Fixed relative path to work when presubmit run from any directory

Bug: 1111829
Change-Id: I75c616f4fa9c16c19e732e43ba740bb67f3eb827
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2327839Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Saagar Sanghavi <saagarsanghavi@google.com>

presubmit_support.py

@@ -1529,10 +1529,10 @@ class PresubmitExecuter(object):
     Return:
       A list of result objects, empty if no problems.
     """
-
     # Change to the presubmit file's directory to support local imports.
     main_path = os.getcwd()
-    os.chdir(os.path.dirname(presubmit_path))
+    presubmit_dir = os.path.dirname(presubmit_path)
+    os.chdir(presubmit_dir)
 
     # Load the presubmit script into context.
     input_api = InputApi(self.change, presubmit_path, self.committing,
@@ -1560,8 +1560,10 @@ class PresubmitExecuter(object):
 
         # TODO (crbug.com/1106943): Dive into each of the individual checks
 
-        rel_path = os.path.relpath(os.getcwd(), main_path)
+        # Get path of presubmit directory relative to repository root.
         # Always use forward slashes, so that path is same in *nix and Windows
+        root = input_api.change.RepositoryRoot()
+        rel_path = os.path.relpath(presubmit_dir, root)
         rel_path = rel_path.replace(os.path.sep, '/')
 
         with rdb_wrapper.setup_rdb(function_name, rel_path) as my_status:

tests/presubmit_unittest.py

@@ -964,25 +964,27 @@ def CheckChangeOnCommit(input_api, output_api):
         presubmit.main(
             ['--root', self.fake_root_dir, 'random_file.txt', '--post_upload']))
 
-  @mock.patch(
-      'presubmit_support.ListRelevantPresubmitFiles',
-      return_value=['PRESUBMIT.py'])
+  @mock.patch('presubmit_support.ListRelevantPresubmitFiles')
   def testMainUnversioned(self, *_mocks):
     gclient_utils.FileRead.return_value = ''
     scm.determine_scm.return_value = None
+    presubmit.ListRelevantPresubmitFiles.return_value = [
+        os.path.join(self.fake_root_dir, 'PRESUBMIT.py')
+    ]
 
     self.assertEqual(
         0,
         presubmit.main(['--root', self.fake_root_dir, 'random_file.txt']))
 
-  @mock.patch(
-      'presubmit_support.ListRelevantPresubmitFiles',
-      return_value=['PRESUBMIT.py'])
+  @mock.patch('presubmit_support.ListRelevantPresubmitFiles')
   def testMainUnversionedChecksFail(self, *_mocks):
     gclient_utils.FileRead.return_value = (
         'def CheckChangeOnUpload(input_api, output_api):\n'
         '  return [output_api.PresubmitError("!!")]\n')
     scm.determine_scm.return_value = None
+    presubmit.ListRelevantPresubmitFiles.return_value = [
+        os.path.join(self.fake_root_dir, 'PRESUBMIT.py')
+    ]
 
     self.assertEqual(
         1,