Ensure workers terminated fully before reading unknownRefs

head.go

@@ -233,16 +233,20 @@ func (h *Head) ReadWAL() error {
 	// They are connected through a ring of channels which ensures that all sample batches
 	// read from the WAL are processed in order.
 	var (
+		wg         sync.WaitGroup
 		n          = runtime.GOMAXPROCS(0)
 		firstInput = make(chan []RefSample, 300)
 		input      = firstInput
 	)
+	wg.Add(n)
+
 	for i := 0; i < n; i++ {
 		output := make(chan []RefSample, 300)
 
 		go func(i int, input <-chan []RefSample, output chan<- []RefSample) {
 			unknown := h.processWALSamples(mint, uint64(i), uint64(n), input, output)
 			atomic.AddUint64(&unknownRefs, unknown)
+			wg.Done()
 		}(i, input, output)
 
 		// The output feeds the next worker goroutine. For the last worker,
@@ -288,6 +292,8 @@ func (h *Head) ReadWAL() error {
 	close(firstInput)
 	for range input {
 	}
+	wg.Wait()
+
 	if err != nil {
 		return errors.Wrap(err, "consume WAL")
 	}