[no-sync] Add method to check for diffs between the current checkout and another commit.

Bug: 1339472

Change-Id: If97d2fad7e5c61ec907e15a00db225b653a62bef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3738365Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Joanna Wang <jojwang@chromium.org>

gclient_scm.py

@@ -536,6 +536,25 @@ class GitWrapper(SCMWrapper):
     if options.reset_patch_ref:
       self._Capture(['reset', '--soft', base_rev])
 
+  def check_diff(self, previous_commit, files=None):
+    # type: (str, Optional[List[str]]) -> bool
+    """Check if a diff exists between the current commit and `previous_commit`.
+
+      Returns True if there were diffs or if an error was encountered.
+    """
+    cmd = ['diff', previous_commit, '--quiet']
+    if files:
+      cmd += ['--'] + files
+    try:
+      self._Capture(cmd)
+      return False
+    except subprocess2.CalledProcessError as e:
+      # git diff --quiet exits with 1 if there were diffs.
+      if e.returncode != 1:
+        self.Print('git returned non-zero exit status %s:\n%s' %
+                   (e.returncode, e.stderr.decode('utf-8')))
+      return True
+
   def update(self, options, args, file_list):
     """Runs git to update or transparently checkout the working copy.
 

tests/gclient_scm_test.py

@@ -1482,6 +1482,54 @@ class GerritChangesTest(fake_repos.FakeReposTestBase):
     self.assertEqual(self.githash('repo_1', 5), self.gitrevparse(self.root_dir))
 
 
+class DepsChangesFakeRepo(fake_repos.FakeReposBase):
+  def populateGit(self):
+    self._commit_git('repo_1', {'DEPS': 'versionA', 'doesnotmatter': 'B'})
+    self._commit_git('repo_1', {'DEPS': 'versionA', 'doesnotmatter': 'C'})
+
+    self._commit_git('repo_1', {'DEPS': 'versionB'})
+    self._commit_git('repo_1', {'DEPS': 'versionA', 'doesnotmatter': 'C'})
+    self._create_ref('repo_1', 'refs/heads/main', 4)
+
+
+class CheckDiffTest(fake_repos.FakeReposTestBase):
+  FAKE_REPOS_CLASS = DepsChangesFakeRepo
+
+  def setUp(self):
+    super(CheckDiffTest, self).setUp()
+    self.enabled = self.FAKE_REPOS.set_up_git()
+    self.options = BaseGitWrapperTestCase.OptionsObject()
+    self.url = self.git_base + 'repo_1'
+    self.mirror = None
+    mock.patch('sys.stdout', StringIO()).start()
+    self.addCleanup(mock.patch.stopall)
+
+  def setUpMirror(self):
+    self.mirror = tempfile.mkdtemp()
+    git_cache.Mirror.SetCachePath(self.mirror)
+    self.addCleanup(gclient_utils.rmtree, self.mirror)
+    self.addCleanup(git_cache.Mirror.SetCachePath, None)
+
+  def testCheckDiff(self):
+    """Correctly check for diffs."""
+    scm = gclient_scm.GitWrapper(self.url, self.root_dir, '.')
+    file_list = []
+
+    # Make sure we don't specify a revision.
+    self.options.revision = None
+    scm.update(self.options, None, file_list)
+    self.assertEqual(self.githash('repo_1', 4), self.gitrevparse(self.root_dir))
+
+    self.assertFalse(scm.check_diff(self.githash('repo_1', 1), files=['DEPS']))
+    self.assertTrue(scm.check_diff(self.githash('repo_1', 1)))
+    self.assertTrue(scm.check_diff(self.githash('repo_1', 3), files=['DEPS']))
+
+    self.assertFalse(
+        scm.check_diff(self.githash('repo_1', 2),
+                       files=['DEPS', 'doesnotmatter']))
+    self.assertFalse(scm.check_diff(self.githash('repo_1', 2)))
+
+
 if 'unittest.util' in __import__('sys').modules:
   # Show full diff in self.assertEqual.
   __import__('sys').modules['unittest.util']._MAX_LENGTH = 999999999