Fix wrong check with non-None value when format group

This patch is same to the bug-fix 'Fix can't generate config sample
with non-None value'[1].

There is pitfall in statement like:
  if getattr(opt.type, 'min', None)
The check returns False When opt.type.min equals 0,
We should check this with None.

This patch will fix it and add unit tests for the change.

[1]https://review.openstack.org/#/c/267566/2

Change-Id: Iec6d8d70c67f641ae53976bde13697ce0f4982c1

oslo_config/sphinxext.py

@@ -108,9 +108,9 @@ def _format_group(app, namespace, group_name, group_obj, opt_list):
             if default:
                 default = '``' + default + '``'
             yield _indent(':Default: %s' % default)
-        if getattr(opt.type, 'min', None):
+        if getattr(opt.type, 'min', None) is not None:
             yield _indent(':Minimum Value: %s' % opt.type.min)
-        if getattr(opt.type, 'max', None):
+        if getattr(opt.type, 'max', None) is not None:
             yield _indent(':Maximum Value: %s' % opt.type.max)
         if getattr(opt.type, 'choices', None):
             choices_text = ', '.join([_get_choice_text(choice)

oslo_config/tests/test_sphinxext.py

@@ -91,6 +91,28 @@ class FormatGroupTest(base.BaseTestCase):
 
         ''').lstrip(), results)
 
+    def test_with_min_0(self):
+        results = '\n'.join(list(sphinxext._format_group(
+            app=mock.Mock(),
+            namespace=None,
+            group_name=None,
+            group_obj=None,
+            opt_list=[
+                cfg.IntOpt('opt_name',
+                           min=0),
+            ],
+        )))
+        self.assertEqual(textwrap.dedent('''
+        .. oslo.config:group:: DEFAULT
+
+        .. oslo.config:option:: opt_name
+
+          :Type: integer
+          :Default: ``<None>``
+          :Minimum Value: 0
+
+        ''').lstrip(), results)
+
     def test_with_max(self):
         results = '\n'.join(list(sphinxext._format_group(
             app=mock.Mock(),
@@ -113,6 +135,28 @@ class FormatGroupTest(base.BaseTestCase):
 
         ''').lstrip(), results)
 
+    def test_with_max_0(self):
+        results = '\n'.join(list(sphinxext._format_group(
+            app=mock.Mock(),
+            namespace=None,
+            group_name=None,
+            group_obj=None,
+            opt_list=[
+                cfg.IntOpt('opt_name',
+                           max=0),
+            ],
+        )))
+        self.assertEqual(textwrap.dedent('''
+        .. oslo.config:group:: DEFAULT
+
+        .. oslo.config:option:: opt_name
+
+          :Type: integer
+          :Default: ``<None>``
+          :Maximum Value: 0
+
+        ''').lstrip(), results)
+
     def test_with_choices(self):
         results = '\n'.join(list(sphinxext._format_group(
             app=mock.Mock(),