lxc: make use of filter python3 compatible

_detect_nbd_devices uses the filter
builtin internally to filter valid devices.

In python 2, filter returns a list. In python 3,
filter returns an iterable or generator function.
This change eagerly converts the result of calling filter
to a list to preserve the python 2 behaviour under python 3.

Closes-Bug: #1840068

Change-Id: I25616c5761ea625a15d725777ae58175651558f8

Author: SeanMooney

nova/tests/unit/virt/disk/mount/test_nbd.py

@@ -51,18 +51,40 @@ def _fake_exists_all_used(path):
     return ORIG_EXISTS(path)
 
 
-def _fake_detect_nbd_devices_none(self):
+def _fake_detect_nbd_devices_none():
     return []
 
 
-def _fake_detect_nbd_devices(self):
+def _fake_detect_nbd_devices():
     return ['nbd0', 'nbd1']
 
 
 def _fake_noop(*args, **kwargs):
     return
 
 
+class NbdTestCaseNoStub(test.NoDBTestCase):
+
+    @mock.patch('os.listdir')
+    def test_detect_nbd_devices(self, list_dir_mock):
+        list_dir_mock.return_value = _fake_detect_nbd_devices()
+        result = nbd.NbdMount._detect_nbd_devices()
+        self.assertIsNotNone(result)
+        self.assertIsInstance(result, list)
+        self.assertEqual(len(list_dir_mock.return_value), len(result))
+        for path in list_dir_mock.return_value:
+            self.assertIn(path, result)
+
+    @mock.patch('os.listdir')
+    def test_detect_nbd_devices_empty(self, list_dir_mock):
+        list_dir_mock.return_value = [
+            "nbdz", "fake0", "not-nbd1"]
+        result = nbd.NbdMount._detect_nbd_devices()
+        self.assertIsNotNone(result)
+        self.assertIsInstance(result, list)
+        self.assertEqual(0, len(result))
+
+
 class NbdTestCase(test.NoDBTestCase):
     def setUp(self):
         super(NbdTestCase, self).setUp()
@@ -274,7 +296,7 @@ def test_device_creation_race(self, mock_exists, mock_nbd_disconnect,
         # they cannot choose the same nbd number (see bug 1207422)
 
         tempdir = self.useFixture(fixtures.TempDir()).path
-        free_devices = _fake_detect_nbd_devices(None)[:]
+        free_devices = _fake_detect_nbd_devices()[:]
         chosen_devices = []
 
         def fake_find_unused(self):

nova/virt/disk/mount/nbd.py

@@ -39,9 +39,10 @@ class NbdMount(api.Mount):
     """qemu-nbd support disk images."""
     mode = 'nbd'
 
-    def _detect_nbd_devices(self):
+    @staticmethod
+    def _detect_nbd_devices():
         """Detect nbd device files."""
-        return filter(NBD_DEVICE_RE.match, os.listdir('/sys/block/'))
+        return list(filter(NBD_DEVICE_RE.match, os.listdir('/sys/block/')))
 
     def _find_unused(self, devices):
         for device in devices: