counter: fix mutex copy issue

counter/counter_test.go

@@ -5,7 +5,9 @@ import (
 	"testing"
 )
 
-func assertCounter(t testing.TB, got Counter, want int) {
+// assertCounter
+// XXX: a copy of Counter is created, but we shouldn't copy a mutex. Use pointer instead.
+func assertCounter(t testing.TB, got *Counter, want int) {
 	t.Helper()
 	if got.Value() != want {
 		t.Errorf("got %d, want %d", got.Value(), want)
@@ -20,7 +22,7 @@ func TestCounter(t *testing.T) {
 		counter.Inc()
 		counter.Inc()
 
-		assertCounter(t, counter, 3)
+		assertCounter(t, &counter, 3)
 	})
 
 	t.Run("it runs safely concurrently", func(t *testing.T) {
@@ -37,6 +39,6 @@ func TestCounter(t *testing.T) {
 			}()
 		}
 		wg.Wait()
-		assertCounter(t, counter, wantedCount)
+		assertCounter(t, &counter, wantedCount)
 	})
 }