Rollback snapshot's status when backup failed

When create backup from snapshot, if the backup action failed
in driver layer. The snapshot's status will be backing-up forever.

We should rollback the snapshot's status to its previous status
(available) like volume does when backup failed.

Change-Id: I77ac4e546c9a33003b7a52a5c924f70ad809cac0
Closes-bug: #1683258

cinder/backup/manager.py

@@ -418,9 +418,14 @@ class BackupManager(manager.ThreadPoolManager):
             self._run_backup(context, backup, volume)
         except Exception as err:
             with excutils.save_and_reraise_exception():
-                self.db.volume_update(context, volume_id,
-                                      {'status': previous_status,
-                                       'previous_status': 'error_backing-up'})
+                if snapshot_id:
+                    snapshot.status = fields.SnapshotStatus.AVAILABLE
+                    snapshot.save()
+                else:
+                    self.db.volume_update(
+                        context, volume_id,
+                        {'status': previous_status,
+                         'previous_status': 'error_backing-up'})
                 self._update_backup_error(backup, six.text_type(err))
 
         # Restore the original status.

cinder/tests/unit/backup/test_backup.py

@@ -573,6 +573,27 @@ class BackupTestCase(BaseBackupTest):
         self.assertEqual(fields.BackupStatus.ERROR, backup['status'])
         self.assertTrue(mock_run_backup.called)
 
+    @mock.patch('cinder.backup.manager.BackupManager._run_backup',
+                side_effect=FakeBackupException(str(uuid.uuid4())))
+    def test_create_backup_with_snapshot_error(self, mock_run_backup):
+        """Test error handling when error occurs during backup creation."""
+        vol_id = self._create_volume_db_entry(size=1)
+        snapshot = self._create_snapshot_db_entry(status='backing-up',
+                                                  volume_id=vol_id)
+        backup = self._create_backup_db_entry(volume_id=vol_id,
+                                              snapshot_id=snapshot.id)
+        self.assertRaises(FakeBackupException,
+                          self.backup_mgr.create_backup,
+                          self.ctxt,
+                          backup)
+
+        snapshot.refresh()
+        self.assertEqual('available', snapshot.status)
+
+        backup.refresh()
+        self.assertEqual(fields.BackupStatus.ERROR, backup.status)
+        self.assertTrue(mock_run_backup.called)
+
     @mock.patch('cinder.utils.brick_get_connector_properties')
     @mock.patch('cinder.volume.rpcapi.VolumeAPI.get_backup_device')
     @mock.patch('cinder.utils.temporary_chown')