progress: extract progress updating into Updater struct

This allows reusing the code to create periodic progress updates.

internal/ui/progress/updater_test.go

@@ -0,0 +1,52 @@
+package progress_test
+
+import (
+	"testing"
+	"time"
+
+	"github.com/restic/restic/internal/test"
+	"github.com/restic/restic/internal/ui/progress"
+)
+
+func TestUpdater(t *testing.T) {
+	finalSeen := false
+	var ncalls int
+
+	report := func(d time.Duration, final bool) {
+		if final {
+			finalSeen = true
+		}
+		ncalls++
+	}
+	c := progress.NewUpdater(10*time.Millisecond, report)
+	time.Sleep(100 * time.Millisecond)
+	c.Done()
+
+	test.Assert(t, finalSeen, "final call did not happen")
+	test.Assert(t, ncalls > 0, "no progress was reported")
+}
+
+func TestUpdaterStopTwice(t *testing.T) {
+	c := progress.NewUpdater(0, func(runtime time.Duration, final bool) {})
+	c.Done()
+	c.Done()
+}
+
+func TestUpdaterNoTick(t *testing.T) {
+	finalSeen := false
+	otherSeen := false
+
+	report := func(d time.Duration, final bool) {
+		if final {
+			finalSeen = true
+		} else {
+			otherSeen = true
+		}
+	}
+	c := progress.NewUpdater(0, report)
+	time.Sleep(time.Millisecond)
+	c.Done()
+
+	test.Assert(t, finalSeen, "final call did not happen")
+	test.Assert(t, !otherSeen, "unexpected status update")
+}