kd/machine: Add supervised client. - #10306

instead of using another timer what about creating the context with a deadline?

this context is already cancelled when the function returns

I guess this is due to unreadable branch - one would expect to handle error not success. How about:

func (s *Supervised) Context() context.Context {
	c, err := s.dcf()
	if err != nil {
		ctx, cancel := context.WithCancel(context.Background())
		cancel()
		return ctx
	}

	return c.Context()
}

yes, this is intended to return cancelled context which will prevent any other logic to run (since we don't have a valid client). This type is also going to be extended with cancellation logic so the method will change a little.

defer timer.Stop()

nit: formatting, feels to me claustrophobic, wdyt?

func (s *Supervised) CurrentUser() (user string, err error) {
    fn := func(c Client) error {
        user, err = c.CurrentUser()
        return err
    }

    _, err = s.call(fn)
    return
}

"want err != client.ErrDisconnected" is enough msg, if it fails it's obvious why

(the real reason is that I always need to lookup the doc whether positional arguments are 0- or 1-index thus I try to avoid it if possible)

Actually that's easy to remember:

t.Fatalf(arg0, arg1, arg2, arg3) // everything always starts from 0.

// arg0 is a formatting string so: %[1]v <=> arg1

I will change this anyway.