//Create an alternative done function
newDoneFunc := func (string, bool, time.Time) {
	println("Hi!")
}

message := pubsub.Message{}

//Get a reflectable value of message
messageValue := reflect.ValueOf(message)

//Get message's doneFunc field
doneFuncField := messageValue.FieldByName("doneFunc")

//Get the address of the field
doneFuncFieldAddress := doneFuncField.UnsafeAddr()

//Create a pointer based on the address
doneFuncFieldPointer := unsafe.Pointer(doneFuncFieldAddress)

//Create a new, exported field element that points to the original
accessibleDoneFuncField := reflect.NewAt(doneFuncField.Type(), doneFuncFieldPointer).Elem()

//Set the field with the alternative doneFunc
accessibleDoneFuncField.Set(reflect.ValueOf(newDoneFunc))