net/http: add Transport.Clone

Fixes #26013

Change-Id: I2c82bd90ea7ce6f7a8e5b6c460d3982dca681a93
Reviewed-on: https://go-review.googlesource.com/c/go/+/174597Reviewed-by: Andrew Bonventre <andybons@golang.org>

src/net/http/transport.go

@@ -261,13 +261,14 @@ type Transport struct {
 
 	// ReadBufferSize specifies the size of the read buffer used
 	// when reading from the transport.
-	//If zero, a default (currently 4KB) is used.
+	// If zero, a default (currently 4KB) is used.
 	ReadBufferSize int
 
 	// nextProtoOnce guards initialization of TLSNextProto and
 	// h2transport (via onceSetNextProtoDefaults)
-	nextProtoOnce sync.Once
-	h2transport   h2Transport // non-nil if http2 wired up
+	nextProtoOnce      sync.Once
+	h2transport        h2Transport // non-nil if http2 wired up
+	tlsNextProtoWasNil bool        // whether TLSNextProto was nil when the Once fired
 
 	// ForceAttemptHTTP2 controls whether HTTP/2 is enabled when a non-zero
 	// TLSClientConfig or Dial, DialTLS or DialContext func is provided. By default, use of any those fields conservatively
@@ -290,6 +291,40 @@ func (t *Transport) readBufferSize() int {
 	return 4 << 10
 }
 
+// Clone returns a deep copy of t's exported fields.
+func (t *Transport) Clone() *Transport {
+	t.nextProtoOnce.Do(t.onceSetNextProtoDefaults)
+	t2 := &Transport{
+		Proxy:                  t.Proxy,
+		DialContext:            t.DialContext,
+		Dial:                   t.Dial,
+		DialTLS:                t.DialTLS,
+		TLSClientConfig:        t.TLSClientConfig.Clone(),
+		TLSHandshakeTimeout:    t.TLSHandshakeTimeout,
+		DisableKeepAlives:      t.DisableKeepAlives,
+		DisableCompression:     t.DisableCompression,
+		MaxIdleConns:           t.MaxIdleConns,
+		MaxIdleConnsPerHost:    t.MaxIdleConnsPerHost,
+		MaxConnsPerHost:        t.MaxConnsPerHost,
+		IdleConnTimeout:        t.IdleConnTimeout,
+		ResponseHeaderTimeout:  t.ResponseHeaderTimeout,
+		ExpectContinueTimeout:  t.ExpectContinueTimeout,
+		ProxyConnectHeader:     t.ProxyConnectHeader.Clone(),
+		MaxResponseHeaderBytes: t.MaxResponseHeaderBytes,
+		ForceAttemptHTTP2:      t.ForceAttemptHTTP2,
+		WriteBufferSize:        t.WriteBufferSize,
+		ReadBufferSize:         t.ReadBufferSize,
+	}
+	if !t.tlsNextProtoWasNil {
+		npm := map[string]func(authority string, c *tls.Conn) RoundTripper{}
+		for k, v := range t.TLSNextProto {
+			npm[k] = v
+		}
+		t2.TLSNextProto = npm
+	}
+	return t2
+}
+
 // h2Transport is the interface we expect to be able to call from
 // net/http against an *http2.Transport that's either bundled into
 // h2_bundle.go or supplied by the user via x/net/http2.
@@ -303,6 +338,7 @@ type h2Transport interface {
 // onceSetNextProtoDefaults initializes TLSNextProto.
 // It must be called via t.nextProtoOnce.Do.
 func (t *Transport) onceSetNextProtoDefaults() {
+	t.tlsNextProtoWasNil = (t.TLSNextProto == nil)
 	if strings.Contains(os.Getenv("GODEBUG"), "http2client=0") {
 		return
 	}

src/net/http/transport_test.go

@@ -20,6 +20,7 @@ import (
 	"encoding/binary"
 	"errors"
 	"fmt"
+	"go/token"
 	"internal/nettrace"
 	"internal/testenv"
 	"io"
@@ -5320,3 +5321,53 @@ func TestTransportRequestWriteRoundTrip(t *testing.T) {
 		})
 	}
 }
+
+func TestTransportClone(t *testing.T) {
+	tr := &Transport{
+		Proxy:                  func(*Request) (*url.URL, error) { panic("") },
+		DialContext:            func(ctx context.Context, network, addr string) (net.Conn, error) { panic("") },
+		Dial:                   func(network, addr string) (net.Conn, error) { panic("") },
+		DialTLS:                func(network, addr string) (net.Conn, error) { panic("") },
+		TLSClientConfig:        new(tls.Config),
+		TLSHandshakeTimeout:    time.Second,
+		DisableKeepAlives:      true,
+		DisableCompression:     true,
+		MaxIdleConns:           1,
+		MaxIdleConnsPerHost:    1,
+		MaxConnsPerHost:        1,
+		IdleConnTimeout:        time.Second,
+		ResponseHeaderTimeout:  time.Second,
+		ExpectContinueTimeout:  time.Second,
+		ProxyConnectHeader:     Header{},
+		MaxResponseHeaderBytes: 1,
+		ForceAttemptHTTP2:      true,
+		TLSNextProto: map[string]func(authority string, c *tls.Conn) RoundTripper{
+			"foo": func(authority string, c *tls.Conn) RoundTripper { panic("") },
+		},
+		ReadBufferSize:  1,
+		WriteBufferSize: 1,
+	}
+	tr2 := tr.Clone()
+	rv := reflect.ValueOf(tr2).Elem()
+	rt := rv.Type()
+	for i := 0; i < rt.NumField(); i++ {
+		sf := rt.Field(i)
+		if !token.IsExported(sf.Name) {
+			continue
+		}
+		if rv.Field(i).IsZero() {
+			t.Errorf("cloned field t2.%s is zero", sf.Name)
+		}
+	}
+
+	if _, ok := tr2.TLSNextProto["foo"]; !ok {
+		t.Errorf("cloned Transport lacked TLSNextProto 'foo' key")
+	}
+
+	// But test that a nil TLSNextProto is kept nil:
+	tr = new(Transport)
+	tr2 = tr.Clone()
+	if tr2.TLSNextProto != nil {
+		t.Errorf("Transport.TLSNextProto unexpected non-nil")
+	}
+}