enable using load multiple times

persist.go

@@ -37,6 +37,10 @@ func Save(f *os.File, v interface{}) error {
 // Use os.IsNotExist() to see if the returned error is due
 // to the file being missing.
 func Load(f *os.File, v interface{}) error {
-	err := json.NewDecoder(f).Decode(v)
+	_, err := f.Seek(0, 0)
+	if err != nil {
+		return errors.Wrap(err, "error reetting reader")
+	}
+	err = json.NewDecoder(f).Decode(v)
 	return errors.Wrap(err, "error decoding value")
 }

persist_test.go

@@ -40,4 +40,14 @@ func TestPersist(t *testing.T) {
 	is.Equal(o.Name, o2.Name)
 	is.Equal(o.Number, o2.Number)
 	is.True(o.When.Equal(o2.When))
+
+	// load it, twice
+	o2.Name = ""
+	o2.Number = 0
+	err = persist.Load(f, &o2)
+	is.NoErr(err)
+
+	is.Equal(o.Name, o2.Name)
+	is.Equal(o.Number, o2.Number)
+	is.True(o.When.Equal(o2.When))
 }