middleware: Handle method not allowed errors and redirect

- These could happen when a user tries to reload a page that only allows POST
requests.

- Or when the generic exception handling logic in the middleware redirects the
user to a page that only allows POST.

Tests:

- Insert a exception in the diagnose() of the 'users' app. Without the patch,
running diagnostics on the users app leads to a blank page. With the patch, the
user is redirected to Diagnostics app page and the original error is shown as a
alert message.

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

plinth/middleware.py

@@ -11,6 +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.shortcuts import redirect, render
 from django.template.response import SimpleTemplateResponse
 from django.utils.deprecation import MiddlewareMixin
@@ -154,6 +155,20 @@ class CommonErrorMiddleware(MiddlewareMixin):
 
         return None
 
+    @staticmethod
+    def process_response(request, response):
+        """Handle 405 method not allowed errors.
+
+        These errors may happen when we redirect to a page that does not allow
+        GET.
+        """
+        if isinstance(response, HttpResponseNotAllowed):
+            redirect_url = CommonErrorMiddleware._get_redirect_url_on_error(
+                request)
+            return redirect(redirect_url)
+
+        return response
+
     @staticmethod
     def _get_redirect_url_on_error(request):
         """Return the URL to redirect to after an error."""

plinth/tests/test_middleware.py

@@ -9,7 +9,8 @@ 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, HttpResponseRedirect
+from django.http import (HttpResponse, HttpResponseNotAllowed,
+                         HttpResponseRedirect)
 from django.test.client import RequestFactory
 from django.urls import resolve
 from stronghold.decorators import public
@@ -327,3 +328,13 @@ class TestCommonErrorMiddleware:
         response = middleware.process_exception(web_request, other_error)
         assert not response
         messages_error.assert_not_called()
+
+    @staticmethod
+    @patch('django.contrib.messages.error')
+    def test_405_error(messages_error, middleware, web_request, test_menu):
+        """Test that method not allowed errors are handled."""
+        response = middleware.process_response(
+            web_request, HttpResponseNotAllowed(['POST']))
+        assert isinstance(response, HttpResponseRedirect)
+        assert response.url == '/apps/'
+        messages_error.assert_not_called()