Source file src/cmd/vendor/golang.org/x/sync/semaphore/semaphore.go

     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  			isFront := s.waiters.Front() == elem
    71  			s.waiters.Remove(elem)
    72  			// If we're at the front and there're extra tokens left, notify other waiters.
    73  			if isFront && s.size > s.cur {
    74  				s.notifyWaiters()
    75  			}
    76  		}
    77  		s.mu.Unlock()
    78  		return err
    79  
    80  	case <-ready:
    81  		return nil
    82  	}
    83  }
    84  
    85  // TryAcquire acquires the semaphore with a weight of n without blocking.
    86  // On success, returns true. On failure, returns false and leaves the semaphore unchanged.
    87  func (s *Weighted) TryAcquire(n int64) bool {
    88  	s.mu.Lock()
    89  	success := s.size-s.cur >= n && s.waiters.Len() == 0
    90  	if success {
    91  		s.cur += n
    92  	}
    93  	s.mu.Unlock()
    94  	return success
    95  }
    96  
    97  // Release releases the semaphore with a weight of n.
    98  func (s *Weighted) Release(n int64) {
    99  	s.mu.Lock()
   100  	s.cur -= n
   101  	if s.cur < 0 {
   102  		s.mu.Unlock()
   103  		panic("semaphore: released more than held")
   104  	}
   105  	s.notifyWaiters()
   106  	s.mu.Unlock()
   107  }
   108  
   109  func (s *Weighted) notifyWaiters() {
   110  	for {
   111  		next := s.waiters.Front()
   112  		if next == nil {
   113  			break // No more waiters blocked.
   114  		}
   115  
   116  		w := next.Value.(waiter)
   117  		if s.size-s.cur < w.n {
   118  			// Not enough tokens for the next waiter.  We could keep going (to try to
   119  			// find a waiter with a smaller request), but under load that could cause
   120  			// starvation for large requests; instead, we leave all remaining waiters
   121  			// blocked.
   122  			//
   123  			// Consider a semaphore used as a read-write lock, with N tokens, N
   124  			// readers, and one writer.  Each reader can Acquire(1) to obtain a read
   125  			// lock.  The writer can Acquire(N) to obtain a write lock, excluding all
   126  			// of the readers.  If we allow the readers to jump ahead in the queue,
   127  			// the writer will starve — there is always one token available for every
   128  			// reader.
   129  			break
   130  		}
   131  
   132  		s.cur += w.n
   133  		s.waiters.Remove(next)
   134  		close(w.ready)
   135  	}
   136  }
   137  

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