net/http: enable HTTP/2 support in DefaultTransport

The GODEBUG option remains, for now, but only for turning it off.
We'll decide what to do with it before release.

This CL includes the dependent http2 change (https://golang.org/cl/16692)
in the http2 bundle (h2_bundle.go).

Updates #6891

Change-Id: If9723ef627c7ba4f7343dc8cb89ca88ef0fbcb10
Reviewed-on: https://go-review.googlesource.com/16693
Reviewed-by: Blake Mizerany <[email protected]>
Run-TryBot: Brad Fitzpatrick <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: bradfitz

src/net/http/h2_bundle.go

@@ -44,23 +44,21 @@ var DefaultTransport RoundTripper = &Transport{
 	ExpectContinueTimeout: 1 * time.Second,
 }
 
+// Wire up HTTP/2 support to the DefaultTransport, unless GODEBUG=h2client=0.
 func init() {
-	// TODO(bradfitz,adg): remove the following line before Go 1.6
-	// ships.  This just gives us a mechanism to temporarily
-	// enable the http2 client during development.
-	if !strings.Contains(os.Getenv("GODEBUG"), "h2client=1") {
+	if strings.Contains(os.Getenv("GODEBUG"), "h2client=0") {
 		return
 	}
-
 	t := DefaultTransport.(*Transport)
-
-	// TODO(bradfitz,adg): move all this up to DefaultTransport before Go 1.6:
 	t.RegisterProtocol("https", noDialH2Transport{h2DefaultTransport})
 	t.TLSClientConfig = &tls.Config{
 		NextProtos: []string{"h2"},
 	}
 	t.TLSNextProto = map[string]func(string, *tls.Conn) RoundTripper{
-		"h2": http2TransportForConn,
+		"h2": func(authority string, c *tls.Conn) RoundTripper {
+			h2DefaultTransport.AddIdleConn(authority, c)
+			return h2DefaultTransport
+		},
 	}
 }
 
@@ -69,14 +67,11 @@ func init() {
 type noDialH2Transport struct{ rt *http2Transport }
 
 func (t noDialH2Transport) RoundTrip(req *Request) (*Response, error) {
-	// TODO(bradfitz): wire up http2.Transport
-	return nil, ErrSkipAltProtocol
-}
-
-func http2TransportForConn(authority string, c *tls.Conn) RoundTripper {
-	// TODO(bradfitz): donate c to h2DefaultTransport:
-	// h2DefaultTransport.AddIdleConn(authority, c)
-	return h2DefaultTransport
+	res, err := t.rt.RoundTripOpt(req, http2RoundTripOpt{OnlyCachedConn: true})
+	if err == http2ErrNoCachedConn {
+		return nil, ErrSkipAltProtocol
+	}
+	return res, err
 }
 
 // DefaultMaxIdleConnsPerHost is the default value of Transport's