middleware: Handle page not found errors specially

- Show a different message for them.

Test:

- Try to visit page like /plinth/apps/sharing/foo/edit/ where a share named
'foo' does not exist. The common error handling middleware is triggered and an
alert message 'Page not found' with exception trace back is shown.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Veiko Aasa <veiko17@disroot.org>

plinth/middleware.py

@@ -11,7 +11,7 @@ from django.contrib import messages
 from django.contrib.auth.decorators import login_required
 from django.core.exceptions import PermissionDenied
 from django.db.utils import OperationalError
-from django.http import HttpResponseNotAllowed
+from django.http import Http404, HttpResponseNotAllowed
 from django.shortcuts import redirect, render
 from django.template.response import SimpleTemplateResponse
 from django.utils.deprecation import MiddlewareMixin
@@ -143,12 +143,19 @@ class CommonErrorMiddleware(MiddlewareMixin):
                 # to infinite redirects.
                 return None
 
-            if request.method == 'POST':
+            if isinstance(exception, Http404):
+                message = _('Page not found: {url}').format(url=request.path)
+                exception = None  # Don't show exception details
+            elif request.method == 'POST':
                 message = _('Error running operation.')
             else:
                 message = _('Error loading page.')
 
-            views.messages_error(request, message, exception)
+            if exception:
+                views.messages_error(request, message, exception)
+            else:
+                messages.error(request, message)
+
             redirect_url = CommonErrorMiddleware._get_redirect_url_on_error(
                 request)
             return redirect(redirect_url)

plinth/tests/test_middleware.py

@@ -9,7 +9,7 @@ import pytest
 from django.contrib.auth.models import AnonymousUser, Group, User
 from django.core.exceptions import PermissionDenied
 from django.db.utils import OperationalError
-from django.http import (HttpResponse, HttpResponseNotAllowed,
+from django.http import (Http404, HttpResponse, HttpResponseNotAllowed,
                          HttpResponseRedirect)
 from django.test.client import RequestFactory
 from django.urls import resolve
@@ -294,6 +294,18 @@ class TestCommonErrorMiddleware:
         assert response.template_name == 'error.html'
         assert 'message' in response.context_data
 
+    @staticmethod
+    @patch('django.contrib.messages.error')
+    def test_404_error_get(messages_error, middleware, web_request, test_menu):
+        """Test that 404 page not found errors are handled."""
+        response = middleware.process_exception(web_request, Http404())
+        assert isinstance(response, HttpResponseRedirect)
+        assert response.url == '/apps/'
+        messages_error.assert_called_once()
+        assert messages_error.call_args.args[0] == web_request
+        assert messages_error.call_args.args[1].startswith(
+            'Page not found: /apps/testapp/')
+
     @staticmethod
     @patch('django.contrib.messages.error')
     def test_other_error_get(messages_error, middleware, web_request,