Add beginnings of new cr command

go/src/infra/tools/cr/lib/subcommand/subcommand.go

Index: go/src/infra/tools/cr/lib/subcommand/subcommand.go
diff --git a/go/src/infra/tools/cr/lib/subcommand/subcommand.go b/go/src/infra/tools/cr/lib/subcommand/subcommand.go
new file mode 100644
index 0000000000000000000000000000000000000000..f36c8472a693a917412444cc81c46e66939e6b17
--- /dev/null
+++ b/go/src/infra/tools/cr/lib/subcommand/subcommand.go
@@ -0,0 +1,107 @@
+// 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"
+	"sort"
+	"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
+}
+
+// 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
+}
+
+// Private registry holding a map of subcommands. Users of this library only
+// have access to a default instance of the registry via the New and Tabulate
+// functions, but tests can instantiate one directly.
+type registry struct {
+	subcommands map[string]*Subcommand
+}
+
+func (r *registry) new(name string, shortHelp string, longHelp string, flagFn func(*flag.FlagSet), runFn func(*flag.FlagSet) error) *Subcommand {
+	if _, ok := r.subcommands[name]; ok {
+		panic(fmt.Errorf("attempted to double-register subcommand %q", name))
+	}
+	ret := &Subcommand{shortHelp, longHelp, flagFn, runFn}
+	r.subcommands[name] = ret
+	return ret
+}
+
+func (r *registry) get(name string) *Subcommand {
+	s, ok := r.subcommands[name]
+	if !ok {
+		return nil
+	}
+	return s
+}
+
+func (r *registry) tabulate() {
+	colwriter := tabwriter.NewWriter(os.Stdout, 0, 0, 2, ' ', 0)
+	keys := make([]string, 0, len(r.subcommands))
+	for key := range r.subcommands {
+		keys = append(keys, key)
+	}
+	sort.Strings(keys)
+	for _, key := range keys {
+		fmt.Fprintf(colwriter, "\t%v\t%v\n", key, r.subcommands[key].shortHelp)
+	}
+	colwriter.Flush()
+}
+
+var defaultRegistry = &registry{make(map[string]*Subcommand)}
+
+// New creates, registers, and returns a new Subcommand struct.
+// Panics if you attempt to register a second command with the same name.
+func New(name string, shortHelp string, longHelp string, flagFn func(*flag.FlagSet), runFn func(*flag.FlagSet) error) *Subcommand {
+	return defaultRegistry.new(name, shortHelp, longHelp, flagFn, runFn)
+}
+
+// Get returns a Subcommand struct which has been previously registered.
+func Get(name string) *Subcommand {
+	return defaultRegistry.get(name)
+}
+
+// Tabulate prints out all registered Subcommand names and their
+// short help strings.
+func Tabulate() {
+	defaultRegistry.tabulate()
+}