common/sync/parallel/semaphore.go

// Copyright 2015 The LUCI Authors.
//
// 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
//
//      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 parallel

// SemaphoreToken is a semaphore token.
type SemaphoreToken struct{}

// Semaphore is a sync.Locker that implements a n-semaphore.
//
// Lock the semaphore acquires a semaphore token, possibly blocking until one
// is available.
//
// Unlock releases an owned token, returning it to the semaphore.
//
// For semaphore s, len(s) is the current number of acquired resources, and
// cap(s) is the total resource size of the semaphore.
type Semaphore chan SemaphoreToken

// Lock acquires a semaphore resource, blocking until one is available.
func (s Semaphore) Lock() {
	if cap(s) > 0 {
		s <- SemaphoreToken{}
	}
}

// Unlock releases a single semaphore resource.
func (s Semaphore) Unlock() {
	if cap(s) > 0 {
		<-s
	}
}

// TakeAll blocks until it holds all available semaphore resources. When it
// returns, the caller owns all of the resources in the semaphore.
func (s Semaphore) TakeAll() {
	for i := 0; i < cap(s); i++ {
		s.Lock()
	}
}