Add PRESUBMIT to make sure that perf JSON configs are always up-to-date

tools/perf/generate_perf_json.py

 #!/usr/bin/env python
 # 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.
 
 """Script to generate chromium.perf.json and chromium.perf.fyi.json in
 the src/testing/buildbot directory. Maintaining these files by hand is
 too unwieldy.
 """
-
+import argparse
 import json
 import os
 import sys
 
 from chrome_telemetry_build import chromium_config
 
 sys.path.append(chromium_config.GetTelemetryDir())
 from telemetry import benchmark as benchmark_module
 from telemetry.core import discover
 from telemetry.util import bot_utils
@@ skipping 750 matching lines @@
         if scripts:
           tests[tester_name] = {
             'scripts': sorted(scripts, key=lambda x: x['name'])
           }
 
   for name, config in waterfall['builders'].iteritems():
     tests[name] = config
 
   tests['AAAAA1 AUTOGENERATED FILE DO NOT EDIT'] = {}
   tests['AAAAA2 See //tools/perf/generate_perf_json.py to make changes'] = {}
+  return tests
+
+
+def get_json_config_file_for_waterfall(waterfall):
   filename = '%s.json' % waterfall['name']
+  buildbot_dir = os.path.join(src_dir(), 'testing', 'buildbot')
+  return os.path.join(buildbot_dir, filename)
 
-  buildbot_dir = os.path.join(src_dir(), 'testing', 'buildbot')
-  with open(os.path.join(buildbot_dir, filename), 'w') as fp:
+
+def tests_are_up_to_date(waterfall):
+  tests = generate_all_tests(waterfall)
+  tests_data = json.dumps(tests, indent=2, separators=(',', ': '),
+                          sort_keys=True)
+  config_file = get_json_config_file_for_waterfall(waterfall)
+  with open(config_file, 'r') as fp:
+    config_data = fp.read().strip()
+  return tests_data == config_data
+
+
+def update_all_tests(waterfall):
+  tests = generate_all_tests(waterfall)
+  config_file = get_json_config_file_for_waterfall(waterfall)
+  with open(config_file, 'w') as fp:
     json.dump(tests, fp, indent=2, separators=(',', ': '), sort_keys=True)
     fp.write('\n')
 
+
 def src_dir():
   file_path = os.path.abspath(__file__)
   return os.path.dirname(os.path.dirname(os.path.dirname(file_path)))
 
-def main():
+
+def main(args):
+  parser = argparse.ArgumentParser(
+      description=('Generate perf test\' json config. This need to be done '
+                   'anytime you add/remove any existing benchmarks in '
+                   'tools/perf/benchmarks.'))
+  parser.add_argument(
+      '--validate-only', action='store_true', default=False,
+      help=('Validate whether the perf json generated will be the same as the '
+            'existing configs. This does not change the contain of existing '
+            'configs'))
+  options = parser.parse_args(args)
+
   waterfall = get_waterfall_config()
   waterfall['name'] = 'chromium.perf'
   fyi_waterfall = get_fyi_waterfall_config()
   fyi_waterfall['name'] = 'chromium.perf.fyi'
 
-  generate_all_tests(fyi_waterfall)
-  generate_all_tests(waterfall)
+  if options.validate_only:
+    if tests_are_up_to_date(fyi_waterfall) and tests_are_up_to_date(waterfall):
+      print 'All the perf JSON config files are up-to-date. \\o/'
+      return 0
+    else:
+      print ('The perf JSON config files are not up-to-date. Please run %s '
+             'without --validate-only flag to update the perf JSON '
+             'configs.') % sys.argv[0]
+      return 1
+  else:
+    update_all_tests(fyi_waterfall)
+    update_all_tests(waterfall)
   return 0
 
 if __name__ == '__main__':
-  sys.exit(main())
+  sys.exit(main(sys.argv[1:]))