How do you implement a priority select in Go?

Go has no built-in priority. The usual pattern is a non-blocking check of the high-priority channel first, followed by a blocking select over everything:

for {
	// 1. Give ctx strict priority.
	select {
	case <-ctx.Done():
		return ctx.Err()
	default:
	}

	// 2. Block on all.
	select {
	case <-ctx.Done():
		return ctx.Err()
	case v := <-high:
		handle(v)
	case v := <-low:
		// Before handling low, drain any pending high-priority work.
		for drained := false; !drained; {
			select {
			case h := <-high:
				handle(h)
			default:
				drained = true
			}
		}
		handle(v)
	}
}

Nuance to mention: priority is inherently best-effort in a concurrent system. A high-priority value can arrive a nanosecond after your check. The pattern reduces how often the wrong case wins, but it can't make it impossible. If ordering really matters, merge everything into a single channel with typed messages, or use a real priority queue guarded by a mutex and sync.Cond/signal channel.