middleware: Add new middleware to handle common errors like DB busy

- During database error such as 'database is locked', show a special message
asking users to try again instead of submitting a bug report.

[sunil: Minor formatting, rename the template file name]
Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
[jvalleroy: Fix missing import]
Signed-off-by: James Valleroy <jvalleroy@mailbox.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>
Reviewed-by: Sunil Mohan Adapa <sunil@medhas.org>

plinth/middleware.py

@@ -10,8 +10,11 @@ from django.conf import settings
 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.shortcuts import render
+from django.template.response import SimpleTemplateResponse
 from django.utils.deprecation import MiddlewareMixin
+from django.utils.translation import gettext as _
 from stronghold.utils import is_view_func_public
 
 from plinth import app as app_module
@@ -114,3 +117,19 @@ class FirstSetupMiddleware(MiddlewareMixin):
             'refresh_page_sec': 3
         }
         return render(request, 'first_setup.html', context)
+
+
+class CommonErrorMiddleware(MiddlewareMixin):
+    """Django middleware to handle common errors."""
+
+    @staticmethod
+    def process_exception(request, exception):
+        """Show a custom error page when OperationalError is raised."""
+        if isinstance(exception, OperationalError):
+            message = _(
+                'System is possibly under heavy load. Please retry later.')
+            return SimpleTemplateResponse('error.html',
+                                          context={'message': message},
+                                          status=503)
+
+        return None

plinth/settings.py

@@ -135,6 +135,7 @@ MIDDLEWARE = (
     'django.contrib.auth.middleware.AuthenticationMiddleware',
     'django.contrib.messages.middleware.MessageMiddleware',
     'django.middleware.clickjacking.XFrameOptionsMiddleware',
+    'plinth.middleware.CommonErrorMiddleware',
     'stronghold.middleware.LoginRequiredMiddleware',
     'plinth.middleware.AdminRequiredMiddleware',
     'plinth.middleware.FirstSetupMiddleware',

plinth/templates/error.html

@@ -0,0 +1,16 @@
+{% extends 'base.html' %}
+{% comment %}
+# SPDX-License-Identifier: AGPL-3.0-or-later
+{% endcomment %}
+
+{% load i18n %}
+
+{% block content %}
+
+  <h2>{% trans "Error" %}</h2>
+
+  <p>
+    {{ message }}
+  </p>
+
+{% endblock %}