Add script to cleanup stale entries from fieldtrial configs json

This CL cleans up and submits a script originally written by engedy@chromium.org
with a few speed and convenience improvements:
- use multi-threading to reduce runtime from ~45m to ~8m
- add flags for controlling a few options
- automatically run the config formatter
- reduce the number of false positives
- make the script py2 (since chromium is still on py2)

Bug: 1053702
Change-Id: Ie5d211d0374f5db6352677821e27defa415c7d11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2637405Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Ehsan Kia <ehsankia@google.com>
Cr-Commit-Position: refs/heads/master@{#845695}

tools/variations/cleanup_stale_fieldtrial_configs.py

+# Copyright 2021 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.
+"""Simple script for cleaning up stale configs from fieldtrial_testing_config.
+
+Methodology:
+  Scan for all study names that appear in fieldtrial config file,
+  and removes ones that don't appear anywhere in the codebase.
+  The script ignores WebRTC entries as those often lead to false positives.
+
+Usage:
+  vpython tools/variations/cleanup_stale_fieldtrial_configs.py
+
+Run with --help to get a complete list of options this script runs with.
+
+If this script removes features that appear to be used in the codebase,
+double-check the study or feature name for typos or case differences.
+"""
+
+from __future__ import print_function
+
+import json
+import optparse
+import os
+import subprocess
+import sys
+import threading
+
+CONFIG_PATH = 'testing/variations/fieldtrial_testing_config.json'
+PRESUBMIT_SCRIPT = 'testing/variations/PRESUBMIT.py'
+THREAD_COUNT = 16
+
+_LITERAL_CACHE = {}
+
+
+def is_literal_used(literal):
+  """Check if a given string literal is used in the codebase."""
+  if literal in _LITERAL_CACHE:
+    return _LITERAL_CACHE[literal]
+
+  git_grep_cmd = ('git', 'grep', '--threads', '2', '-l', '\"%s\"' % literal)
+  git_grep_proc = subprocess.Popen(git_grep_cmd, stdout=subprocess.PIPE)
+  # Check for >1 since fieldtrial_testing_config.json will always be a result.
+  if len(git_grep_proc.stdout.read().splitlines()) > 1:
+    _LITERAL_CACHE[literal] = True
+    return True
+
+  bash_find_cmd = ('bash', '-c', 'find', '.', '-type', 'f', '|', 'grep', '-E',
+                   '\"\\.(h|cc)$\"', '|', 'grep', '-E',
+                   '\"(/out/|/build/|/gen/)\"', '|', 'xargs', 'grep', '-l',
+                   '\\\"%s\\\"' % literal)
+  bash_find_proc = subprocess.Popen(bash_find_cmd, stdout=subprocess.PIPE)
+  used = len(bash_find_proc.stdout.read().splitlines()) > 0
+  _LITERAL_CACHE[literal] = used
+  if not used:
+    print('Did not find', repr(literal))
+
+  return used
+
+
+def is_study_used(study_name, configs):
+  """Checks if a given study is used in the codebase."""
+  if study_name.startswith('WebRTC-'):
+    return True  # Skip webrtc studies which give false positives.
+
+  if is_literal_used(study_name):
+    return True
+  for config in configs:
+    for experiment in config.get('experiments', []):
+      for feature in experiment.get('enable_features', []):
+        if is_literal_used(feature):
+          return True
+      for feature in experiment.get('disable_features', []):
+        if is_literal_used(feature):
+          return True
+  return False
+
+
+def thread_func(thread_limiter, studies_map, study_name, configs):
+  """Runs a limited number of tasks and updates the map with the results.
+
+  Args:
+    thread_limited: A lock used to limit the number of active threads.
+    studies_map: The map where confirmed studies are added to.
+    study_name: The name of the study to check.
+    configs: The configs for the given study.
+
+  Side-effect:
+    This function adds the study to |studies_map| if it used.
+  """
+  thread_limiter.acquire()
+  try:
+    if is_study_used(study_name, configs):
+      studies_map[study_name] = configs
+  finally:
+    thread_limiter.release()
+
+
+def main():
+  parser = optparse.OptionParser()
+  parser.add_option('--input_path',
+                    help='Path to the fieldtrial config file to clean.')
+  parser.add_option('--output_path',
+                    help='Path to write cleaned up fieldtrial config file.')
+  parser.add_option('--thread_count',
+                    type='int',
+                    help='The number of threads to use for scanning.')
+
+  opts, _ = parser.parse_args()
+  input_path = os.path.expanduser(opts.input_path or CONFIG_PATH)
+  output_path = os.path.expanduser(opts.output_path or CONFIG_PATH)
+  thread_limiter = threading.BoundedSemaphore(opts.thread_count or THREAD_COUNT)
+
+  with open(input_path) as fin:
+    studies = json.load(fin)
+  print('Loaded config from', input_path)
+
+  threads = []
+  clean_studies = {}
+  for study_name, configs in studies.items():
+    args = (thread_limiter, clean_studies, study_name, configs)
+    threads.append(threading.Thread(target=thread_func, args=args))
+
+  # Start all threads, then join all threads.
+  for t in threads:
+    t.start()
+  for t in threads:
+    t.join()
+
+  with open(output_path, 'wt') as fout:
+    json.dump(clean_studies, fout)
+  print('Wrote cleaned config to', output_path)
+
+  # Run presubmit script to format config file.
+  retcode = subprocess.call(['vpython', PRESUBMIT_SCRIPT, output_path])
+  if retcode != 0:
+    print('Failed to format output, manually run:')
+    print('vpython', PRESUBMIT_SCRIPT, output_path)
+
+
+if __name__ == '__main__':
+  sys.exit(main())