diff options
Diffstat (limited to 'src/cmd/vendor/golang.org/x/sync/semaphore/semaphore.go')
-rw-r--r-- | src/cmd/vendor/golang.org/x/sync/semaphore/semaphore.go | 42 |
1 files changed, 33 insertions, 9 deletions
diff --git a/src/cmd/vendor/golang.org/x/sync/semaphore/semaphore.go b/src/cmd/vendor/golang.org/x/sync/semaphore/semaphore.go index 30f632c577..b618162aab 100644 --- a/src/cmd/vendor/golang.org/x/sync/semaphore/semaphore.go +++ b/src/cmd/vendor/golang.org/x/sync/semaphore/semaphore.go @@ -35,11 +35,25 @@ type Weighted struct { // Acquire acquires the semaphore with a weight of n, blocking until resources // are available or 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 { + done := ctx.Done() + s.mu.Lock() + select { + case <-done: + // ctx becoming done has "happened before" acquiring the semaphore, + // whether it became done before the call began or while we were + // waiting for the mutex. We prefer to fail even if we could acquire + // the mutex without blocking. + s.mu.Unlock() + return ctx.Err() + default: + } if s.size-s.cur >= n && s.waiters.Len() == 0 { + // Since we hold s.mu and haven't synchronized since checking done, if + // ctx becomes done before we return here, it becoming done must have + // "happened concurrently" with this call - it cannot "happen before" + // we return in this branch. So, we're ok to always acquire here. s.cur += n s.mu.Unlock() return nil @@ -48,7 +62,7 @@ func (s *Weighted) Acquire(ctx context.Context, n int64) error { if n > s.size { // Don't make other Acquire calls block on one that's doomed to fail. s.mu.Unlock() - <-ctx.Done() + <-done return ctx.Err() } @@ -58,14 +72,14 @@ func (s *Weighted) Acquire(ctx context.Context, n int64) error { s.mu.Unlock() select { - case <-ctx.Done(): - err := ctx.Err() + case <-done: 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 + // Acquired the semaphore after we were canceled. + // Pretend we didn't and put the tokens back. + s.cur -= n + s.notifyWaiters() default: isFront := s.waiters.Front() == elem s.waiters.Remove(elem) @@ -75,9 +89,19 @@ func (s *Weighted) Acquire(ctx context.Context, n int64) error { } } s.mu.Unlock() - return err + return ctx.Err() case <-ready: + // Acquired the semaphore. Check that ctx isn't already done. + // We check the done channel instead of calling ctx.Err because we + // already have the channel, and ctx.Err is O(n) with the nesting + // depth of ctx. + select { + case <-done: + s.Release(n) + return ctx.Err() + default: + } return nil } } |