Fix pylxd hard dependencies

Since introduction of the LXD/LXC first-party manila driver [1],
generating configuration and running unit tests fail unless pylxd is
installed.  This hard depency is likely unwanted as it is problematic
for distributions that do not include LXD and which will not therefore
distribute a packaged version of pylxd.

This commit converts the pylxd dependency to a soft dependency.  Config
generation succeeds and lxd driver unit tests are skipped if pylxd
cannot be imported.

Additionally, we handle the pylxd exceptions encountered using
pylxd 2.x, as reported here [1].

[1] https://review.openstack.org/#/c/289899/

Change-Id: I8436de2fd994fc73f81f6f3a43c658c874550c10
Closes-bug: #1553280
Closes-bug: #1553208

manila/share/drivers/lxd.py

@@ -25,14 +25,30 @@ import re
 
 from oslo_config import cfg
 from oslo_log import log
-from pylxd import api as lxd_api
+# NOTE(tbarron): pylxd module is unavailable in some distribtutions so
-from pylxd import exceptions as pylxd_exc
+# handle this circumstance gracefully.  Also handle failure to import
+# pylxd_exc directly in versions >= pylxd 2.
+try:
+    from pylxd import api as lxd_api
+    try:
+        from pylxd import exceptions as pylxd_exc
+        NO_LXD = False
+    except ImportError:
+        try:
+            # pylint: disable=E0611
+            from pylxd.deprecated import exceptions as pylxd_exc
+            NO_LXD = False
+        except ImportError:
+            NO_LXD = True
+except ImportError:
+    NO_LXD = True
 import six
 
 from manila.common import constants as const
 from manila import context
 from manila import exception
 from manila.i18n import _
+from manila.i18n import _LI
 from manila.i18n import _LW
 from manila.share import driver
 from manila import utils
@@ -92,6 +108,10 @@ CONF.register_opts(lv_opts)
 class LXDHelper(object):
 
     def __init__(self, lxd_api, config):
+        if NO_LXD:
+            LOG.info(_LI('pylxd modules are not present on this system: LXD '
+                         'driver will not function.'))
+            return
         super(LXDHelper, self).__init__()
         self.api = lxd_api
         self.conf = config

manila/tests/share/drivers/test_lxd.py

@@ -17,6 +17,7 @@
 import functools
 import mock
 from oslo_config import cfg
+import testtools
 
 from manila.common import constants as const
 from manila import context
@@ -28,7 +29,6 @@ from manila.tests.db import fakes as db_fakes
 from manila.tests import fake_utils
 from manila import utils
 
-
 CONF = cfg.CONF
 
 
@@ -71,6 +71,7 @@ def fake_network(**kwargs):
     return db_fakes.FakeModel(network)
 
 
+@testtools.skipIf(lxd.NO_LXD, "pylxd is unavailable")
 class LXDHelperTestCase(test.TestCase):
     """Tests LXDUnfs3Helper"""
 
@@ -219,6 +220,7 @@ class LXDHelperTestCase(test.TestCase):
         fake_stream.close.assert_called_once_with()
 
 
+@testtools.skipIf(lxd.NO_LXD, "pylxd is unavailable")
 class LXDUnfs3HelperTestCase(test.TestCase):
     """Tests LXDUnfs3Helper"""
 
@@ -432,6 +434,7 @@ class LXDUnfs3HelperTestCase(test.TestCase):
         self.assertFalse(self.UNFS3Helper._deny_access.called)
 
 
+@testtools.skipIf(lxd.NO_LXD, "pylxd is unavailable")
 class LXDCIFSHelperTestCase(test.TestCase):
     """Tests LXDCIFSHelper"""
 
@@ -671,6 +674,7 @@ class LXDCIFSHelperTestCase(test.TestCase):
         self.assertFalse(self.CIFSHelper._deny_access.called)
 
 
+@testtools.skipIf(lxd.NO_LXD, "pylxd is unavailable")
 class LXDDriverTestCase(test.TestCase):
     """Tests LXDDriver."""