Add --test to gen_tasks.go, add presubmit check

infra/bots/gen_tasks.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.
 
 package main
 
 /*
         Generate the tasks.json file.
 */
 
 import (
         "bytes"
         "encoding/json"
+        "flag"
         "fmt"
         "io/ioutil"
         "os"
         "path"
         "path/filepath"
         "sort"
         "strings"
 
         "github.com/skia-dev/glog"
         "go.skia.org/infra/go/common"
@@ skipping 31 matching lines @@
 
         // Defines the structure of job names.
         jobNameSchema *JobNameSchema
 
         // Caches CIPD package info so that we don't have to re-read VERSION
         // files.
         cipdPackages = map[string]*specs.CipdPackage{}
 
         // Path to the infra/bots directory.
         infrabotsDir = ""
+
+        // Flags.
+        testing = flag.Bool("test", false, "Run in test mode: verify that the output hasn't changed.")
 )
 
 // deriveCompileTaskName returns the name of a compile task based on the given
 // job name.
 func deriveCompileTaskName(jobName string, parts map[string]string) string {
         if parts["role"] == "Housekeeper" {
                 return "Build-Ubuntu-GCC-x86_64-Release-Shared"
         } else if parts["role"] == "Test" || parts["role"] == "Perf" {
                 task_os := parts["os"]
                 ec := parts["extra_config"]
@@ skipping 410 matching lines @@
         for _, j := range JOBS {
                 process(cfg, j)
         }
 
         // Validate the config.
         if err := cfg.Validate(); err != nil {
                 glog.Fatal(err)
         }
 
         // Write the tasks.json file.
-        outFile := path.Join(root, specs.TASKS_CFG_FILE)
         b, err := json.MarshalIndent(cfg, "", "  ")
         if err != nil {
                 glog.Fatal(err)
         }
         // The json package escapes HTML characters, which makes our output
         // much less readable. Replace the escape characters with the real
         // character.
         b = bytes.Replace(b, []byte("\\u003c"), []byte("<"), -1)
-        if err := ioutil.WriteFile(outFile, b, os.ModePerm); err != nil {
-                glog.Fatal(err)
+
+        outFile := path.Join(root, specs.TASKS_CFG_FILE)
+        if *testing {
+                // Don't write the file; read it and compare.
+                expect, err := ioutil.ReadFile(outFile)
+                if err != nil {
+                        glog.Fatal(err)
+                }
+                if !bytes.Equal(expect, b) {
+                        glog.Fatalf("Expected no changes, but changes were found!")
+                }
+        } else {
+                if err := ioutil.WriteFile(outFile, b, os.ModePerm); err != nil {
+                        glog.Fatal(err)
+                }
         }
 }
 
 // TODO(borenet): The below really belongs in its own file, probably next to the
 // builder_name_schema.json file.
 
 // JobNameSchema is a struct used for (de)constructing Job names in a
 // predictable format.
 type JobNameSchema struct {
         Schema map[string][]string `json:"builder_name_schema"`
@@ skipping 64 matching lines @@
                 if !ok {
                         return "", fmt.Errorf("Invalid job parts; missing %q", k)
                 }
                 rvParts = append(rvParts, v)
         }
         if _, ok := parts["extra_config"]; ok {
                 rvParts = append(rvParts, parts["extra_config"])
         }
         return strings.Join(rvParts, s.Sep), nil
 }