LogDog: Implement prefix registration in Butler.

client/cmd/logdog_butler/output.go

 // Copyright 2015 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.
 
 package main
 
 import (
+        "sort"
+
         "github.com/luci/luci-go/client/internal/logdog/butler/output"
         "github.com/luci/luci-go/common/flag/multiflag"
 )
 
 type outputFactory interface {
         option() multiflag.Option
         configOutput(a *application) (output.Output, error)
+        scopes() []string
 }
 
 // outputConfigFlag instance that produces a MessageOutput instance when run.
 type outputConfigFlag struct {
         multiflag.MultiFlag
 }
 
 // Adds an output factory to this outputConfigFlag instance.
 func (ocf *outputConfigFlag) AddFactory(f outputFactory) {
         ocf.Options = append(ocf.Options, f.option())
@@ skipping 38 matching lines @@
 // Registers a new output option. This is meant to be called by 'init()' methods
 // of each option.
 func registerOutputFactory(f outputFactory) {
         outputFactories = append(outputFactories, f)
 }
 
 // Returns a slice of registered output options.
 func getOutputFactories() []outputFactory {
         return outputFactories
 }
+
+func allOutputScopes() []string {
+        scopes := make(map[string]struct{})
+        for _, of := range outputFactories {
+                for _, scope := range of.scopes() {
+                        scopes[scope] = struct{}{}
+                }
+        }
+
+        allScopes := make([]string, 0, len(scopes))
+        for scope := range scopes {
+                allScopes = append(allScopes, scope)
+        }
+        sort.Strings(allScopes)
+        return allScopes
+}