5a06fca2c3
This package provides a weighted semaphore that is context-aware. The code is derived from a similar package inside Google. Change-Id: Id1dad96d79e8ccfd289e4299e8265aa5bdad3a5b Reviewed-on: https://go-review.googlesource.com/38298 Reviewed-by: Ian Lance Taylor <iant@golang.org> Reviewed-by: Damien Neil <dneil@google.com> Run-TryBot: Ian Lance Taylor <iant@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org>
131 lines
3.4 KiB
Go
131 lines
3.4 KiB
Go
// 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"
|
|
"sync"
|
|
|
|
// Use the old context because packages that depend on this one
|
|
// (e.g. cloud.google.com/go/...) must run on Go 1.6.
|
|
// TODO(jba): update to "context" when possible.
|
|
"golang.org/x/net/context"
|
|
)
|
|
|
|
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 only until 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:
|
|
s.waiters.Remove(elem)
|
|
}
|
|
s.mu.Unlock()
|
|
return err
|
|
|
|
case <-ready:
|
|
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: bad release")
|
|
}
|
|
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)
|
|
}
|
|
s.mu.Unlock()
|
|
}
|