Making save_to_well_known_file() save with 0600 permission.

Fixes #144.

H/T to http://stackoverflow.com/a/5624691/1068170 and
       http://stackoverflow.com/a/5337329/1068170

oauth2client/client.py

@@ -28,7 +28,9 @@ import logging
 import os
 import socket
 import sys
+import tempfile
 import time
+import shutil
 import six
 from six.moves import urllib
 
@@ -1208,6 +1210,21 @@ class GoogleCredentials(OAuth2Credentials):
           'method should point to a file.')
 
 
+def _save_private_file(filename, json_contents):
+  """Saves a file with read-write permissions on for the owner.
+
+  Args:
+    filename: String. Absolute path to file.
+    json_contents: JSON serializable object to be saved.
+  """
+  temp_filename = tempfile.mktemp()
+  file_desc = os.open(temp_filename, os.O_WRONLY | os.O_CREAT, 0o600)
+  with os.fdopen(file_desc, 'w') as file_handle:
+    json.dump(json_contents, file_handle, sort_keys=True,
+              indent=2, separators=(',', ': '))
+  shutil.move(temp_filename, filename)
+
+
 def save_to_well_known_file(credentials, well_known_file=None):
   """Save the provided GoogleCredentials to the well known file.
 
@@ -1226,9 +1243,7 @@ def save_to_well_known_file(credentials, well_known_file=None):
     well_known_file = _get_well_known_file()
 
   credentials_data = credentials.serialization_data
-
-  with open(well_known_file, 'w') as f:
-    json.dump(credentials_data, f, sort_keys=True, indent=2, separators=(',', ': '))
+  _save_private_file(well_known_file, credentials_data)
 
 
 def _get_environment_variable_file():

tests/test_oauth2client.py

@@ -1112,5 +1112,35 @@ class MemoryCacheTests(unittest.TestCase):
     self.assertEqual(None, m.get('foo'))
 
 
+class Test__save_private_file(unittest.TestCase):
+
+  def _save_helper(self, filename):
+    contents = []
+    contents_str = '[]'
+    client._save_private_file(filename, contents)
+    with open(filename, 'r') as f:
+      stored_contents = f.read()
+    self.assertEqual(stored_contents, contents_str)
+
+    stat_mode = os.stat(filename).st_mode
+    # Octal 777, only last 3 positions matter for permissions mask.
+    stat_mode &= 0o777
+    self.assertEqual(stat_mode, 0o600)
+
+  def test_new(self):
+    import tempfile
+    filename = tempfile.mktemp()
+    self.assertFalse(os.path.exists(filename))
+    self._save_helper(filename)
+
+  def test_existing(self):
+    import tempfile
+    filename = tempfile.mktemp()
+    with open(filename, 'w') as f:
+      f.write('a bunch of nonsense longer than []')
+    self.assertTrue(os.path.exists(filename))
+    self._save_helper(filename)
+
+
 if __name__ == '__main__':
   unittest.main()