Make UpgradeAwareProxyHandler use transport.Dial if provided

This change is required for the handler to work with sshtunnels.
Without it, `kubectl exec` and `kubectl port-forward` are broken
when an ssh proxy is used (see #9292). I manually verified this
fixes that issue, e2e test coming shortly.

pkg/registry/generic/rest/proxy.go

@@ -174,24 +174,57 @@ func (h *UpgradeAwareProxyHandler) tryUpgrade(w http.ResponseWriter, req *http.R
 func (h *UpgradeAwareProxyHandler) dialURL() (net.Conn, error) {
 	dialAddr := netutil.CanonicalAddr(h.Location)
 
+	var dialer func(network, addr string) (net.Conn, error)
+	if httpTransport, ok := h.Transport.(*http.Transport); ok && httpTransport.Dial != nil {
+		dialer = httpTransport.Dial
+	}
+
 	switch h.Location.Scheme {
 	case "http":
+		if dialer != nil {
+			return dialer("tcp", dialAddr)
+		}
 		return net.Dial("tcp", dialAddr)
 	case "https":
+		// TODO: this TLS logic can probably be cleaned up; it's messy in an attempt
+		// to preserve behavior that we don't know for sure is exercised.
+
 		// Get the tls config from the transport if we recognize it
 		var tlsConfig *tls.Config
+		var tlsConn *tls.Conn
+		var err error
 		if h.Transport != nil {
 			httpTransport, ok := h.Transport.(*http.Transport)
 			if ok {
 				tlsConfig = httpTransport.TLSClientConfig
 			}
 		}
-
+		if dialer != nil {
-		// Dial
+			// We have a dialer; use it to open the connection, then
-		tlsConn, err := tls.Dial("tcp", dialAddr, tlsConfig)
+			// create a tls client using the connection.
+			netConn, err := dialer("tcp", dialAddr)
 			if err != nil {
 				return nil, err
 			}
+			// tls.Client requires non-nil config
+			if tlsConfig == nil {
+				glog.Warningf("using custom dialer with no TLSClientConfig. Defaulting to InsecureSkipVerify")
+				tlsConfig = &tls.Config{
+					InsecureSkipVerify: true,
+				}
+			}
+			tlsConn = tls.Client(netConn, tlsConfig)
+			if err := tlsConn.Handshake(); err != nil {
+				return nil, err
+			}
+
+		} else {
+			// Dial
+			tlsConn, err = tls.Dial("tcp", dialAddr, tlsConfig)
+			if err != nil {
+				return nil, err
+			}
+		}
 
 		// Verify
 		host, _, _ := net.SplitHostPort(dialAddr)
@@ -202,7 +235,7 @@ func (h *UpgradeAwareProxyHandler) dialURL() (net.Conn, error) {
 
 		return tlsConn, nil
 	default:
-		return nil, fmt.Errorf("Unknown scheme: %s", h.Location.Scheme)
+		return nil, fmt.Errorf("unknown scheme: %s", h.Location.Scheme)
 	}
 }