Done channel is used to prevent goroutine leaking. For example, what happen if previous generator example doesn't close channel? That means there is a goroutine is running forever. Furthermore, what happen if you call this generator 1,000 times? Then, you will waste a lot of resources.
In the example, you could see I choose to close done channel to send message because this characteristic Can read data from closed channel.
We could also use context.WithCancel() to create context which could be called by cancel(). Then you could use case <-ctx.Done() to receive the cancellation message. In the following examples, I'll use cancelable context to avoid goroutine leaking.
Beside the done channel, there is also a timeout mechanism to avoid execution time is too long. Use case <- time.After(10 * time.Seconds) in select could receive message after 10 seconds, then you could do return to exit the goroutine.