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"
+        "os"
+        "text/tabwriter"
+)
+
+// 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 {
+        shortHelp string
+        longHelp  string
+        flagFn    func(*flag.FlagSet)
+        runFn     func(*flag.FlagSet) error
+}
+
+// Subcommands is a map of the string names of all registered subcommands to
+// the Subcommand structs holding their vital data.
+var Subcommands map[string]*Subcommand

[seanmccullough1, 2016/05/06 00:43:54]

var Subcommands = make(map[string]*Subcommand)

and you can get rid of the init() below.

[agable, 2016/05/10 00:34:11]

...I could have sworn I tried exactly that, and only did the init as a last
resort. Guess I somehow missed that, or messed something else up at the same
time. Thanks!

+
+// TabulateSubcommands prints out all registered Subcommand names and their
+// short help strings. If no formatter is supplied, it uses a sane default.

[seanmccullough1, 2016/05/06 00:43:54]

Might want to mention that it flushes formatter before returning, or just not
make it a parameter until you actually need to (if ever). 

[agable, 2016/05/10 00:34:11]

Yeah, removed the parameter.

+func TabulateSubcommands(formatter *tabwriter.Writer) {
+        if formatter == nil {
+                formatter = tabwriter.NewWriter(os.Stdout, 0, 0, 2, ' ', 0)
+        }
+        for key, value := range Subcommands {
+                fmt.Fprintf(formatter, "\t%v\t%v\n", key, value.shortHelp)
+        }
+        formatter.Flush()
+}
+
+// New creates a new Subcommand struct and returns a reference to it.
+func New(name string, shortHelp string, longHelp string, flagFn func(*flag.FlagSet), runFn func(*flag.FlagSet) error) *Subcommand {
+        ret := &Subcommand{shortHelp, longHelp, flagFn, runFn}
+        Subcommands[name] = ret

[seanmccullough1, 2016/05/06 00:43:54]

bonus points: check Subcommands for existence of name before adding it, and
return an error indicating a collision in that case. 

[agable, 2016/05/10 00:34:11]

Done

+        return ret
+}
+
+// Help prints the short and long help messages, as well as the autogenerated
+// flags documentation.
+func (c *Subcommand) Help(flags *flag.FlagSet) {
+        fmt.Printf("%s\n\n%s\n\n", c.shortHelp, c.longHelp)
+        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
+}
+
+func init() {
+        Subcommands = map[string]*Subcommand{}
+}