LogDog: Use server/auth for authentication.

logdog/server/service/rt.go

Index: logdog/server/service/rt.go
diff --git a/logdog/server/service/rt.go b/logdog/server/service/rt.go
new file mode 100644
index 0000000000000000000000000000000000000000..4630b84c33071ea709c92f675572cb322f76e2bc
--- /dev/null
+++ b/logdog/server/service/rt.go
@@ -0,0 +1,40 @@
+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package service
+
+import (
+	"net/http"
+	"strings"
+
+	commonAuth "github.com/luci/luci-go/common/auth"
+)
+
+type serviceModifyingTransport struct {
+	userAgent string
+}
+
+func (smt *serviceModifyingTransport) roundTripper(base http.RoundTripper) http.RoundTripper {
+	if base == nil {
+		base = http.DefaultTransport
+	}
+
+	return commonAuth.NewModifyingTransport(base, func(req *http.Request) error {
+		// Set a user agent based on our service. If an existing user agent is
+		// specified, add it onto the end of the string.
+		parts := make([]string, 0, 2)
+		for _, part := range []string{
+			smt.userAgent,
+			req.UserAgent(),
+		} {
+			if part != "" {
+				parts = append(parts, part)
+			}
+		}
+		if len(parts) > 0 {
+			req.Header.Set("User-Agent", strings.Join(parts, " / "))
+		}
+		return nil
+	})
+}