ooni-probe-cli/internal/engine/netx/dialer/proxy.go

package dialer

import (
	"context"
	"errors"
	"net"
	"net/url"

	"golang.org/x/net/proxy"
)

// ProxyDialer is a dialer that uses a proxy. If the ProxyURL is not configured, this
// dialer is a passthrough for the next Dialer in chain. Otherwise, it will internally
// create a SOCKS5 dialer that will connect to the proxy using the underlying Dialer.
//
// As a special case, you can force a proxy to be used only extemporarily. To this end,
// you can use the WithProxyURL function, to store the proxy URL in the context. This
// will take precedence over any otherwise configured proxy. The use case for this
// functionality is when you need a tunnel to contact OONI probe services.
type ProxyDialer struct {
	Dialer
	ProxyURL *url.URL
}

type proxyKey struct{}

// ContextProxyURL retrieves the proxy URL from the context. This is mainly used
// to force a tunnel when we fail contacting OONI probe services otherwise.
func ContextProxyURL(ctx context.Context) *url.URL {
	url, _ := ctx.Value(proxyKey{}).(*url.URL)
	return url
}

// WithProxyURL assigns the proxy URL to the context
func WithProxyURL(ctx context.Context, url *url.URL) context.Context {
	return context.WithValue(ctx, proxyKey{}, url)
}

// DialContext implements Dialer.DialContext
func (d ProxyDialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
	url := ContextProxyURL(ctx) // context URL takes precedence
	if url == nil {
		url = d.ProxyURL
	}
	if url == nil {
		return d.Dialer.DialContext(ctx, network, address)
	}
	if url.Scheme != "socks5" {
		return nil, errors.New("Scheme is not socks5")
	}
	// the code at proxy/socks5.go never fails; see https://git.io/JfJ4g
	child, _ := proxy.SOCKS5(
		network, url.Host, nil, proxyDialerWrapper{Dialer: d.Dialer})
	return d.dial(ctx, child, network, address)
}

func (d ProxyDialer) dial(
	ctx context.Context, child proxy.Dialer, network, address string) (net.Conn, error) {
	connch := make(chan net.Conn)
	errch := make(chan error, 1)
	go func() {
		conn, err := child.Dial(network, address)
		if err != nil {
			errch <- err
			return
		}
		select {
		case connch <- conn:
		default:
			conn.Close()
		}
	}()
	select {
	case <-ctx.Done():
		return nil, ctx.Err()
	case err := <-errch:
		return nil, err
	case conn := <-connch:
		return conn, nil
	}
}

// proxyDialerWrapper is required because SOCKS5 expects a Dialer.Dial type but internally
// it checks whether DialContext is available and prefers that. So, we need to use this
// structure to cast our inner Dialer the way in which SOCKS5 likes it.
//
// See https://git.io/JfJ4g.
type proxyDialerWrapper struct {
	Dialer
}

func (d proxyDialerWrapper) Dial(network, address string) (net.Conn, error) {
	return d.DialContext(context.Background(), network, address)
}