Add standalone udevadm_settle function

Add a standalone udevadm_settle that can be called alone. This is
to enable it to be called by the ceph-osd charm. I think the
subprocess calls should be switched to check_call in the longer
term but that may be a disruptive change if udevadm does not
always return 0 so I propose to leave them as they are for the
moment.

Change-Id: Iec5932a4d819ad87e54c2af391abe1befe84f164
Partial-Bug: #1812925

ceph/utils.py

@@ -935,6 +935,11 @@ def start_osds(devices):
                 subprocess.check_call(['ceph-disk', 'activate', dev_or_path])
 
 
+def udevadm_settle():
+    cmd = ['udevadm', 'settle']
+    subprocess.call(cmd)
+
+
 def rescan_osd_devices():
     cmd = [
         'udevadm', 'trigger',
@@ -943,8 +948,7 @@ def rescan_osd_devices():
 
     subprocess.call(cmd)
 
-    cmd = ['udevadm', 'settle']
-    subprocess.call(cmd)
+    udevadm_settle()
 
 
 _bootstrap_keyring = "/var/lib/ceph/bootstrap-osd/ceph.keyring"

unit_tests/test_general_utils.py

@@ -0,0 +1,40 @@
+# Copyright 2019 Canonical Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#  http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import unittest
+
+from mock import patch
+
+import ceph.utils
+
+
+class GeneralUtilsTestCase(unittest.TestCase):
+    def setUp(self):
+        super(GeneralUtilsTestCase, self).setUp()
+
+    @patch.object(ceph.utils.subprocess, 'call')
+    def test_udevadm_settle(self, _call):
+        ceph.utils.udevadm_settle()
+        _call.assert_called_once_with(['udevadm', 'settle'])
+
+    @patch.object(ceph.utils, 'udevadm_settle')
+    @patch.object(ceph.utils.subprocess, 'call')
+    def test_rescan_osd_devices(self, _call, _udevadm_settle):
+        ceph.utils.rescan_osd_devices()
+        _call.assert_called_once_with([
+            'udevadm',
+            'trigger',
+            '--subsystem-match=block',
+            '--action=add'])
+        _udevadm_settle.assert_called_once_with()