Implement presubmit API to get the old contents of an affected file.

BUG=702851

Change-Id: I6f005d19524cbb8b361d3fcb0b91912885c46e00
Reviewed-on: https://chromium-review.googlesource.com/456755
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>

presubmit_support.py

@@ -584,6 +584,10 @@ class _DiffCache(object):
     """Get the diff for a particular path."""
     raise NotImplementedError()
 
+  def GetOldContents(self, path, local_root):
+    """Get the old version for a particular path."""
+    raise NotImplementedError()
+
 
 class _GitDiffCache(_DiffCache):
   """DiffCache implementation for git; gets all file diffs at once."""
@@ -626,6 +630,9 @@ class _GitDiffCache(_DiffCache):
 
     return self._diffs_by_file[path]
 
+  def GetOldContents(self, path, local_root):
+    return scm.GIT.GetOldContents(local_root, path, branch=self._upstream)
+
 
 class AffectedFile(object):
   """Representation of a file in a change."""
@@ -670,6 +677,18 @@ class AffectedFile(object):
     """An alias to IsTestableFile for backwards compatibility."""
     return self.IsTestableFile()
 
+  def OldContents(self):
+    """Returns an iterator over the lines in the old version of file.
+
+    The new version is the file in the user's workspace, i.e. the "right hand
+    side".
+
+    Contents will be empty if the file is a directory or does not exist.
+    Note: The carriage returns (LF or CR) are stripped off.
+    """
+    return self._diff_cache.GetOldContents(self.LocalPath(),
+                                           self._local_root).splitlines()
+
   def NewContents(self):
     """Returns an iterator over the lines in the new version of file.
 

scm.py

@@ -254,6 +254,13 @@ class GIT(object):
         upstream_branch = ''.join(remote_ref)
     return upstream_branch
 
+  @staticmethod
+  def GetOldContents(cwd, filename, branch=None):
+    if not branch:
+      branch = GIT.GetUpstreamBranch(cwd)
+    command = ['show', '%s:%s' % (branch, filename)]
+    return GIT.Capture(command, cwd=cwd, strip_out=False)
+
   @staticmethod
   def GenerateDiff(cwd, branch=None, branch_head='HEAD', full_move=False,
                    files=None):

tests/presubmit_unittest.py

@@ -1482,7 +1482,7 @@ class AffectedFileUnittest(PresubmitTestsBase):
     members = [
       'AbsoluteLocalPath', 'Action', 'ChangedContents', 'DIFF_CACHE',
       'GenerateScmDiff', 'IsTestableFile', 'IsTextFile', 'LocalPath',
-      'NewContents',
+      'NewContents', 'OldContents',
     ]
     # If this test fails, you should add the relevant test.
     self.compareMembers(

tests/scm_unittest.py

@@ -86,6 +86,7 @@ class GitWrapperTestCase(BaseSCMTestCase):
         'GetDifferentFiles',
         'GetEmail',
         'GetGitDir',
+        'GetOldContents',
         'GetPatchName',
         'GetUpstreamBranch',
         'IsDirectoryVersioned',