net/http: document GODEBUG settings, update bundled http2

Document the three GODEBUG environment variables in the package doc.

Updates the bundled http2 to x/net git rev 415f1917
for https://golang.org/cl/18372.

Fixes #13611

Change-Id: I3116c5d7de70d3d15242d7198f3758b1fb7d94b9
Reviewed-on: https://go-review.googlesource.com/18373Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/doc.go

@@ -76,5 +76,20 @@ custom Server:
 		MaxHeaderBytes: 1 << 20,
 	}
 	log.Fatal(s.ListenAndServe())
+
+The http package has transparent support for the HTTP/2 protocol when
+using HTTPS. Programs that must disable HTTP/2 can do so by setting
+Transport.TLSNextProto (for clients) or Server.TLSNextProto (for
+servers) to a non-nil, empty map. Alternatively, the following GODEBUG
+environment variables are currently supported:
+
+	GODEBUG=http2client=0  # disable HTTP/2 client support
+	GODEBUG=http2server=0  # disable HTTP/2 server support
+	GODEBUG=http2debug=1   # enable verbose HTTP/2 debug logs
+	GODEBUG=http2debug=2   # ... even more verbose, with frame dumps
+
+The GODEBUG variables are not covered by Go's API compatibility promise.
+HTTP/2 support was added in Go 1.6. Please report any issues instead of
+disabling HTTP/2 support: https://golang.org/s/http2bug
 */
 package http

src/net/http/server.go

@@ -2267,6 +2267,9 @@ func (srv *Server) setupHTTP2() error {
 // configured otherwise. (by setting srv.TLSNextProto non-nil)
 // It must only be called via srv.nextProtoOnce (use srv.setupHTTP2).
 func (srv *Server) onceSetNextProtoDefaults() {
+	if strings.Contains(os.Getenv("GODEBUG"), "http2server=0") {
+		return
+	}
 	// Enable HTTP/2 by default if the user hasn't otherwise
 	// configured their TLSNextProto map.
 	if srv.TLSNextProto == nil {

src/net/http/transport.go

@@ -151,7 +151,7 @@ type Transport struct {
 // onceSetNextProtoDefaults initializes TLSNextProto.
 // It must be called via t.nextProtoOnce.Do.
 func (t *Transport) onceSetNextProtoDefaults() {
-	if strings.Contains(os.Getenv("GODEBUG"), "h2client=0") {
+	if strings.Contains(os.Getenv("GODEBUG"), "http2client=0") {
 		return
 	}
 	if t.TLSNextProto != nil {

src/vendor/golang.org/x/net/http2/hpack/hpack.go

@@ -41,6 +41,14 @@ type HeaderField struct {
 	Sensitive bool
 }
 
+func (hf HeaderField) String() string {
+	var suffix string
+	if hf.Sensitive {
+		suffix = " (sensitive)"
+	}
+	return fmt.Sprintf("header field %q = %q%s", hf.Name, hf.Value, suffix)
+}
+
 func (hf *HeaderField) size() uint32 {
 	// http://http2.github.io/http2-spec/compression.html#rfc.section.4.1
 	// "The size of the dynamic table is the sum of the size of