How would you merge two contexts so the result is canceled when either one is?

The standard library has no merge function, but WithCancelCause plus AfterFunc makes one cheap, without a dedicated watcher goroutine when both inputs are stdlib contexts. A typical need: a request context combined with a server shutdown context, or a stream's context combined with a per-message context.

// Merge returns a context canceled when a or b is done.
// Values and Deadline come from a only.
func Merge(a, b context.Context) (context.Context, context.CancelFunc) {
	ctx, cancel := context.WithCancelCause(a)
	stop := context.AfterFunc(b, func() {
		cancel(context.Cause(b))
	})
	return ctx, func() {
		stop()
		cancel(context.Canceled)
	}
}

func main() {
	a, cancelA := context.WithCancel(context.Background())
	defer cancelA()
	b, cancelB := context.WithTimeout(context.Background(), 10*time.Millisecond)
	defer cancelB()

	m, cancel := Merge(a, b)
	defer cancel()
	<-m.Done()
	fmt.Println(m.Err(), "|", context.Cause(m)) // context canceled | context deadline exceeded
}

Discussion points:

• Err() is Canceled because we canceled manually. Cause keeps the real reason from b.
• Deadline() reports only a's deadline. If that matters, compute the earlier of the two and wrap the result in WithDeadline.
• Value lookups see only a. Merging values is ambiguous, which is one reason the stdlib does not provide merging.
• The returned cancel must call stop(), or the callback stays registered on b.