Check form class from Page edit handler for correct type

Forms for Page classes must subclass WagtailAdminPageForm. If they do
not, an error will be thrown for invalid arguments when the Page editor
is opened.

Partial fix for #2267.

wagtail/wagtailadmin/tests/test_edit_handlers.py

@@ -118,7 +118,21 @@ class TestGetFormForModel(TestCase):
         self.assertEqual(type(form.fields['date_from'].widget), forms.PasswordInput)
 
 
+def clear_edit_handler(page_cls):
+    def decorator(fn):
+        def decorated(self):
+            # Clear any old EditHandlers generated
+            page_cls.get_edit_handler.cache_clear()
+            fn(self)
+            # Clear the bad EditHandler generated just now
+            page_cls.get_edit_handler.cache_clear()
+        return decorated
+    return decorator
+
+
 class TestPageEditHandlers(TestCase):
+
+    @clear_edit_handler(EventPage)
     def test_get_edit_handler(self):
         """
         Forms for pages should have a base class of WagtailAdminPageForm.
@@ -129,6 +143,7 @@ class TestPageEditHandlers(TestCase):
         # The generated form should inherit from WagtailAdminPageForm
         self.assertTrue(issubclass(EventPageForm, WagtailAdminPageForm))
 
+    @clear_edit_handler(ValidatedPage)
     def test_get_form_for_page_with_custom_base(self):
         """
         ValidatedPage sets a custom base_form_class. This should be used as the
@@ -141,26 +156,40 @@ class TestPageEditHandlers(TestCase):
         # ValidatedPage.base_form_class == ValidatedPageForm
         self.assertTrue(issubclass(GeneratedValidatedPageForm, ValidatedPageForm))
 
+    @clear_edit_handler(ValidatedPage)
     def test_check_invalid_base_form_class(self):
         class BadFormClass(object):
             pass
 
-        # Clear any old EditHandlers generated
-        ValidatedPage.get_edit_handler.cache_clear()
+        invalid_base_form = checks.Error(
+            "ValidatedPage.base_form_class does not extend WagtailAdminPageForm",
+            hint="Ensure that wagtail.wagtailadmin.tests.test_edit_handlers.BadFormClass extends WagtailAdminPageForm",
+            obj=ValidatedPage,
+            id='wagtailcore.E002')
+
+        invalid_edit_handler = checks.Error(
+            "ValidatedPage.get_edit_handler().get_form_class(ValidatedPage) does not extend WagtailAdminPageForm",
+            hint="Ensure that the EditHandler for ValidatedPage creates a subclass of WagtailAdminPageForm",
+            obj=ValidatedPage,
+            id='wagtailcore.E003')
 
         with mock.patch.object(ValidatedPage, 'base_form_class', new=BadFormClass):
             errors = ValidatedPage.check()
-            self.assertEqual(len(errors), 1)
+            self.assertEqual(errors, [invalid_base_form, invalid_edit_handler])
 
-            error = errors[0]
-            self.assertEqual(error, checks.Error(
-                "base_form_class does not extend WagtailAdminPageForm",
-                hint="Ensure that wagtail.wagtailadmin.tests.test_edit_handlers.BadFormClass extends WagtailAdminPageForm",
-                obj=ValidatedPage,
-                id='wagtailcore.E002'))
-
-        # Clear the bad EditHandler generated just now
-        ValidatedPage.get_edit_handler.cache_clear()
+    @clear_edit_handler(ValidatedPage)
+    def test_custom_edit_handler_form_class(self):
+        """
+        Set a custom edit handler on a Page class, but dont customise
+        ValidatedPage.base_form_class, or provide a custom form class for the
+        edit handler. Check the generated form class is of the correct type.
+        """
+        ValidatedPage.edit_handler = TabbedInterface([])
+        with mock.patch.object(ValidatedPage, 'edit_handler', new=TabbedInterface([]), create=True):
+            form_class = ValidatedPage.get_edit_handler().get_form_class(ValidatedPage)
+            self.assertTrue(issubclass(form_class, WagtailAdminPageForm))
+            errors = ValidatedPage.check()
+            self.assertEqual(errors, [])
 
 
 class TestExtractPanelDefinitionsFromModelClass(TestCase):

wagtail/wagtailcore/models.py

@@ -560,15 +560,23 @@ class Page(six.with_metaclass(PageBase, MP_Node, ClusterableModel, index.Indexed
         from wagtail.wagtailadmin.forms import WagtailAdminPageForm
         if not issubclass(cls.base_form_class, WagtailAdminPageForm):
             errors.append(checks.Error(
-                "base_form_class does not extend WagtailAdminPageForm",
+                "{}.base_form_class does not extend WagtailAdminPageForm".format(
+                    cls.__name__),
                 hint="Ensure that {}.{} extends WagtailAdminPageForm".format(
                     cls.base_form_class.__module__,
                     cls.base_form_class.__name__),
                 obj=cls,
                 id='wagtailcore.E002'))
-        # Sadly, there is no way of checking the form class returned from
-        # cls.get_edit_handler().get_form_class(cls), as these calls can hit
-        # the DB in order to fetch content types.
+
+        edit_handler = cls.get_edit_handler()
+        if not issubclass(edit_handler.get_form_class(cls), WagtailAdminPageForm):
+            errors.append(checks.Error(
+                "{cls}.get_edit_handler().get_form_class({cls}) does not extend WagtailAdminPageForm".format(
+                    cls=cls.__name__),
+                hint="Ensure that the EditHandler for {cls} creates a subclass of WagtailAdminPageForm".format(
+                    cls=cls.__name__),
+                obj=cls,
+                id='wagtailcore.E003'))
 
         return errors