[Findit] Add a sub-pipeline to analyze failures caused by DEPS rolls.

appengine/findit/common/chromium_deps.py

Index: appengine/findit/common/chromium_deps.py
diff --git a/appengine/findit/common/chromium_deps.py b/appengine/findit/common/chromium_deps.py
index 079d67e2dd063fcd74b513c0b80d0510fb626c2d..eb9edcc4bd6cc9262181b8a708f402be1d3a1c8f 100644
--- a/appengine/findit/common/chromium_deps.py
+++ b/appengine/findit/common/chromium_deps.py
@@ -8,8 +8,12 @@ from common import dependency
 from common import deps_parser
 
 
+_CHROMIUM_ROOT_DIR = 'src/'
+_CHROMIUM_REPO_MASTER = 'https://chromium.googlesource.com/chromium/src.git'
+
+
 class DEPSDownloader(deps_parser.DEPSLoader):
-  """Download DEPS from remote GIT repo."""
+  """Downloads DEPS from remote Git repo."""
 
   def Load(self, repo_url, revision, deps_file):
     http_client = http_client_appengine.HttpClientAppengine()
@@ -18,13 +22,13 @@ class DEPSDownloader(deps_parser.DEPSLoader):
     content = None
 
     # Try .DEPS.git first if the given deps_file is "DEPS", because before
-    # migration from SVN to GIT, .DEPS.git contains dependencies from GIT while
+    # migration from SVN to Git, .DEPS.git contains dependencies from Git while
     # DEPS contains those from SVN.
     if deps_file == 'DEPS':
       content = repo.GetSource('.DEPS.git', revision)
 
     # If .DEPS.git is not found, use DEPS. Assume it is a commit after migration
-    # from SVN to GIT.
+    # from SVN to Git.
     if content is None:
       content = repo.GetSource(deps_file, revision)
 
@@ -35,7 +39,7 @@ class DEPSDownloader(deps_parser.DEPSLoader):
 
 
 def GetChromeDependency(revision, os_platform):
-  """Return all dependencies of Chrome as a dict for the given revision and os.
+  """Returns all dependencies of Chrome as a dict for the given revision and OS.
 
   Args:
     revision (str): The revision of a Chrome build.
@@ -46,19 +50,50 @@ def GetChromeDependency(revision, os_platform):
     A map from dependency path to the dependency info.
   """
   root_dep = dependency.Dependency(
-      'src/', 'https://chromium.googlesource.com/chromium/src.git', revision,
-      'DEPS')
+      _CHROMIUM_ROOT_DIR, _CHROMIUM_REPO_MASTER, revision, 'DEPS')
 
   deps_parser.UpdateDependencyTree(root_dep, [os_platform], DEPSDownloader())
 
   dependencies = {}
 
   # Flatten the dependency tree into a one-level dict.
-  def _FlattenDepTree(dep):
+  def FlattenDepTree(dep):
     dependencies[dep.path] = dep
     for child in dep.children.values():
-      _FlattenDepTree(child)
+      FlattenDepTree(child)
 
-  _FlattenDepTree(root_dep)
+  FlattenDepTree(root_dep)
 
   return dependencies
+
+
+def GetChromiumDEPSRolls(old_cr_revision, new_cr_revision, os_platform):
+  """Returns a list of dependency rolls between the given Chromium revisions.
+
+  Args:
+    old_cr_revision (str): The Git commit hash for the old Chromium revision.
+    new_cr_revision (str): The Git commit hash for the new Chromium revision.
+    os_platform (str): The target OS platform of the Chrome or test binary.
+  """
+  old_deps = GetChromeDependency(old_cr_revision, os_platform)
+  new_deps = GetChromeDependency(new_cr_revision, os_platform)
+
+  rolls = []
+
+  for path, new_dep in new_deps.iteritems():
+    if path == _CHROMIUM_ROOT_DIR:  # Skip the root dependency -- chromium.
+      continue
+
+    old_revision = None
+    if path in old_deps:
+      old_revision = old_deps[path].revision
+
+    if old_revision != new_dep.revision:
+      rolls.append(
+          dependency.DependencyRoll(
+              path, new_dep.repo_url, old_revision, new_dep.revision))
+
+  # Note: dependencies could be deleted too. However, regressions caused by that
+  # seems rare. Thus, ignore them for now.
+
+  return rolls