[auto_update] Add control script, allow disable.

Enable a sentinel file to enable/disable "depot_tools" auto-updating.
This file, if present, indicates that automatic updating should not
occur.

Add a simple control script to enable/disable automatic updating.

BUG=chromium:727917
TEST=local
  - Ran locally, --enable, --disable.

R=iannucci@chromium.org, maruel@chromium.org, nodir@chromium.org

Change-Id: Id30a17586248228849d046f79d7e0bc445d97f97
Reviewed-on: https://chromium-review.googlesource.com/521884
Commit-Queue: Daniel Jacques <dnj@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>

.gitignore

@@ -6,6 +6,9 @@
 /.cipd_client*
 /.versions
 
+# Ignore "disable auto update" sentinel file.
+.disable_auto_update
+
 # Ignore the batch files produced by the Windows bootstrapping.
 /git-bash
 /git.bat

update_depot_tools

@@ -39,6 +39,10 @@ else
   fi
 fi
 
+if [ -e $base_dir/.disable_auto_update ]; then
+  exit
+fi
+
 # We want to update the bundled tools even under MinGW.
 if [ $MINGW = 0 ]; then
   $COMSPEC /c `cygpath -w "$base_dir/bootstrap/win/win_tools.bat"`

update_depot_tools.bat

@@ -19,6 +19,8 @@ IF "%~nx0"=="update_depot_tools.bat" (
 set DEPOT_TOOLS_DIR=%~1
 SHIFT
 
+IF EXIST "%DEPOT_TOOLS_DIR%.disable_auto_update" GOTO :EOF
+
 set GIT_URL=https://chromium.googlesource.com/chromium/tools/depot_tools.git
 
 :: Will download git, svn and python.

update_depot_tools_toggle.py

@@ -0,0 +1,38 @@
+#!/usr/bin/env python
+# Copyright (c) 2017 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.
+
+"""Small utility script to enable/disable `depot_tools` automatic updating."""
+
+import argparse
+import datetime
+import os
+import sys
+
+
+DEPOT_TOOLS_ROOT = os.path.abspath(os.path.dirname(__file__))
+SENTINEL_PATH = os.path.join(DEPOT_TOOLS_ROOT, '.disable_auto_update')
+
+
+def main():
+  parser = argparse.ArgumentParser()
+  group = parser.add_mutually_exclusive_group(required=True)
+  group.add_argument('--enable', action='store_true',
+      help='Enable auto-updating.')
+  group.add_argument('--disable', action='store_true',
+      help='Disable auto-updating.')
+  args = parser.parse_args()
+
+  if args.enable:
+    if os.path.exists(SENTINEL_PATH):
+      os.unlink(SENTINEL_PATH)
+  if args.disable:
+    if not os.path.exists(SENTINEL_PATH):
+      with open(SENTINEL_PATH, 'w') as fd:
+        fd.write('Disabled by %s at %s\n' % (__file__, datetime.datetime.now()))
+  return 0
+
+
+if __name__ == '__main__':
+  sys.exit(main())