Add beginnings of new cr command

go/src/infra/tools/cr/help.go

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// The 'cr help' command. It is implemented here, rather than in the cmd/
+// package, because it needs to be able to reference the other subcommands.
+
+package main
+
+import (
+        "flag"
+        "fmt"
+
+        "infra/tools/cr/lib/subcommand"
+)
+
+var shortHelp = "Print help for a subcommand."
+
+var longHelp = `The help subcommand prints long-form help for the top-level cr command
+and its subcommands. Run 'cr help' for a list of available commands.
+
+Examples:
+  cr help             # print top-level help and list of commands
+  cr help firstrun    # print help for the 'firstrun' subcommand
+  cr firstrun --help  # same as above`
+
+func helpRun(flags *flag.FlagSet) error {
+        if flags.NArg() == 0 {
+                printCrHelp()
+                return nil
+        }
+        helpcmd := subcommands[flags.Arg(0)]

[seanmccullough1, 2016/04/29 00:54:36]

You could also say 

if helpcmd, ok := subcommands[...]; ok {
   helpcmd.InitFlags(...)
   ...
} else {
  return fmt.Errorf(...)
}

It's a little more idiomatic but it does increase the indent level.

(And map element access is one place where exact return value count is not
enforced, so the 'ok' value is always optional.)

[agable, 2016/05/05 23:59:43]

Shiny, didn't know about that. Done.

+        if helpcmd == nil {
+                return fmt.Errorf("Unrecognized subcommand for help '%v'.\n", flags.Arg(0))
+        }
+        helpcmd.InitFlags(flags)
+        helpcmd.Help(flags)
+        return nil
+}
+
+var helpCmd = subcommand.New(shortHelp, longHelp, nil, helpRun)

[seanmccullough1, 2016/04/29 00:54:36]

The existence of helpCmd here and helpcmd (lower c) above is kind of confusing,
but I think I see what you're doing now. Maybe rename helpcmd above as
cmdForHelp or something else more distinct?

[agable, 2016/05/05 23:59:43]

Good idea, done.