Don Newton | 379ae25 | 2019-04-01 12:17:06 -0400 | [diff] [blame^] | 1 | // Copyright 2017 The Go Authors. All rights reserved. |
| 2 | // Use of this source code is governed by a BSD-style |
| 3 | // license that can be found in the LICENSE file. |
| 4 | |
| 5 | // Package semaphore provides a weighted semaphore implementation. |
| 6 | package semaphore // import "golang.org/x/sync/semaphore" |
| 7 | |
| 8 | import ( |
| 9 | "container/list" |
| 10 | "context" |
| 11 | "sync" |
| 12 | ) |
| 13 | |
| 14 | type waiter struct { |
| 15 | n int64 |
| 16 | ready chan<- struct{} // Closed when semaphore acquired. |
| 17 | } |
| 18 | |
| 19 | // NewWeighted creates a new weighted semaphore with the given |
| 20 | // maximum combined weight for concurrent access. |
| 21 | func NewWeighted(n int64) *Weighted { |
| 22 | w := &Weighted{size: n} |
| 23 | return w |
| 24 | } |
| 25 | |
| 26 | // Weighted provides a way to bound concurrent access to a resource. |
| 27 | // The callers can request access with a given weight. |
| 28 | type Weighted struct { |
| 29 | size int64 |
| 30 | cur int64 |
| 31 | mu sync.Mutex |
| 32 | waiters list.List |
| 33 | } |
| 34 | |
| 35 | // Acquire acquires the semaphore with a weight of n, blocking until resources |
| 36 | // are available or ctx is done. On success, returns nil. On failure, returns |
| 37 | // ctx.Err() and leaves the semaphore unchanged. |
| 38 | // |
| 39 | // If ctx is already done, Acquire may still succeed without blocking. |
| 40 | func (s *Weighted) Acquire(ctx context.Context, n int64) error { |
| 41 | s.mu.Lock() |
| 42 | if s.size-s.cur >= n && s.waiters.Len() == 0 { |
| 43 | s.cur += n |
| 44 | s.mu.Unlock() |
| 45 | return nil |
| 46 | } |
| 47 | |
| 48 | if n > s.size { |
| 49 | // Don't make other Acquire calls block on one that's doomed to fail. |
| 50 | s.mu.Unlock() |
| 51 | <-ctx.Done() |
| 52 | return ctx.Err() |
| 53 | } |
| 54 | |
| 55 | ready := make(chan struct{}) |
| 56 | w := waiter{n: n, ready: ready} |
| 57 | elem := s.waiters.PushBack(w) |
| 58 | s.mu.Unlock() |
| 59 | |
| 60 | select { |
| 61 | case <-ctx.Done(): |
| 62 | err := ctx.Err() |
| 63 | s.mu.Lock() |
| 64 | select { |
| 65 | case <-ready: |
| 66 | // Acquired the semaphore after we were canceled. Rather than trying to |
| 67 | // fix up the queue, just pretend we didn't notice the cancelation. |
| 68 | err = nil |
| 69 | default: |
| 70 | s.waiters.Remove(elem) |
| 71 | } |
| 72 | s.mu.Unlock() |
| 73 | return err |
| 74 | |
| 75 | case <-ready: |
| 76 | return nil |
| 77 | } |
| 78 | } |
| 79 | |
| 80 | // TryAcquire acquires the semaphore with a weight of n without blocking. |
| 81 | // On success, returns true. On failure, returns false and leaves the semaphore unchanged. |
| 82 | func (s *Weighted) TryAcquire(n int64) bool { |
| 83 | s.mu.Lock() |
| 84 | success := s.size-s.cur >= n && s.waiters.Len() == 0 |
| 85 | if success { |
| 86 | s.cur += n |
| 87 | } |
| 88 | s.mu.Unlock() |
| 89 | return success |
| 90 | } |
| 91 | |
| 92 | // Release releases the semaphore with a weight of n. |
| 93 | func (s *Weighted) Release(n int64) { |
| 94 | s.mu.Lock() |
| 95 | s.cur -= n |
| 96 | if s.cur < 0 { |
| 97 | s.mu.Unlock() |
| 98 | panic("semaphore: bad release") |
| 99 | } |
| 100 | for { |
| 101 | next := s.waiters.Front() |
| 102 | if next == nil { |
| 103 | break // No more waiters blocked. |
| 104 | } |
| 105 | |
| 106 | w := next.Value.(waiter) |
| 107 | if s.size-s.cur < w.n { |
| 108 | // Not enough tokens for the next waiter. We could keep going (to try to |
| 109 | // find a waiter with a smaller request), but under load that could cause |
| 110 | // starvation for large requests; instead, we leave all remaining waiters |
| 111 | // blocked. |
| 112 | // |
| 113 | // Consider a semaphore used as a read-write lock, with N tokens, N |
| 114 | // readers, and one writer. Each reader can Acquire(1) to obtain a read |
| 115 | // lock. The writer can Acquire(N) to obtain a write lock, excluding all |
| 116 | // of the readers. If we allow the readers to jump ahead in the queue, |
| 117 | // the writer will starve — there is always one token available for every |
| 118 | // reader. |
| 119 | break |
| 120 | } |
| 121 | |
| 122 | s.cur += w.n |
| 123 | s.waiters.Remove(next) |
| 124 | close(w.ready) |
| 125 | } |
| 126 | s.mu.Unlock() |
| 127 | } |