go ctx react impl

ctx.go

func resSearch(ctx context.Context, hostname string, rtype, class int) ([]dnsmessage.Resource, error) {
	if ctx.Done() == nil {
		return cgoResSearch(hostname, rtype, class)
	}

	type result struct {
		err error
		res []dnsmessage.Resource
	}

	res := make(chan result, 1)
	go func() {
		r, err := cgoResSearch(hostname, rtype, class)
		res <- result{
			err: err,
			res: r,
		}
	}()

	select {
	case res := <-res:
		return res.res, res.err
	case <-ctx.Done():
		return nil, mapErr(ctx.Err())
	}
}