[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.

[qyearsley, 2015/01/16 22:55:26]

Extra space after """

[stgao, 2015/01/21 20:29:23]

Done.

+
+  Input:
+    It is output of pipeline DetectFirstFailurePipeline.

[qyearsley, 2015/01/16 22:55:26]

"It is" can be omitted, I think -- you could say:
Input:
  The output of DetectFirstFailurePipeline.

Output:
  A dictionary with the following form:
  ...

Again, it might be a bit clearer to put this in Args and Returns sections in the
docstring for run(). During execution, the output of the run() method will be
serialized as JSON, so the "output" of the pipeline class could be said to be a
dict or JSON with some data; but the return value of run() is definitely a dict.

[stgao, 2015/01/21 20:29:23]

Done.

+
+  Output:
+    It is a json like below:

[qyearsley, 2015/01/16 22:55:26]

json -> dict

[stgao, 2015/01/21 20:29:23]

Done.

+    {
+      'git_hash_revision1': model.change_log.ChangeLog.ToJson(),
+      ...
+    }
+  """
+
+  # 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):
+    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