net/http: add Transport.Dialer, plumb RoundTrip contexts to net package

This simply connects the contexts, pushing them down the call stack.
Future CLs will utilize them.

For #12580 (http.Transport tracing/analytics)
Updates #13021

Change-Id: I5b2074d6eb1e87d79a767fc0609c84e7928d1a16
Reviewed-on: https://go-review.googlesource.com/22124Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/transport.go

@@ -12,6 +12,7 @@ package http
 import (
 	"bufio"
 	"compress/gzip"
+	"context"
 	"crypto/tls"
 	"errors"
 	"fmt"
@@ -32,10 +33,10 @@ import (
 // $no_proxy) environment variables.
 var DefaultTransport RoundTripper = &Transport{
 	Proxy: ProxyFromEnvironment,
-	Dial: (&net.Dialer{
+	Dialer: &net.Dialer{
 		Timeout:   30 * time.Second,
 		KeepAlive: 30 * time.Second,
-	}).Dial,
+	},
 	TLSHandshakeTimeout:   10 * time.Second,
 	ExpectContinueTimeout: 1 * time.Second,
 }
@@ -80,10 +81,17 @@ type Transport struct {
 	Proxy func(*Request) (*url.URL, error)
 
 	// Dial specifies the dial function for creating unencrypted
-	// TCP connections.
-	// If Dial is nil, net.Dial is used.
+	// TCP connections. If Dial and Dialer are both nil, net.Dial
+	// is used.
+	//
+	// Deprecated: Use Dialer instead. If both are specified, Dialer
+	// takes precedence.
 	Dial func(network, addr string) (net.Conn, error)
 
+	// Dialer optionally specifies a dialer configuration to use
+	// for new connections.
+	Dialer *net.Dialer
+
 	// DialTLS specifies an optional dial function for creating
 	// TLS connections for non-proxied HTTPS requests.
 	//
@@ -689,7 +697,10 @@ func (t *Transport) replaceReqCanceler(r *Request, fn func()) bool {
 	return true
 }
 
-func (t *Transport) dial(network, addr string) (net.Conn, error) {
+func (t *Transport) dial(ctx context.Context, network, addr string) (net.Conn, error) {
+	if t.Dialer != nil {
+		return t.Dialer.DialContext(ctx, network, addr)
+	}
 	if t.Dial != nil {
 		c, err := t.Dial(network, addr)
 		if c == nil && err == nil {
@@ -705,6 +716,7 @@ func (t *Transport) dial(network, addr string) (net.Conn, error) {
 // and/or setting up TLS.  If this doesn't return an error, the persistConn
 // is ready to write requests to.
 func (t *Transport) getConn(req *Request, cm connectMethod) (*persistConn, error) {
+	ctx := req.Context()
 	if pc := t.getIdleConn(cm); pc != nil {
 		// set request canceler to some non-nil function so we
 		// can detect whether it was cleared between now and when
@@ -738,7 +750,7 @@ func (t *Transport) getConn(req *Request, cm connectMethod) (*persistConn, error
 	t.setReqCanceler(req, func() { close(cancelc) })
 
 	go func() {
-		pc, err := t.dialConn(cm)
+		pc, err := t.dialConn(ctx, cm)
 		dialc <- dialRes{pc, err}
 	}()
 
@@ -767,7 +779,7 @@ func (t *Transport) getConn(req *Request, cm connectMethod) (*persistConn, error
 	}
 }
 
-func (t *Transport) dialConn(cm connectMethod) (*persistConn, error) {
+func (t *Transport) dialConn(ctx context.Context, cm connectMethod) (*persistConn, error) {
 	pconn := &persistConn{
 		t:          t,
 		cacheKey:   cm.key(),
@@ -797,7 +809,7 @@ func (t *Transport) dialConn(cm connectMethod) (*persistConn, error) {
 			pconn.tlsState = &cs
 		}
 	} else {
-		conn, err := t.dial("tcp", cm.addr())
+		conn, err := t.dial(ctx, "tcp", cm.addr())
 		if err != nil {
 			if cm.proxyURL != nil {
 				err = fmt.Errorf("http: error connecting to proxy %s: %v", cm.proxyURL, err)