Fix GCE creds serialization w/ oauth2client >= 4.0 (#195)

Fix GCE creds serialization w/ oauth2client >= 4.0

When updating gsutil to use oauth2client 4.1.2 and apitools 0.5.19,
testing on GCE instances showed that GceAssertionCredentials were not
being written to our credential storage file. Debugging showed that this
was because of a KeyError when looking for the 'scope' attribute in a
serialized credential (no longer included as of oauth2client 3.0). This
change maintains backward compatibility for oauth2client versions that
used the 'scope' attribute while also working for current versions that
don't include 'scope' when constructing a GceAssertionCredentials
object.

apitools/base/py/credentials_lib.py

@@ -268,7 +268,7 @@ def __init__(self, scopes=None, service_account_name='default', **kwds):
         # catch and squelch the warning.
         with warnings.catch_warnings():
             warnings.simplefilter('ignore')
-            super(GceAssertionCredentials, self).__init__(scopes, **kwds)
+            super(GceAssertionCredentials, self).__init__(scope=scopes, **kwds)
 
     @classmethod
     def Get(cls, *args, **kwds):
@@ -412,14 +412,26 @@ def _do_refresh_request(self, unused_http_request):
         if self.store:
             self.store.locked_put(self)
 
+    def to_json(self):
+        # OAuth2Client made gce.AppAssertionCredentials unserializable as of
+        # v3.0, but we need those credentials to be serializable for use with
+        # this library, so we use AppAssertionCredentials' parent's to_json
+        # method.
+        # pylint: disable=bad-super-call
+        return super(gce.AppAssertionCredentials, self).to_json()
+
     @classmethod
     def from_json(cls, json_data):
         data = json.loads(json_data)
         kwargs = {}
         if 'cache_filename' in data.get('kwargs', []):
             kwargs['cache_filename'] = data['kwargs']['cache_filename']
-        credentials = GceAssertionCredentials(scopes=[data['scope']],
-                                              **kwargs)
+        # Newer versions of GceAssertionCredentials don't have a "scope"
+        # attribute.
+        scope_list = None
+        if 'scope' in data:
+            scope_list = [data['scope']]
+        credentials = GceAssertionCredentials(scopes=scope_list, **kwargs)
         if 'access_token' in data:
             credentials.access_token = data['access_token']
         if 'token_expiry' in data:

apitools/base/py/credentials_lib_test.py

@@ -13,6 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import json
 import os.path
 import shutil
 import tempfile
@@ -81,6 +82,18 @@ def testGceServiceAccounts(self):
             service_account_name='my_service_account',
             scopes=scopes)
 
+    def testGceAssertionCredentialsToJson(self):
+        scopes = ['scope1']
+        service_account_name = 'my_service_account'
+        # Ensure that we can obtain a JSON representation of
+        # GceAssertionCredentials to put in a credential Storage object, and
+        # that the JSON representation is valid.
+        original_creds = self._GetServiceCreds(
+            service_account_name=service_account_name,
+            scopes=scopes)
+        original_creds_json_str = original_creds.to_json()
+        json.loads(original_creds_json_str)
+
     @mock.patch.object(util, 'DetectGce', autospec=True)
     def testGceServiceAccountsCached(self, mock_detect):
         mock_detect.return_value = True