[2.2.x] Fixed #30318 -- Added check for importability of arguments of custom error handler views.

Thanks to Jon on Stack Overflow for reporting the issue.

Backport of a5accc0368 from master

django/urls/resolvers.py

@@ -15,7 +15,7 @@ from urllib.parse import quote
 from django.conf import settings
 from django.core.checks import Error, Warning
 from django.core.checks.urls import check_resolver
-from django.core.exceptions import ImproperlyConfigured
+from django.core.exceptions import ImproperlyConfigured, ViewDoesNotExist
 from django.utils.datastructures import MultiValueDict
 from django.utils.functional import cached_property
 from django.utils.http import RFC3986_SUBDELIMS, escape_leading_slashes
@@ -405,7 +405,15 @@ class URLResolver:
         # All handlers take (request, exception) arguments except handler500
         # which takes (request).
         for status_code, num_parameters in [(400, 2), (403, 2), (404, 2), (500, 1)]:
-            handler, param_dict = self.resolve_error_handler(status_code)
+            try:
+                handler, param_dict = self.resolve_error_handler(status_code)
+            except (ImportError, ViewDoesNotExist) as e:
+                path = getattr(self.urlconf_module, 'handler%s' % status_code)
+                msg = (
+                    "The custom handler{status_code} view '{path}' could not be imported."
+                ).format(status_code=status_code, path=path)
+                messages.append(Error(msg, hint=str(e), id='urls.E008'))
+                continue
             signature = inspect.signature(handler)
             args = [None] * num_parameters
             try:

docs/ref/checks.txt

@@ -464,6 +464,8 @@ The following checks are performed on your URL configuration:
   end with a slash.
 * **urls.E007**: The custom ``handlerXXX`` view ``'path.to.view'`` does not
   take the correct number of arguments (…).
+* **urls.E008**: The custom ``handlerXXX`` view ``'path.to.view'`` could not be
+  imported.
 
 ``contrib`` app checks
 ======================

docs/releases/2.2.1.txt

@@ -46,3 +46,6 @@ Bugfixes
 * Fixed a regression in Django 2.2 where
   :class:`~django.contrib.postgres.search.SearchVector` generates SQL that is
   not indexable (:ticket:`30385`).
+
+* Fixed a regression in Django 2.2 that caused an exception to be raised when
+  a custom error handler could not be imported (:ticket:`30318`).

tests/check_framework/test_urls.py

@@ -181,6 +181,29 @@ class CheckCustomErrorHandlersTests(SimpleTestCase):
                     id='urls.E007',
                 ))
 
+    @override_settings(ROOT_URLCONF='check_framework.urls.bad_error_handlers_invalid_path')
+    def test_bad_handlers_invalid_path(self):
+        result = check_url_config(None)
+        paths = [
+            'django.views.bad_handler',
+            'django.invalid_module.bad_handler',
+            'invalid_module.bad_handler',
+            'django',
+        ]
+        hints = [
+            "Could not import '{}'. View does not exist in module django.views.",
+            "Could not import '{}'. Parent module django.invalid_module does not exist.",
+            "No module named 'invalid_module'",
+            "Could not import '{}'. The path must be fully qualified.",
+        ]
+        for code, path, hint, error in zip([400, 403, 404, 500], paths, hints, result):
+            with self.subTest('handler{}'.format(code)):
+                self.assertEqual(error, Error(
+                    "The custom handler{} view '{}' could not be imported.".format(code, path),
+                    hint=hint.format(path),
+                    id='urls.E008',
+                ))
+
     @override_settings(ROOT_URLCONF='check_framework.urls.good_error_handlers')
     def test_good_handlers(self):
         result = check_url_config(None)

tests/check_framework/urls/bad_error_handlers_invalid_path.py

@@ -0,0 +1,6 @@
+urlpatterns = []
+
+handler400 = 'django.views.bad_handler'
+handler403 = 'django.invalid_module.bad_handler'
+handler404 = 'invalid_module.bad_handler'
+handler500 = 'django'