bugfix: updatedAt timestamp file may be empty

Signed-off-by: Michael Wan <zirenwan@gmail.com>

content/local/store_test.go

@@ -36,6 +36,7 @@ import (
 	"github.com/containerd/containerd/content"
 	"github.com/containerd/containerd/content/testsuite"
 	"github.com/containerd/containerd/pkg/testutil"
+
 	"github.com/opencontainers/go-digest"
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
 	"gotest.tools/assert"
@@ -392,3 +393,24 @@ func setupIncompleteWrite(ctx context.Context, t *testing.T, cs content.Store, r
 
 	assert.NilError(t, writer.Close())
 }
+
+func TestWriteReadEmptyFileTimestamp(t *testing.T) {
+	root, err := ioutil.TempDir("", "test-write-read-file-timestamp")
+	if err != nil {
+		t.Errorf("failed to create a tmp dir: %v", err)
+	}
+	defer os.RemoveAll(root)
+
+	emptyFile := filepath.Join(root, "updatedat")
+	if err := writeTimestampFile(emptyFile, time.Time{}); err != nil {
+		t.Errorf("failed to write Zero Time to file: %v", err)
+	}
+
+	timestamp, err := readFileTimestamp(emptyFile)
+	if err != nil {
+		t.Errorf("read empty timestamp file should success, but got error: %v", err)
+	}
+	if !timestamp.IsZero() {
+		t.Errorf("read empty timestamp file should return time.Time{}, but got: %v", timestamp)
+	}
+}