Add presubmit script checking FlatBuffer changes.

This change introduces a presubmit check ensuring that developers modifying
.fbs files in subresource_filter get reminded to change IndexedRuleset format
version (if that is required).

BUG=609747

Review-Url: https://codereview.chromium.org/2850813003
Cr-Commit-Position: refs/heads/master@{#468618}

components/subresource_filter/core/common/PRESUBMIT.py

+# Copyright 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.
+
+"""Presubmit script for subresource_filter component's core/common directory.
+
+See https://www.chromium.org/developers/how-tos/depottools/presubmit-scripts
+for more details about the presubmit API built into depot_tools.
+"""
+
+def CheckIndexedRulesetVersion(input_api, output_api):
+  """ Checks that IndexedRuleset format version is modified when necessary.
+
+  Whenever a *.fbs or indexed_ruleset.cc file is touched in
+  components/subresource_filter/core/common and kIndexedFormatVersion constant
+  is not changed, this check returns a presubmit warning to make sure the value
+  should not be updated.
+  """
+
+  indexed_ruleset_changed = False
+  indexed_ruleset_version_changed = False
+
+  for affected_file in input_api.AffectedFiles():
+    path = affected_file.LocalPath()
+    if not 'components/subresource_filter/core/common' in path:
+      continue
+    basename = input_api.basename(path)
+
+    if basename.endswith('.fbs') or basename == 'indexed_ruleset.cc':
+      indexed_ruleset_changed = True
+    if basename == 'indexed_ruleset.cc':
+      for (_, line) in affected_file.ChangedContents():
+        if 'kIndexedFormatVersion =' in line:
+          indexed_ruleset_version_changed = True
+          break
+
+  if indexed_ruleset_changed and not indexed_ruleset_version_changed:
+    return [output_api.PresubmitPromptWarning(
+        'Please make sure that IndexedRuleset modifications in *.fbs and '
+        'indexed_ruleset.cc do not require updating '
+        'RulesetIndexer::kIndexedFormatVersion.')]
+  return []
+
+def CheckChangeOnUpload(input_api, output_api):
+  return CheckIndexedRulesetVersion(input_api, output_api)

components/subresource_filter/core/common/indexed_ruleset.h

@@ -39,6 +39,12 @@ static_assert(kNGramSize <= sizeof(NGram), "NGram type is too narrow.");
 class RulesetIndexer {
  public:
   // The current binary format version of the indexed ruleset.
+  //
+  // Increase this value when introducing an incompatible change in
+  // IndexedRuleset format, or otherwise willing to nudge clients to rebuild
+  // their ruleset (e.g., a change is compatible, but significantly reduces the
+  // size of the buffer). Note: The PRESUBMIT.py script tries to keep
+  // contributors aware of that.
   static const int kIndexedFormatVersion;
 
   RulesetIndexer();