views: Drop use of private Django utility

We are currently using django.utils.http.is_safe_url which is a private method
and may break API anytime. Replace it with similar but limited implementation.

Tests:

- Unit tests.

- Dismiss a notification and the redirect to the same page happens properly.

- Logout, goto to home page or login page. Change the language and it will
redirect back to home page or login page appropriately.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/tests/test_views.py

@@ -0,0 +1,31 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+"""
+Tests for common FreedomBox views.
+"""
+
+import pytest
+
+from plinth.views import is_safe_url
+
+
+@pytest.mark.parametrize('url', [
+    '/plinth/login/',
+    '/',
+    'safe',
+])
+def test_is_safe_url_valid_url(url):
+    """Test valid URLs for safe URL checks."""
+    assert is_safe_url(url)
+
+
+@pytest.mark.parametrize('url', [
+    '',
+    None,
+    '\\plinth',
+    '///plinth',
+    'https://example.com/plinth/login/',
+    'https:///plinth/login',
+])
+def test_is_safe_url_invalid_url(url):
+    """Test invalid URLs for safe URL checks."""
+    assert not is_safe_url(url)

plinth/views.py

@@ -4,13 +4,13 @@ Main FreedomBox views.
 """
 
 import time
+import urllib.parse
 
 from django.contrib import messages
 from django.core.exceptions import ImproperlyConfigured
 from django.http import Http404, HttpResponseBadRequest, HttpResponseRedirect
 from django.template.response import TemplateResponse
 from django.urls import reverse
-from django.utils.http import is_safe_url
 from django.utils.translation import ugettext as _
 from django.views.generic import TemplateView
 from django.views.generic.edit import FormView
@@ -26,11 +26,35 @@ from . import forms, frontpage
 REDIRECT_FIELD_NAME = 'next'
 
 
+def is_safe_url(url):
+    """Check if the URL is safe to redirect to.
+
+    Based on Django internal utility.
+
+    """
+    if url is not None:
+        url = url.strip()
+
+    if not url:
+        return False
+
+    if '\\' in url or url.startswith('///'):
+        return False
+
+    result = urllib.parse.urlparse(url)
+
+    # Only accept URLs to the same site and scheme.
+    if result.scheme or result.netloc:
+        return False
+
+    return True
+
+
 def _get_redirect_url_from_param(request):
     """Return the redirect URL from 'next' GET/POST param."""
     redirect_to = request.GET.get(REDIRECT_FIELD_NAME, '')
     redirect_to = request.POST.get(REDIRECT_FIELD_NAME, redirect_to)
-    if is_safe_url(url=redirect_to, allowed_hosts={request.get_host()}):
+    if is_safe_url(redirect_to):
         return redirect_to
 
     return reverse('index')