net/http: add Transport.DialTLS hook

Per discussions out of https://golang.org/cl/128930043/
and golang-nuts threads and with agl.

Fixes #8522

LGTM=agl, adg
R=agl, c, adg
CC=c, golang-codereviews
https://golang.org/cl/137940043

src/pkg/net/http/transport.go

@@ -43,8 +43,8 @@ var DefaultTransport RoundTripper = &Transport{
 // MaxIdleConnsPerHost.
 const DefaultMaxIdleConnsPerHost = 2
 
-// Transport is an implementation of RoundTripper that supports http,
-// https, and http proxies (for either http or https with CONNECT).
+// Transport is an implementation of RoundTripper that supports HTTP,
+// HTTPS, and HTTP proxies (for either HTTP or HTTPS with CONNECT).
 // Transport can also cache connections for future re-use.
 type Transport struct {
 	idleMu      sync.Mutex
@@ -61,11 +61,22 @@ type Transport struct {
 	// If Proxy is nil or returns a nil *URL, no proxy is used.
 	Proxy func(*Request) (*url.URL, error)
 
-	// Dial specifies the dial function for creating TCP
-	// connections.
+	// Dial specifies the dial function for creating unencrypted
+	// TCP connections.
 	// If Dial is nil, net.Dial is used.
 	Dial func(network, addr string) (net.Conn, error)
 
+	// DialTLS specifies an optional dial function for creating
+	// TLS connections for non-proxied HTTPS requests.
+	//
+	// If DialTLS is nil, Dial and TLSClientConfig are used.
+	//
+	// If DialTLS is set, the Dial hook is not used for HTTPS
+	// requests and the TLSClientConfig and TLSHandshakeTimeout
+	// are ignored. The returned net.Conn is assumed to already be
+	// past the TLS handshake.
+	DialTLS func(network, addr string) (net.Conn, error)
+
 	// TLSClientConfig specifies the TLS configuration to use with
 	// tls.Client. If nil, the default configuration is used.
 	TLSClientConfig *tls.Config
@@ -504,44 +515,56 @@ func (t *Transport) getConn(req *Request, cm connectMethod) (*persistConn, error
 }
 
 func (t *Transport) dialConn(cm connectMethod) (*persistConn, error) {
-	conn, err := t.dial("tcp", cm.addr())
-	if err != nil {
-		if cm.proxyURL != nil {
-			err = fmt.Errorf("http: error connecting to proxy %s: %v", cm.proxyURL, err)
-		}
-		return nil, err
-	}
-
-	pa := cm.proxyAuth()
-
 	pconn := &persistConn{
 		t:          t,
 		cacheKey:   cm.key(),
-		conn:       conn,
 		reqch:      make(chan requestAndChan, 1),
 		writech:    make(chan writeRequest, 1),
 		closech:    make(chan struct{}),
 		writeErrCh: make(chan error, 1),
 	}
+	tlsDial := t.DialTLS != nil && cm.targetScheme == "https" && cm.proxyURL == nil
+	if tlsDial {
+		var err error
+		pconn.conn, err = t.DialTLS("tcp", cm.addr())
+		if err != nil {
+			return nil, err
+		}
+		if tc, ok := pconn.conn.(*tls.Conn); ok {
+			cs := tc.ConnectionState()
+			pconn.tlsState = &cs
+		}
+	} else {
+		conn, err := t.dial("tcp", cm.addr())
+		if err != nil {
+			if cm.proxyURL != nil {
+				err = fmt.Errorf("http: error connecting to proxy %s: %v", cm.proxyURL, err)
+			}
+			return nil, err
+		}
+		pconn.conn = conn
+	}
 
+	// Proxy setup.
 	switch {
 	case cm.proxyURL == nil:
-		// Do nothing.
+		// Do nothing. Not using a proxy.
 	case cm.targetScheme == "http":
 		pconn.isProxy = true
-		if pa != "" {
+		if pa := cm.proxyAuth(); pa != "" {
 			pconn.mutateHeaderFunc = func(h Header) {
 				h.Set("Proxy-Authorization", pa)
 			}
 		}
 	case cm.targetScheme == "https":
+		conn := pconn.conn
 		connectReq := &Request{
 			Method: "CONNECT",
 			URL:    &url.URL{Opaque: cm.targetAddr},
 			Host:   cm.targetAddr,
 			Header: make(Header),
 		}
-		if pa != "" {
+		if pa := cm.proxyAuth(); pa != "" {
 			connectReq.Header.Set("Proxy-Authorization", pa)
 		}
 		connectReq.Write(conn)
@@ -562,7 +585,7 @@ func (t *Transport) dialConn(cm connectMethod) (*persistConn, error) {
 		}
 	}
 
-	if cm.targetScheme == "https" {
+	if cm.targetScheme == "https" && !tlsDial {
 		// Initiate TLS and check remote host name against certificate.
 		cfg := t.TLSClientConfig
 		if cfg == nil || cfg.ServerName == "" {
@@ -575,7 +598,7 @@ func (t *Transport) dialConn(cm connectMethod) (*persistConn, error) {
 				cfg = &clone
 			}
 		}
-		plainConn := conn
+		plainConn := pconn.conn
 		tlsConn := tls.Client(plainConn, cfg)
 		errc := make(chan error, 2)
 		var timer *time.Timer // for canceling TLS handshake

src/pkg/net/http/transport_test.go

@@ -2096,6 +2096,46 @@ func TestTransportClosesBodyOnError(t *testing.T) {
 	}
 }
 
+func TestTransportDialTLS(t *testing.T) {
+	var mu sync.Mutex // guards following
+	var gotReq, didDial bool
+
+	ts := httptest.NewTLSServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		mu.Lock()
+		gotReq = true
+		mu.Unlock()
+	}))
+	defer ts.Close()
+	tr := &Transport{
+		DialTLS: func(netw, addr string) (net.Conn, error) {
+			mu.Lock()
+			didDial = true
+			mu.Unlock()
+			c, err := tls.Dial(netw, addr, &tls.Config{
+				InsecureSkipVerify: true,
+			})
+			if err != nil {
+				return nil, err
+			}
+			return c, c.Handshake()
+		},
+	}
+	defer tr.CloseIdleConnections()
+	client := &Client{Transport: tr}
+	res, err := client.Get(ts.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	res.Body.Close()
+	mu.Lock()
+	if !gotReq {
+		t.Error("didn't get request")
+	}
+	if !didDial {
+		t.Error("didn't use dial hook")
+	}
+}
+
 func wantBody(res *http.Response, err error, want string) error {
 	if err != nil {
 		return err