Fixed #21473 -- Limited language preservation to logout

Current language is no longer saved to session by LocaleMiddleware
on  every response (the behavior introduced in #14825).
Instead language stored in session is reintroduced into new session
after logout.

Forward port of c558a43fd6 to master.

4 files changed, 26 insertions, 54 deletions

diff --git a/django/contrib/auth/__init__.py b/django/contrib/auth/__init__.py
index 4ef5c0b2cd..83aa445bac 100644
--- a/django/contrib/auth/__init__.py
+++ b/django/contrib/auth/__init__.py
@@ -105,7 +105,15 @@ def logout(request):
         user = None
     user_logged_out.send(sender=user.__class__, request=request, user=user)
 
+    # remember language choice saved to session
+    # for backwards compatibility django_language is also checked (remove in 1.8)
+    language = request.session.get('_language', request.session.get('django_language'))
+
     request.session.flush()
+
+    if language is not None:
+        request.session['_language'] = language
+
     if hasattr(request, 'user'):
         from django.contrib.auth.models import AnonymousUser
         request.user = AnonymousUser()
diff --git a/django/contrib/auth/tests/test_views.py b/django/contrib/auth/tests/test_views.py
index 060f0086ad..4a1783b14d 100644
--- a/django/contrib/auth/tests/test_views.py
+++ b/django/contrib/auth/tests/test_views.py
@@ -1,3 +1,4 @@
+from importlib import import_module
 import itertools
 import os
 import re
@@ -710,6 +711,18 @@ class LogoutTest(AuthViewsTestCase):
                             "%s should be allowed" % good_url)
             self.confirm_logged_out()
 
+    def test_logout_preserve_language(self):
+        """Check that language stored in session is preserved after logout"""
+        # Create a new session with language
+        engine = import_module(settings.SESSION_ENGINE)
+        session = engine.SessionStore()
+        session['_language'] = 'pl'
+        session.save()
+        self.client.cookies[settings.SESSION_COOKIE_NAME] = session.session_key
+
+        self.client.get('/logout/')
+        self.assertEqual(self.client.session['_language'], 'pl')
+
 
 @skipIfCustomUser
 @override_settings(
diff --git a/django/middleware/locale.py b/django/middleware/locale.py
index 87f904d7dd..1f64387d78 100644
--- a/django/middleware/locale.py
+++ b/django/middleware/locale.py
@@ -55,16 +55,6 @@ class LocaleMiddleware(object):
                     request.get_full_path())
                 return self.response_redirect_class(language_url)
 
-        # Store language back into session if it is not present
-        if hasattr(request, 'session') and '_language' not in request.session:
-            # Backwards compatibility check on django_language (remove in 1.8);
-            # revert to: `request.session.setdefault('_language', language)`.
-            if 'django_language' in request.session:
-                request.session['_language'] = request.session['django_language']
-                del request.session['django_language']
-            else:
-                request.session['_language'] = language
-
         if not (self.is_language_prefix_patterns_used()
                 and language_from_path):
             patch_vary_headers(response, ('Accept-Language',))
diff --git a/tests/i18n/tests.py b/tests/i18n/tests.py
index 471f434467..77b7d18455 100644
--- a/tests/i18n/tests.py
+++ b/tests/i18n/tests.py
@@ -1247,51 +1247,12 @@ class LocaleMiddlewareTests(TransRealMixin, TestCase):
             'django.middleware.common.CommonMiddleware',
         ),
     )
-    def test_session_language(self):
-        """
-        Check that language is stored in session if missing.
-        """
-        # Create an empty session
-        engine = import_module(settings.SESSION_ENGINE)
-        session = engine.SessionStore()
-        session.save()
-        self.client.cookies[settings.SESSION_COOKIE_NAME] = session.session_key
-
-        # Clear the session data before request
-        session.save()
-        self.client.get('/en/simple/')
-        self.assertEqual(self.client.session['_language'], 'en')
-
-        # Clear the session data before request
-        session.save()
-        self.client.get('/fr/simple/')
-        self.assertEqual(self.client.session['_language'], 'fr')
-
-        # Check that language is not changed in session
-        self.client.get('/en/simple/')
-        self.assertEqual(self.client.session['_language'], 'fr')
-
-    @override_settings(
-        MIDDLEWARE_CLASSES=(
-            'django.contrib.sessions.middleware.SessionMiddleware',
-            'django.middleware.locale.LocaleMiddleware',
-            'django.middleware.common.CommonMiddleware',
-        ),
-    )
-    def test_backwards_session_language(self):
-        """
-        Check that language is stored in session if missing.
-        """
-        # Create session with old language key name
-        engine = import_module(settings.SESSION_ENGINE)
-        session = engine.SessionStore()
-        session['django_language'] = 'en'
-        session.save()
-        self.client.cookies[settings.SESSION_COOKIE_NAME] = session.session_key
-
-        # request other language; should default to old language key value
+    def test_language_not_saved_to_session(self):
+        """Checks that current language is not automatically saved to
+        session on every request."""
+        # Regression test for #21473
         self.client.get('/fr/simple/')
-        self.assertEqual(self.client.session['_language'], 'en')
+        self.assertNotIn('_language', self.client.session)
 
 
 @override_settings(