Make user agent set via envvar, not CLI option

cipd/client/cmd/cipd/main.go

 // Copyright 2014 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 cipd implements a client of for Chrome Infra Package Deployer.
 //
 // Subcommand starting with 'pkg-' are low level commands operating on package
 // files on disk.
 package main
 
@@ skipping 217 matching lines @@
 
 ////////////////////////////////////////////////////////////////////////////////
 // ClientOptions mixin.
 
 // ClientOptions defines command line arguments related to CIPD client creation.
 // Subcommands that need a CIPD client embed it.
 type ClientOptions struct {
         authFlags  authcli.Flags
         serviceURL string
         cacheDir   string
-        userAgent  string
 }
 
 func (opts *ClientOptions) registerFlags(f *flag.FlagSet) {
         f.StringVar(&opts.serviceURL, "service-url", "", "URL of a backend to use instead of the default one.")
-        f.StringVar(&opts.userAgent, "http-user-agent", "",
-                "User Agent text to add. If specified, the UA will be '<this_option>/"+cipd.UserAgent+"'.")
         f.StringVar(&opts.cacheDir, "cache-dir", "", "Directory for shared cache")
         opts.authFlags.Register(f, auth.Options{})
 }
 
 func (opts *ClientOptions) makeCipdClient(ctx context.Context, root string) (cipd.Client, error) {
         authOpts, err := opts.authFlags.Options()
         if err != nil {
                 return nil, err
         }
         client, err := auth.NewAuthenticator(ctx, auth.OptionalLogin, authOpts).Client()
         if err != nil {
                 return nil, err
         }
         ua := cipd.UserAgent
-        if opts.userAgent != "" {
-                ua = fmt.Sprintf("%s/%s", opts.userAgent, ua)
+        if prefix := os.Getenv("CIPD_HTTP_USER_AGENT_PREFIX"); prefix != "" {
+                ua = fmt.Sprintf("%s/%s", prefix, ua)
         }
         return cipd.NewClient(cipd.ClientOptions{
                 ServiceURL:          opts.serviceURL,
                 Root:                root,
                 UserAgent:           ua,
                 CacheDir:            opts.cacheDir,
                 AuthenticatedClient: client,
                 AnonymousClient:     http.DefaultClient,
         }), nil
 }
@@ skipping 1807 matching lines @@
 
 func main() {
         var seed int64
         if err := binary.Read(cryptorand.Reader, binary.LittleEndian, &seed); err != nil {
                 panic(err)
         }
         rand.Seed(seed)
 
         os.Exit(subcommands.Run(application, fixFlagsPosition(os.Args[1:])))
 }