Add separate goroutine that closes the output chan

This allows iterating over the output channel without having to start
another Goroutine outside of the worker pool. This also removes the need
for calling Wait().

src/restic/worker/pool.go

@@ -1,10 +1,5 @@
 package worker
 
-import (
-	"fmt"
-	"sync"
-)
-
 // Job is one unit of work. It is given to a Func, and the returned result and
 // error are stored in Result and Error.
 type Job struct {
@@ -20,9 +15,12 @@ type Func func(job Job, done <-chan struct{}) (result interface{}, err error)
 type Pool struct {
 	f     Func
 	done  chan struct{}
-	wg    *sync.WaitGroup
 	jobCh <-chan Job
 	resCh chan<- Job
+
+	numWorkers     int
+	workersExit    chan struct{}
+	allWorkersDone chan struct{}
 }
 
 // New returns a new worker pool with n goroutines, each running the function
@@ -31,22 +29,39 @@ func New(n int, f Func, jobChan <-chan Job, resultChan chan<- Job) *Pool {
 	p := &Pool{
 		f:              f,
 		done:           make(chan struct{}),
-		wg:    &sync.WaitGroup{},
+		workersExit:    make(chan struct{}),
+		allWorkersDone: make(chan struct{}),
+		numWorkers:     n,
 		jobCh:          jobChan,
 		resCh:          resultChan,
 	}
 
 	for i := 0; i < n; i++ {
-		p.wg.Add(1)
 		go p.runWorker(i)
 	}
 
+	go p.waitForExit()
+
 	return p
 }
 
+// waitForExit receives from p.workersExit until all worker functions have
+// exited, then closes the result channel.
+func (p *Pool) waitForExit() {
+	n := p.numWorkers
+	for n > 0 {
+		<-p.workersExit
+		n--
+	}
+	close(p.allWorkersDone)
+	close(p.resCh)
+}
+
 // runWorker runs a worker function.
 func (p *Pool) runWorker(numWorker int) {
-	defer p.wg.Done()
+	defer func() {
+		p.workersExit <- struct{}{}
+	}()
 
 	var (
 		// enable the input channel when starting up a new goroutine
@@ -65,7 +80,6 @@ func (p *Pool) runWorker(numWorker int) {
 
 		case job, ok = <-inCh:
 			if !ok {
-				fmt.Printf("in channel closed, worker exiting\n")
 				return
 			}
 
@@ -88,6 +102,5 @@ func (p *Pool) Cancel() {
 // Wait waits for all worker goroutines to terminate, afterwards the output
 // channel is closed.
 func (p *Pool) Wait() {
-	p.wg.Wait()
-	close(p.resCh)
+	<-p.allWorkersDone
 }