Merge pull request #32231 from timothysc/re-re-http2

Automatic merge from submit-queue

Default HTTP2 on, post fixes from #29001

This reverts commit 8cb799c.

Enables HTTP2 on by default post fixes from #29001 for 1.5 

NOTE:  We are nearing lb connection limits at current thresholds. 

/cc @bradfitz @lavalamp @smarterclayton

pkg/genericapiserver/genericapiserver.go

@@ -251,6 +251,9 @@ func (s *GenericAPIServer) Run(options *options.ServerRunOptions) {
 			secureServer.TLSConfig.ClientAuth = tls.RequestClientCert
 			// Specify allowed CAs for client certificates
 			secureServer.TLSConfig.ClientCAs = clientCAs
+			// "h2" NextProtos is necessary for enabling HTTP2 for go's 1.7 HTTP Server
+			secureServer.TLSConfig.NextProtos = []string{"h2"}
+
 		}
 
 		glog.Infof("Serving securely on %s", secureLocation)

pkg/util/net/http.go

@@ -77,8 +77,10 @@ func SetOldTransportDefaults(t *http.Transport) *http.Transport {
 // for the Proxy, Dial, and TLSHandshakeTimeout fields if unset
 func SetTransportDefaults(t *http.Transport) *http.Transport {
 	t = SetOldTransportDefaults(t)
-	// Allow HTTP2 clients but default off for now
-	if s := os.Getenv("ENABLE_HTTP2"); len(s) > 0 {
+	// Allow clients to disable http2 if needed.
+	if s := os.Getenv("DISABLE_HTTP2"); len(s) > 0 {
+		glog.Infof("HTTP2 has been explicitly disabled")
+	} else {
 		if err := http2.ConfigureTransport(t); err != nil {
 			glog.Warningf("Transport failed http2 configuration: %v", err)
 		}