[Extern Generation] Add a presubmit script to check externs not being updated

extensions/common/api/externs_checker.py

Index: extensions/common/api/externs_checker.py
diff --git a/extensions/common/api/externs_checker.py b/extensions/common/api/externs_checker.py
new file mode 100644
index 0000000000000000000000000000000000000000..3c59a81420507d3dbdd038f151afeb0e02330d32
--- /dev/null
+++ b/extensions/common/api/externs_checker.py
@@ -0,0 +1,34 @@
+# 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.
+
+class ExternsChecker(object):
+  _UPDATE_MESSAGE = """To update the externs, run:
+ src/ $ python tools/json_schema_compiler/compiler.py\
+ %s --root=. --generator=externs > %s"""
+
+  def __init__(self, input_api, output_api, api_pairs):
+    self._input_api = input_api
+    self._output_api = output_api
+    self._api_pairs = api_pairs
+
+  def _GetWarningText(self, entries):
+    replacements = (('<source_file>', '<output_file>') if len(entries) > 1 else
+        (entries[0]['source'], entries[0]['extern']))
+    return self._UPDATE_MESSAGE % replacements
+
+  def RunChecks(self):
+    bad_files = []
+    affected = [f.AbsoluteLocalPath() for f in self._input_api.AffectedFiles()]
+    for path in affected:
+      pair = self._api_pairs.get(path)
+      if pair != None and pair not in affected:
+        bad_files.append({'source': path, 'extern': pair})
+    results = []
+    if bad_files:
+      results.append(self._output_api.PresubmitPromptWarning(
+          str('Found updated extension api files without updated extern files. '
+              'Please update the extern files.'),
+          [f['source'] for f in bad_files],
+          self._GetWarningText(bad_files)))
+    return results