Use native Pub/Sub library primitives.

client/cmd/logdog_butler/main.go

Index: client/cmd/logdog_butler/main.go
diff --git a/client/cmd/logdog_butler/main.go b/client/cmd/logdog_butler/main.go
index e4a39c7e8fbba611650282d743a7ca3cec08f7fb..fcd50a4e8028d446b9466d63933ffda5d4dd9faf 100644
--- a/client/cmd/logdog_butler/main.go
+++ b/client/cmd/logdog_butler/main.go
@@ -29,6 +29,7 @@ import (
 	"github.com/luci/luci-go/common/paniccatcher"
 	"github.com/maruel/subcommands"
 	"golang.org/x/net/context"
+	"golang.org/x/oauth2"
 )
 
 const (
@@ -117,15 +118,23 @@ func (a *application) addFlags(fs *flag.FlagSet) {
 			"of wire-format efficiency.")
 }
 
+func (a *application) authenticator(ctx context.Context) (*auth.Authenticator, error) {
+	opts, err := a.authFlags.Options()
+	if err != nil {
+		return nil, err
+	}
+	opts.Context = ctx
+	return auth.NewAuthenticator(auth.SilentLogin, opts), nil
+}
+
 func (a *application) authenticatedClient(ctx context.Context) (*http.Client, error) {
 	if a.client == nil {
-		opts, err := a.authFlags.Options()
+		authenticator, err := a.authenticator(ctx)
 		if err != nil {
 			return nil, err
 		}
-		opts.Context = ctx
 
-		client, err := auth.NewAuthenticator(auth.SilentLogin, opts).Client()
+		client, err := authenticator.Client()
 		if err != nil {
 			return nil, err
 		}
@@ -134,6 +143,14 @@ func (a *application) authenticatedClient(ctx context.Context) (*http.Client, er
 	return a.client, nil
 }
 
+func (a *application) tokenSource(ctx context.Context) (oauth2.TokenSource, error) {
+	authenticator, err := a.authenticator(ctx)
+	if err != nil {
+		return nil, err
+	}
+	return authenticator.TokenSource(), nil
+}
+
 func (a *application) configOutput() (output.Output, error) {
 	factory := a.outputConfig.getFactory()
 	if factory == nil {