[Findit] Add three sub-pipelines to analyze build failure.

appengine/findit/waterfall/pull_changelog_pipeline.py

+# Copyright 2015 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.
+
+from pipeline_utils.appengine_third_party_pipeline_src_pipeline import pipeline
+
+from common.git_repository import GitRepository
+from common.http_client_appengine import HttpClientAppengine as HttpClient
+from waterfall.base_pipeline import BasePipeline
+
+
+class PullChangelogPipeline(BasePipeline):
+  """A pipeline to pull change log of CLs."""
+
+  # TODO: for files in dependencies(blink, v8, skia, etc), use blame first.
+  GIT_REPO = GitRepository(
+      'https://chromium.googlesource.com/chromium/src', HttpClient())
+
+  # Arguments number differs from overridden method - pylint: disable=W0221
+  def run(self, failure_info):
+    """
+    Args:
+      failure_info (dict): Output of pipeline DetectFirstFailurePipeline.run().
+
+    Returns:
+      A dict with the following form:
+      {
+        'git_hash_revision1': common.change_log.ChangeLog.ToJson(),
+        ...
+      }
+    """
+    change_logs = {}
+
+    for build in failure_info['builds'].values():
+      for revision in build['blame_list']:
+        change_log = self.GIT_REPO.GetChangeLog(revision)
+        if not change_log:  # pragma: no cover
+          raise pipeline.Retry('Failed to get change log for %s' % revision)
+
+        # TODO: save in datastore?
+        change_logs[revision] = change_log.ToJson()
+
+    return change_logs