Fixed #21446 -- Allowed not performing redirect in set_language view

Thanks Claude Paroz and Tim Graham for polishing the patch.

django/views/i18n.py

@@ -34,14 +34,15 @@ def set_language(request):
     any state.
     """
     next = request.POST.get('next', request.GET.get('next'))
-    if not is_safe_url(url=next, host=request.get_host()):
+    if (next or not request.is_ajax()) and not is_safe_url(url=next, host=request.get_host()):
         next = request.META.get('HTTP_REFERER')
         if not is_safe_url(url=next, host=request.get_host()):
             next = '/'
-    response = http.HttpResponseRedirect(next)
+    response = http.HttpResponseRedirect(next) if next else http.HttpResponse(status=204)
     if request.method == 'POST':
         lang_code = request.POST.get(LANGUAGE_QUERY_PARAMETER)
         if lang_code and check_for_language(lang_code):
+            if next:
                 next_trans = translate_url(next, lang_code)
                 if next_trans != next:
                     response = http.HttpResponseRedirect(next_trans)

docs/releases/1.10.txt

@@ -267,6 +267,10 @@ Internationalization
   :func:`~django.conf.urls.i18n.i18n_patterns` to ``False``, you can allow
   accessing the default language without a URL prefix.
 
+* :func:`~django.views.i18n.set_language` now returns a 204 status code (No
+  Content) for AJAX requests when there is no ``next`` parameter in ``POST`` or
+  ``GET``.
+
 Management Commands
 ~~~~~~~~~~~~~~~~~~~
 
@@ -695,6 +699,9 @@ Miscellaneous
   :meth:`~django.test.Client.login()` method no longer always rejects inactive
   users but instead delegates this decision to the authentication backend.
 
+* :func:`django.views.i18n.set_language` may now return a 204 status code for
+  AJAX requests.
+
 .. _deprecated-features-1.10:
 
 Features deprecated in 1.10

docs/topics/i18n/translation.txt

@@ -1788,14 +1788,21 @@ saves the language choice in the user's session. Otherwise, it saves the
 language choice in a cookie that is by default named ``django_language``.
 (The name can be changed through the :setting:`LANGUAGE_COOKIE_NAME` setting.)
 
-After setting the language choice, Django redirects the user, following this
-algorithm:
+After setting the language choice, Django looks for a ``next`` parameter in the
+``POST`` or ``GET`` data. If that is found and Django considers it to be a safe
+URL (i.e. it doesn't point to a different host and uses a safe scheme), a
+redirect to that URL will be performed. Otherwise, Django may fall back to
+redirecting the user to the URL from the ``Referer`` header or, if it is not
+set, to ``/``, depending on the nature of the request:
 
-* Django looks for a ``next`` parameter in the ``POST`` data.
-* If that doesn't exist, or is empty, Django tries the URL in the
-  ``Referrer`` header.
-* If that's empty -- say, if a user's browser suppresses that header --
-  then the user will be redirected to ``/`` (the site root) as a fallback.
+* For AJAX requests, the fallback will be performed only if the ``next``
+  parameter was set. Otherwise a 204 status code (No Content) will be returned.
+* For non-AJAX requests, the fallback will always be performed.
+
+.. versionchanged:: 1.10
+
+    Returning a 204 status code for AJAX requests when no redirect is specified
+    is new.
 
 Here's example HTML template code:
 

tests/view_tests/tests/test_i18n.py

@@ -13,7 +13,9 @@ from django.test.selenium import SeleniumTestCase
 from django.urls import reverse
 from django.utils import six
 from django.utils._os import upath
-from django.utils.translation import LANGUAGE_SESSION_KEY, override
+from django.utils.translation import (
+    LANGUAGE_SESSION_KEY, get_language, override,
+)
 
 from ..urls import locale_dir
 
@@ -22,15 +24,20 @@ from ..urls import locale_dir
 class I18NTests(TestCase):
     """ Tests django views in django/views/i18n.py """
 
+    def _get_inactive_language_code(self):
+        """Return language code for a language which is not activated."""
+        current_language = get_language()
+        return [code for code, name in settings.LANGUAGES if not code == current_language][0]
+
     def test_setlang(self):
         """
         The set_language view can be used to change the session language.
 
         The user is redirected to the 'next' argument if provided.
         """
-        for lang_code, lang_name in settings.LANGUAGES:
+        lang_code = self._get_inactive_language_code()
         post_data = dict(language=lang_code, next='/')
-            response = self.client.post('/i18n/setlang/', data=post_data)
+        response = self.client.post('/i18n/setlang/', post_data, HTTP_REFERER='/i_should_not_be_used/')
         self.assertRedirects(response, '/')
         self.assertEqual(self.client.session[LANGUAGE_SESSION_KEY], lang_code)
 
@@ -39,12 +46,76 @@ class I18NTests(TestCase):
         The set_language view only redirects to the 'next' argument if it is
         "safe".
         """
-        lang_code, lang_name = settings.LANGUAGES[0]
+        lang_code = self._get_inactive_language_code()
         post_data = dict(language=lang_code, next='//unsafe/redirection/')
         response = self.client.post('/i18n/setlang/', data=post_data)
         self.assertEqual(response.url, '/')
         self.assertEqual(self.client.session[LANGUAGE_SESSION_KEY], lang_code)
 
+    def test_setlang_redirect_to_referer(self):
+        """
+        The set_language view redirects to the URL in the referer header when
+        there isn't a "next" parameter.
+        """
+        lang_code = self._get_inactive_language_code()
+        post_data = dict(language=lang_code)
+        response = self.client.post('/i18n/setlang/', post_data, HTTP_REFERER='/i18n/')
+        self.assertRedirects(response, '/i18n/', fetch_redirect_response=False)
+        self.assertEqual(self.client.session[LANGUAGE_SESSION_KEY], lang_code)
+
+    def test_setlang_default_redirect(self):
+        """
+        The set_language view redirects to '/' when there isn't a referer or
+        "next" parameter.
+        """
+        lang_code = self._get_inactive_language_code()
+        post_data = dict(language=lang_code)
+        response = self.client.post('/i18n/setlang/', post_data)
+        self.assertRedirects(response, '/')
+        self.assertEqual(self.client.session[LANGUAGE_SESSION_KEY], lang_code)
+
+    def test_setlang_performs_redirect_for_ajax_if_explicitly_requested(self):
+        """
+        The set_language view redirects to the "next" parameter for AJAX calls.
+        """
+        lang_code = self._get_inactive_language_code()
+        post_data = dict(language=lang_code, next='/')
+        response = self.client.post('/i18n/setlang/', post_data, HTTP_X_REQUESTED_WITH='XMLHttpRequest')
+        self.assertRedirects(response, '/')
+        self.assertEqual(self.client.session[LANGUAGE_SESSION_KEY], lang_code)
+
+    def test_setlang_doesnt_perform_a_redirect_to_referer_for_ajax(self):
+        """
+        The set_language view doesn't redirect to the HTTP referer header for
+        AJAX calls.
+        """
+        lang_code = self._get_inactive_language_code()
+        post_data = dict(language=lang_code)
+        headers = {'HTTP_REFERER': '/', 'HTTP_X_REQUESTED_WITH': 'XMLHttpRequest'}
+        response = self.client.post('/i18n/setlang/', post_data, **headers)
+        self.assertEqual(response.status_code, 204)
+        self.assertEqual(self.client.session[LANGUAGE_SESSION_KEY], lang_code)
+
+    def test_setlang_doesnt_perform_a_default_redirect_for_ajax(self):
+        """
+        The set_language view returns 204 for AJAX calls by default.
+        """
+        lang_code = self._get_inactive_language_code()
+        post_data = dict(language=lang_code)
+        response = self.client.post('/i18n/setlang/', post_data, HTTP_X_REQUESTED_WITH='XMLHttpRequest')
+        self.assertEqual(response.status_code, 204)
+        self.assertEqual(self.client.session[LANGUAGE_SESSION_KEY], lang_code)
+
+    def test_setlang_unsafe_next_for_ajax(self):
+        """
+        The fallback to root URL for the set_language view works for AJAX calls.
+        """
+        lang_code = self._get_inactive_language_code()
+        post_data = dict(language=lang_code, next='//unsafe/redirection/')
+        response = self.client.post('/i18n/setlang/', post_data, HTTP_X_REQUESTED_WITH='XMLHttpRequest')
+        self.assertEqual(response.url, '/')
+        self.assertEqual(self.client.session[LANGUAGE_SESSION_KEY], lang_code)
+
     def test_setlang_reversal(self):
         self.assertEqual(reverse('set_language'), '/i18n/setlang/')