Add locking to enable single process code execution

The code in OpenStackSnap.setup() creates users, files,
directories, symlinks, etc. Locking support is added to ensure
that setup() code is only executed by a single process at a time.

Change-Id: Ia7ea6251d68284f5ea6e726e474f65d393b296ea

requirements.txt

@@ -7,3 +7,4 @@ pbr>=1.6 # Apache-2.0
 # Left unversioned as designed to align with OpenStack component being snapped
 jinja2
 jsonschema
+oslo.concurrency

snap_openstack/base.py

@@ -19,6 +19,8 @@ import os
 import shutil
 import yaml
 
+from oslo_concurrency import lockutils
+
 from snap_openstack.renderer import SnapFileRenderer
 
 LOG = logging.getLogger(__name__)
@@ -79,6 +81,8 @@ class OpenStackSnap(object):
             self.configuration = yaml.load(config)
         self.snap_env = snap_env()
 
+    @lockutils.synchronized('setup.lock', external=True,
+                            lock_path="/var/lock/snap-openstack")
     def setup(self):
         '''Perform any pre-execution snap setup