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package syncutil
import (
"fmt"
"log"
"sync"
)
type debugT bool
var debug = debugT(false)
func (d debugT) Printf(format string, args ...interface{}) {
if bool(d) {
log.Printf(format, args...)
}
}
// Sem implements a semaphore that can have multiple units acquired/released
// at a time.
type Sem struct {
c *sync.Cond // Protects size
max, free int64
}
// NewSem creates a semaphore with max units available for acquisition.
func NewSem(max int64) *Sem {
return &Sem{
c: sync.NewCond(new(sync.Mutex)),
free: max,
max: max,
}
}
// Acquire will deduct n units from the semaphore. If the deduction would
// result in the available units falling below zero, the call will block until
// another go routine returns units via a call to Release. If more units are
// requested than the semaphore is configured to hold, error will be non-nil.
func (s *Sem) Acquire(n int64) error {
if n > s.max {
return fmt.Errorf("sem: attempt to acquire more units than semaphore size %d > %d", n, s.max)
}
s.c.L.Lock()
defer s.c.L.Unlock()
for {
debug.Printf("Acquire check max %d free %d, n %d", s.max, s.free, n)
if s.free >= n {
s.free -= n
return nil
}
debug.Printf("Acquire Wait max %d free %d, n %d", s.max, s.free, n)
s.c.Wait()
}
}
// Release will return n units to the semaphore and notify any currently
// blocking Acquire calls.
func (s *Sem) Release(n int64) {
s.c.L.Lock()
defer s.c.L.Unlock()
debug.Printf("Release max %d free %d, n %d", s.max, s.free, n)
s.free += n
s.c.Broadcast()
}
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