cleanup(netx): remove unused proxy-via-context codepath (#367)

We always set the proxy explicitly now. So, let us remove this
extra bit of code we're not using.

Part of https://github.com/ooni/probe/issues/1507.

internal/engine/netx/dialer/proxy.go

@@ -12,36 +12,14 @@ import (
 // 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
-	}
+	url := d.ProxyURL
 	if url == nil {
 		return d.Dialer.DialContext(ctx, network, address)
 	}

internal/engine/netx/dialer/proxy_test.go

@@ -24,23 +24,6 @@ func TestProxyDialerDialContextNoProxyURL(t *testing.T) {
 	}
 }
 
-func TestProxyDialerContextTakesPrecedence(t *testing.T) {
-	expected := errors.New("mocked error")
-	d := dialer.ProxyDialer{
-		Dialer:   dialer.FakeDialer{Err: expected},
-		ProxyURL: &url.URL{Scheme: "antani"},
-	}
-	ctx := context.Background()
-	ctx = dialer.WithProxyURL(ctx, &url.URL{Scheme: "socks5", Host: "[::1]:443"})
-	conn, err := d.DialContext(ctx, "tcp", "www.google.com:443")
-	if !errors.Is(err, expected) {
-		t.Fatal("not the error we expected")
-	}
-	if conn != nil {
-		t.Fatal("conn is not nil")
-	}
-}
-
 func TestProxyDialerDialContextInvalidScheme(t *testing.T) {
 	d := dialer.ProxyDialer{
 		Dialer:   dialer.FakeDialer{},