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// Based upon sync.WaitGroup, SizedWaitGroup allows to start multiple
// routines and to wait for their end using the simple API.
// SizedWaitGroup adds the feature of limiting the maximum number of
// concurrently started routines. It could for example be used to start
// multiples routines querying a database but without sending too much
// queries in order to not overload the given database.
//
// Rémy Mathieu © 2016
package sizedwaitgroup
import (
"context"
"math"
"sync"
)
// SizedWaitGroup has the same role and close to the
// same API as the Golang sync.WaitGroup but adds a limit of
// the amount of goroutines started concurrently.
type SizedWaitGroup struct {
Size int
current chan struct{}
wg sync.WaitGroup
}
// New creates a SizedWaitGroup.
// The limit parameter is the maximum amount of
// goroutines which can be started concurrently.
func New(limit int) SizedWaitGroup {
size := math.MaxInt32 // 2^32 - 1
if limit > 0 {
size = limit
}
return SizedWaitGroup{
Size: size,
current: make(chan struct{}, size),
wg: sync.WaitGroup{},
}
}
// Add increments the internal WaitGroup counter.
// It can be blocking if the limit of spawned goroutines
// has been reached. It will stop blocking when Done is
// been called.
//
// See sync.WaitGroup documentation for more information.
func (s *SizedWaitGroup) Add() {
s.AddWithContext(context.Background())
}
// AddWithContext increments the internal WaitGroup counter.
// It can be blocking if the limit of spawned goroutines
// has been reached. It will stop blocking when Done is
// been called, or when the context is canceled. Returns nil on
// success or an error if the context is canceled before the lock
// is acquired.
//
// See sync.WaitGroup documentation for more information.
func (s *SizedWaitGroup) AddWithContext(ctx context.Context) error {
select {
case <-ctx.Done():
return ctx.Err()
case s.current <- struct{}{}:
break
}
s.wg.Add(1)
return nil
}
// Done decrements the SizedWaitGroup counter.
// See sync.WaitGroup documentation for more information.
func (s *SizedWaitGroup) Done() {
<-s.current
s.wg.Done()
}
// Wait blocks until the SizedWaitGroup counter is zero.
// See sync.WaitGroup documentation for more information.
func (s *SizedWaitGroup) Wait() {
s.wg.Wait()
}
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