1package syncutil
2
3import (
4	"fmt"
5	"log"
6	"sync"
7)
8
9type debugT bool
10
11var debug = debugT(false)
12
13func (d debugT) Printf(format string, args ...interface{}) {
14	if bool(d) {
15		log.Printf(format, args...)
16	}
17}
18
19// Sem implements a semaphore that can have multiple units acquired/released
20// at a time.
21type Sem struct {
22	c         *sync.Cond // Protects size
23	max, free int64
24}
25
26// NewSem creates a semaphore with max units available for acquisition.
27func NewSem(max int64) *Sem {
28	return &Sem{
29		c:    sync.NewCond(new(sync.Mutex)),
30		free: max,
31		max:  max,
32	}
33}
34
35// Acquire will deduct n units from the semaphore.  If the deduction would
36// result in the available units falling below zero, the call will block until
37// another go routine returns units via a call to Release.  If more units are
38// requested than the semaphore is configured to hold, error will be non-nil.
39func (s *Sem) Acquire(n int64) error {
40	if n > s.max {
41		return fmt.Errorf("sem: attempt to acquire more units than semaphore size %d > %d", n, s.max)
42	}
43	s.c.L.Lock()
44	defer s.c.L.Unlock()
45	for {
46		debug.Printf("Acquire check max %d free %d, n %d", s.max, s.free, n)
47		if s.free >= n {
48			s.free -= n
49			return nil
50		}
51		debug.Printf("Acquire Wait max %d free %d, n %d", s.max, s.free, n)
52		s.c.Wait()
53	}
54}
55
56// Release will return n units to the semaphore and notify any currently
57// blocking Acquire calls.
58func (s *Sem) Release(n int64) {
59	s.c.L.Lock()
60	defer s.c.L.Unlock()
61	debug.Printf("Release max %d free %d, n %d", s.max, s.free, n)
62	s.free += n
63	s.c.Broadcast()
64}
65