UUID sentinel

Add a private _UUIDSentinels() class to oslo_utils.fixture that behaves
like a mock.sentinel, but returns specifically UUIDs. Expose a
(singleton) instance of it called uuidsentinel. This is mostly copied
from [1], with the following differences:

- We don't do the enforced-singleton thing [2]. But importing the
  uuidsentinel global yields the same behavior.
- We don't do the local import thing [3][4], because we're already in
  the right lib, so no issues with circular imports.
- Locking is added to make this threadsafe. (See discussion at [5])

Note that there is some question as to whether it is more appropriate
for this to live here or in oslotest [6]. This has been discussed on the
dev ML [7] and it was concluded [8] that it should live here.

[1] https://github.com/openstack/nova/blob/722d5b477219f0a2435a9f4ad4d54c61b83219f1/nova/tests/uuidsentinel.py
[2] https://github.com/openstack/nova/blob/722d5b477219f0a2435a9f4ad4d54c61b83219f1/nova/tests/uuidsentinel.py#L30
[3] https://github.com/openstack/nova/blob/722d5b477219f0a2435a9f4ad4d54c61b83219f1/nova/tests/uuidsentinel.py#L18-L19
[4] https://github.com/openstack/nova/blob/722d5b477219f0a2435a9f4ad4d54c61b83219f1/nova/tests/uuidsentinel.py#L26
[5] http://eavesdrop.openstack.org/irclogs/%23openstack-oslo/%23openstack-oslo.2018-08-20.log.html#t2018-08-20T20:10:33
[6] https://review.openstack.org/#/c/594068/
[7] http://lists.openstack.org/pipermail/openstack-dev/2018-August/133670.html
[8] http://lists.openstack.org/pipermail/openstack-dev/2018-August/133861.html

Change-Id: I214ff21b461fa1ca4b83476e1d0a763efe986217

2 files changed, 62 insertions, 0 deletions

diff --git a/oslo_utils/fixture.py b/oslo_utils/fixture.py
index 9526154..1872c70 100644
--- a/oslo_utils/fixture.py
+++ b/oslo_utils/fixture.py
@@ -20,9 +20,12 @@ Test fixtures.
 .. versionadded:: 1.3
 """
 
+import threading
+
 import fixtures
 
 from oslo_utils import timeutils
+from oslo_utils import uuidutils
 
 
 class TimeFixture(fixtures.Fixture):
@@ -49,3 +52,41 @@ class TimeFixture(fixtures.Fixture):
     def advance_time_seconds(self, seconds):
         """Advance overridden time by seconds."""
         timeutils.advance_time_seconds(seconds)
+
+
+class _UUIDSentinels(object):
+    """Registry of dynamically created, named, random UUID strings.
+
+    An instance of this class will dynamically generate attributes as they are
+    referenced, associating a random UUID string with each. Thereafter,
+    referring to the same attribute will give the same UUID for the life of the
+    instance. Plan accordingly.
+
+    Usage:
+
+        from oslo_utils.fixture import uuidsentinel as uuids
+        ...
+        foo = uuids.foo
+        do_a_thing(foo)
+        # Referencing the same sentinel again gives the same value
+        assert foo == uuids.foo
+        # But a different one will be different
+        assert foo != uuids.bar
+    """
+    def __init__(self):
+        self._sentinels = {}
+        self._lock = threading.Lock()
+
+    def __getattr__(self, name):
+        if name.startswith('_'):
+            raise ValueError('Sentinels must not start with _')
+        with self._lock:
+            if name not in self._sentinels:
+                self._sentinels[name] = uuidutils.generate_uuid()
+        return self._sentinels[name]
+
+
+# Singleton sentinel instance. Caveat emptor: using this multiple times in the
+# same process (including across multiple modules) will result in the same
+# values
+uuidsentinel = _UUIDSentinels()
diff --git a/oslo_utils/tests/test_fixture.py b/oslo_utils/tests/test_fixture.py
index 3e0898a..dafac2f 100644
--- a/oslo_utils/tests/test_fixture.py
+++ b/oslo_utils/tests/test_fixture.py
@@ -17,9 +17,12 @@
 import datetime
 
 from oslotest import base as test_base
+import six
 
 from oslo_utils import fixture
+from oslo_utils.fixture import uuidsentinel as uuids
 from oslo_utils import timeutils
+from oslo_utils import uuidutils
 
 
 class TimeFixtureTest(test_base.BaseTestCase):
@@ -61,3 +64,21 @@ class TimeFixtureTest(test_base.BaseTestCase):
         time_fixture.advance_time_seconds(2)
         expected_time = datetime.datetime(2015, 1, 2, 3, 4, 8, 7)
         self.assertEqual(expected_time, timeutils.utcnow())
+
+
+class UUIDSentinelsTest(test_base.BaseTestCase):
+
+    def test_different_sentinel(self):
+        uuid1 = uuids.foobar
+        uuid2 = uuids.barfoo
+        self.assertNotEqual(uuid1, uuid2)
+
+    def test_returns_uuid(self):
+        self.assertTrue(uuidutils.is_uuid_like(uuids.foo))
+
+    def test_returns_string(self):
+        self.assertIsInstance(uuids.foo, str)
+
+    def test_with_underline_prefix(self):
+        ex = self.assertRaises(ValueError, getattr, uuids, '_foo')
+        self.assertIn("Sentinels must not start with _", six.text_type(ex))