Fix retry mechanism for generator results

Both v1 and v2 of the glance client return python generators in some
cases (rather than fully fleshed out lists) which thwarts our retry
mechanism. Convert generator results to a list, so that any potential
exceptions get raised earlier rather than later, allowing for retries.

Conflicts:

    /nova/tests/unit/image/test_glance.py

Conflict notes: I couldn't add the following assert from master because
client.api_servers didn't exist then. It was added in commit
590b2c3007. Instead, I added one that
follows the pattern at that time.

  self.assertEqual(str(client.api_server), "https://host2:9293")

Change-Id: Ibc84f1596d4eaabdef0a48f6cf4da2d1323843a8
Closes-Bug: #1557584
(cherry picked from commit ae6d868e2f)

nova/image/glance.py

@@ -18,6 +18,7 @@
 from __future__ import absolute_import
 
 import copy
+import inspect
 import itertools
 import random
 import sys
@@ -227,7 +228,12 @@ class GlanceClientWrapper(object):
             client = self.client or self._create_onetime_client(context,
                                                                 version)
             try:
-                return getattr(client.images, method)(*args, **kwargs)
+                result = getattr(client.images, method)(*args, **kwargs)
+                if inspect.isgenerator(result):
+                    # Convert generator results to a list, so that we can
+                    # catch any potential exceptions now and retry the call.
+                    return list(result)
+                return result
             except retry_excs as e:
                 host = self.host
                 port = self.port

nova/tests/unit/image/test_glance.py

@@ -415,6 +415,45 @@ class TestGlanceClientWrapper(test.NoDBTestCase):
         )
         sleep_mock.assert_called_once_with(1)
 
+    @mock.patch('random.shuffle')
+    @mock.patch('time.sleep')
+    @mock.patch('nova.image.glance._create_glance_client')
+    def test_retry_works_with_generators(self, create_client_mock,
+                                         sleep_mock, shuffle_mock):
+        def some_generator(exception):
+            if exception:
+                raise glanceclient.exc.CommunicationError('Boom!')
+            yield 'something'
+
+        api_servers = [
+            'host1:9292',
+            'https://host2:9293',
+            'http://host3:9294'
+        ]
+        client_mock = mock.MagicMock()
+        images_mock = mock.MagicMock()
+        images_mock.list.side_effect = [
+            some_generator(exception=True),
+            some_generator(exception=False),
+        ]
+        type(client_mock).images = mock.PropertyMock(return_value=images_mock)
+        create_client_mock.return_value = client_mock
+
+        self.flags(num_retries=1, group='glance')
+        self.flags(api_servers=api_servers, group='glance')
+
+        ctx = context.RequestContext('fake', 'fake')
+        client = glance.GlanceClientWrapper()
+        client.call(ctx, 1, 'list', 'meow')
+
+        create_client_mock.assert_has_calls(
+            [
+                mock.call(ctx, 'host1', 9292, False, 1),
+                mock.call(ctx, 'host2', 9293, True, 1),
+            ]
+        )
+        sleep_mock.assert_called_once_with(1)
+
     @mock.patch('oslo_service.sslutils.is_enabled')
     @mock.patch('glanceclient.Client')
     def test_create_glance_client_with_ssl(self, client_mock,