Add beginnings of new cr command

go/src/infra/tools/cr/lib/subcommand/subcommand.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.
+
+/*
+Library for defining subcommands in a structured way.
+*/
+
+// Package subcommand provides a simple framework for defining subcommands.
+package subcommand
+
+import (
+        "flag"
+        "fmt"
+)
+
+// Subcommand represents a single subcommand, with short and long versions of
+// its help (usage) text, a function which adds command-line flags to a
+// pre-existing flag.FlagSet, and a function which will be called to actually
+// execute the subcommand.
+type Subcommand struct {

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

SubCmd would be a fine name too.

+        ShortHelp string

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

godoc for ShortHelp or don't export it (doesn't appear to be a necessary export,
FWIW). Exported structs with no public fields (just public methods) are actually
pretty common. 

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

main.go directly access ShortHelp to print the list of all available
subcommands. But since I moved the list of Subcommands into the subcommand
package, I no longer need to export either of these.

+        LongHelp  string
+        flagFn    func(*flag.FlagSet)
+        runFn     func(*flag.FlagSet) error
+}
+
+// New creates a new Subcommand struct and returns a reference to it.
+func New(shortHelp string, longHelp string, flagFn func(*flag.FlagSet), runFn func(*flag.FlagSet) error) *Subcommand {
+        return &Subcommand{shortHelp, longHelp, flagFn, runFn}
+}
+
+// Help prints the short and long help messages, as well as the autogenerated
+// flags documentation.
+func (c *Subcommand) Help(flags *flag.FlagSet) {
+        fmt.Println(c.ShortHelp)

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

fmt.Printf("%s\n\n%s\n\n", c.ShortHelp, c.LongHelp)

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

Done.

+        fmt.Println("")
+        fmt.Println(c.LongHelp)
+        fmt.Println("")
+        flags.PrintDefaults()
+}
+
+// InitFlags adds the subcommand's flags to a FlagSet, if flagFn is defined.
+func (c *Subcommand) InitFlags(flags *flag.FlagSet) {
+        if c.flagFn != nil {
+                c.flagFn(flags)
+        }
+}
+
+// Run executes the subcommand's runFn, if it is defined.
+func (c *Subcommand) Run(flags *flag.FlagSet) error {
+        if c.runFn != nil {
+                return c.runFn(flags)
+        }
+        return nil
+}