Refactor batcher to fix race issues

- Replace done channel with a WaitGroup to fix a race condition and to
  properly wait for *all* active batches during shutdown
- Use time.AfterFunc() instead of manually spawning a goroutine and
  calling sleep()
- Improve a few method names and doc comments
- move some logic from batch() to submitWork() so it's always obvious
  what is or isn't guarded by the lock

batcher/batcher.go

@@ -16,16 +16,17 @@ type Batcher struct {
 	timeout   time.Duration
 	prefilter func(interface{}) error
 
-	lock   sync.Mutex
-	submit chan *work
-	doWork func([]interface{}) error
-	done   chan bool
+	lock         sync.Mutex
+	submit       chan *work
+	doWork       func([]interface{}) error
+	batchCounter sync.WaitGroup
+	flushTimer   *time.Timer
 }
 
 // New constructs a new batcher that will batch all calls to Run that occur within
 // `timeout` time before calling doWork just once for the entire batch. The doWork
 // function must be safe to run concurrently with itself as this may occur, especially
-// when the timeout is small.
+// when the doWork function is slow, or the timeout is small.
 func New(timeout time.Duration, doWork func([]interface{}) error) *Batcher {
 	return &Batcher{
 		timeout: timeout,
@@ -70,21 +71,23 @@ func (b *Batcher) submitWork(w *work) {
 	b.lock.Lock()
 	defer b.lock.Unlock()
 
+	// kick off a new batch if needed
 	if b.submit == nil {
-		b.done = make(chan bool)
+		b.batchCounter.Add(1)
 		b.submit = make(chan *work, 4)
-		go b.batch()
+		go b.batch(b.submit)
+		b.flushTimer = time.AfterFunc(b.timeout, b.flushCurrentBatch)
 	}
 
+	// then add this work to the current batch
 	b.submit <- w
 }
 
-func (b *Batcher) batch() {
+func (b *Batcher) batch(input <-chan *work) {
+	defer b.batchCounter.Done()
+
 	var params []interface{}
 	var futures []chan error
-	input := b.submit
-
-	go b.timer()
 
 	for work := range input {
 		params = append(params, work.param)
@@ -97,37 +100,33 @@ func (b *Batcher) batch() {
 		future <- ret
 		close(future)
 	}
-	close(b.done)
 }
 
-func (b *Batcher) timer() {
-	time.Sleep(b.timeout)
-
-	b.flush()
-}
-
-// Shutdown flush the changes and wait to be saved
+// Shutdown flushes and executes any pending batches. If wait is true, it also waits for the pending batches
+// to finish executing before it returns. This can be used to avoid waiting for the timeout to expire when
+// gracefully shutting down your application. Calling Run at any point after calling Shutdown will lead to
+// undefined behaviour.
 func (b *Batcher) Shutdown(wait bool) {
-	b.flush()
+	b.flushCurrentBatch()
 
 	if wait {
-		if b.done != nil {
-			// wait done channel
-			<-b.done
-		}
+		b.batchCounter.Wait()
 	}
 }
 
-// Flush saves the changes before the timer expires.
-// It is useful to flush the changes when you shutdown your application
-func (b *Batcher) flush() {
+func (b *Batcher) flushCurrentBatch() {
 	b.lock.Lock()
 	defer b.lock.Unlock()
 
 	if b.submit == nil {
 		return
 	}
 
+	// stop the timer to avoid spurious flushes and trigger immediate cleanup in case this flush was
+	// triggered manually by a call to Shutdown (it has to happen inside the lock, so it can't be done
+	// in the Shutdown method directly)
+	b.flushTimer.Stop()
+
 	close(b.submit)
 	b.submit = nil
 }