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