Allow escalation of UUID validation warning to error

Today UUIDField emits a warning if the provided value is not a valid
UUID. However this warning cannot be escalated to an exception through
the standard warnings module because the validation code also adds a
warning filter to the warning filter list that overrides the existing
filters.

This patch ensures that the 'once' warning filter only added to the
end of the filter list. This way if the client has already specified
another filter for this warning (e.g. and error filter) then that
filter will not be overriden.

Change-Id: I17cb96d16fcd91195478b738fbdda01b47cfd69d
Closes-Bug: #1746966

oslo_versionedobjects/fields.py

@@ -347,7 +347,10 @@ class UUID(StringPattern):
     def coerce(obj, attr, value):
         # FIXME(danms): We should actually verify the UUIDness here
         with warnings.catch_warnings():
-            warnings.simplefilter("once")
+            # Change the warning action only if no other filter exists
+            # for this warning to allow the client to define other action
+            # like 'error' for this warning.
+            warnings.filterwarnings(action="once", append=True)
             try:
                 uuid.UUID(str(value))
             except Exception:

oslo_versionedobjects/tests/test_fields.py

@@ -13,6 +13,7 @@
 #    under the License.
 
 import datetime
+import warnings
 
 import iso8601
 import mock
@@ -296,6 +297,11 @@ def test_validation_enabled(self):
         self.test_from_primitive()
         self.test_to_primitive()
 
+    def test_validation_warning_can_be_escalated_to_exception(self):
+        warnings.filterwarnings(action='error')
+        self.assertRaises(FutureWarning, self.field.coerce, 'obj', 'attr',
+                          'not a uuid')
+
     def test_get_schema(self):
         field = fields.UUIDField()
         schema = field.get_schema()