net/http: update bundled http2, add test for Transport's User-Agent behavior

Adds a test that both http1 and http2's Transport send a default
User-Agent, with the same behavior.

Updates bundled http2 to golang.org/x/net git rev 1ade16a545 (for
https://go-review.googlesource.com/18285)

The http1 behavior changes slightly: if req.Header["User-Agent"] is
defined at all, even if it's nil or a zero-length slice, then the
User-Agent header is omitted. This is a slight behavior change for
http1, but is consistent with how http1 & http2 do optional headers
elsewhere (such as "Date", "Content-Type"). The old behavior (set it
explicitly to "", aka []string{""}) still works as before. And now
there are even tests.

Fixes #13685

Change-Id: I5786a6913b560de4a5f1f90e595fe320ff567adf
Reviewed-on: https://go-review.googlesource.com/18284
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/clientserver_test.go

@@ -734,3 +734,64 @@ func testConnectRequest(t *testing.T, h2 bool) {
 		}
 	}
 }
+
+func TestTransportUserAgent_h1(t *testing.T) { testTransportUserAgent(t, h1Mode) }
+func TestTransportUserAgent_h2(t *testing.T) { testTransportUserAgent(t, h2Mode) }
+func testTransportUserAgent(t *testing.T, h2 bool) {
+	defer afterTest(t)
+	cst := newClientServerTest(t, h2, HandlerFunc(func(w ResponseWriter, r *Request) {
+		fmt.Fprintf(w, "%q", r.Header["User-Agent"])
+	}))
+	defer cst.close()
+
+	either := func(a, b string) string {
+		if h2 {
+			return b
+		}
+		return a
+	}
+
+	tests := []struct {
+		setup func(*Request)
+		want  string
+	}{
+		{
+			func(r *Request) {},
+			either(`["Go-http-client/1.1"]`, `["Go-http-client/2.0"]`),
+		},
+		{
+			func(r *Request) { r.Header.Set("User-Agent", "foo/1.2.3") },
+			`["foo/1.2.3"]`,
+		},
+		{
+			func(r *Request) { r.Header["User-Agent"] = []string{"single", "or", "multiple"} },
+			`["single"]`,
+		},
+		{
+			func(r *Request) { r.Header.Set("User-Agent", "") },
+			`[]`,
+		},
+		{
+			func(r *Request) { r.Header["User-Agent"] = nil },
+			`[]`,
+		},
+	}
+	for i, tt := range tests {
+		req, _ := NewRequest("GET", cst.ts.URL, nil)
+		tt.setup(req)
+		res, err := cst.c.Do(req)
+		if err != nil {
+			t.Errorf("%d. RoundTrip = %v", i, err)
+			continue
+		}
+		slurp, err := ioutil.ReadAll(res.Body)
+		res.Body.Close()
+		if err != nil {
+			t.Errorf("%d. read body = %v", i, err)
+			continue
+		}
+		if string(slurp) != tt.want {
+			t.Errorf("%d. body mismatch.\n got: %s\nwant: %s\n", i, slurp, tt.want)
+		}
+	}
+}

src/net/http/h2_bundle.go

@@ -24,7 +24,6 @@ import (
 	"encoding/binary"
 	"errors"
 	"fmt"
-	"golang.org/x/net/http2/hpack"
 	"io"
 	"io/ioutil"
 	"log"
@@ -38,6 +37,8 @@ import (
 	"strings"
 	"sync"
 	"time"
+
+	"golang.org/x/net/http2/hpack"
 )
 
 // ClientConnPool manages a pool of HTTP/2 client connections.
@@ -4095,6 +4096,8 @@ const (
 	// transportDefaultStreamMinRefresh is the minimum number of bytes we'll send
 	// a stream-level WINDOW_UPDATE for at a time.
 	http2transportDefaultStreamMinRefresh = 4 << 10
+
+	http2defaultUserAgent = "Go-http-client/2.0"
 )
 
 // Transport is an HTTP/2 Transport.
@@ -4794,11 +4797,23 @@ func (cc *http2ClientConn) encodeHeaders(req *Request, addGzipHeader bool, trail
 		cc.writeHeader("trailer", trailers)
 	}
 
+	var didUA bool
 	for k, vv := range req.Header {
 		lowKey := strings.ToLower(k)
 		if lowKey == "host" {
 			continue
 		}
+		if lowKey == "user-agent" {
+
+			didUA = true
+			if len(vv) < 1 {
+				continue
+			}
+			vv = vv[:1]
+			if vv[0] == "" {
+				continue
+			}
+		}
 		for _, v := range vv {
 			cc.writeHeader(lowKey, v)
 		}
@@ -4806,6 +4821,9 @@ func (cc *http2ClientConn) encodeHeaders(req *Request, addGzipHeader bool, trail
 	if addGzipHeader {
 		cc.writeHeader("accept-encoding", "gzip")
 	}
+	if !didUA {
+		cc.writeHeader("user-agent", http2defaultUserAgent)
+	}
 	return cc.hbuf.Bytes()
 }
 

src/net/http/request.go

@@ -427,10 +427,8 @@ func (req *Request) write(w io.Writer, usingProxy bool, extraHeaders Header, wai
 	// Use the defaultUserAgent unless the Header contains one, which
 	// may be blank to not send the header.
 	userAgent := defaultUserAgent
-	if req.Header != nil {
-		if ua := req.Header["User-Agent"]; len(ua) > 0 {
-			userAgent = ua[0]
-		}
+	if _, ok := req.Header["User-Agent"]; ok {
+		userAgent = req.Header.Get("User-Agent")
 	}
 	if userAgent != "" {
 		_, err = fmt.Fprintf(w, "User-Agent: %s\r\n", userAgent)