Fix GitBackend.checkout

unittests/autoroll_test.py

 #!/usr/bin/env python
 # Copyright 2016 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 import json
+import logging
 import os
 import subprocess
 import sys
 import unittest
 
 import repo_test_util
 
 
 class TestAutoroll(repo_test_util.RepoTest):
   def run_roll(self, repo, *args):
     """Runs the autoroll command and returns JSON.
     Does not commit the resulting roll.
     """
     with repo_test_util.in_directory(repo['root']), \
         repo_test_util.temporary_file() as tempfile_path:
-      subprocess.check_output([
-          sys.executable, self._recipe_tool,
-          '--package', os.path.join(
-              repo['root'], 'infra', 'config', 'recipes.cfg'),
-          '--use-bootstrap',
-          'autoroll',
-          '--output-json', tempfile_path
-      ] + list(args), stderr=subprocess.STDOUT)
+      try:
+        subprocess.check_output([
+            sys.executable, self._recipe_tool,
+            '--package', os.path.join(
+                repo['root'], 'infra', 'config', 'recipes.cfg'),
+            '--use-bootstrap',
+            'autoroll',
+            '--output-json', tempfile_path
+        ] + list(args), stderr=subprocess.STDOUT)
+      except subprocess.CalledProcessError as ex:
+        logging.error('Failed subprocess output: %s', ex.output)
+        raise
       with open(tempfile_path) as f:
         return json.load(f)
 
   def test_empty(self):
     """Tests the scenario where there are no roll candidates.
     """
     repos = self.repo_setup({
         'a': [],
     })
 
@@ skipping 463 matching lines @@
         '--project=c',
         '--project=d',
     )
     self.assertFalse(roll_result['success'])
     self.assertFalse(bool(roll_result['roll_details']))
     self.assertFalse(bool(roll_result['rejected_candidates_details']))
 
 
 if __name__ == '__main__':
   sys.exit(unittest.main())