |
| } |