3 files changed, 48 insertions, 4 deletions
diff --git a/django/contrib/auth/__init__.py b/django/contrib/auth/__init__.py
index e73e34b816..87c2bc1b50 100644
--- a/django/contrib/auth/__init__.py
+++ b/django/contrib/auth/__init__.py
@@ -53,10 +53,15 @@ def login(request, user):
     # TODO: It would be nice to support different login methods, like signed cookies.
     user.last_login = datetime.datetime.now()
     user.save()
-    if request.session.get('SESSION_KEY', user.id) != user.id:
-        # To avoid reusing another user's session, create a new, empty session
-        # if the existing session corresponds to a different authenticated user.
-        request.session.flush()
+
+    if SESSION_KEY in request.session:
+        if request.session[SESSION_KEY] != user.id:
+            # To avoid reusing another user's session, create a new, empty
+            # session if the existing session corresponds to a different
+            # authenticated user.
+            request.session.flush()
+    else:
+        request.session.cycle_key()
     request.session[SESSION_KEY] = user.id
     request.session[BACKEND_SESSION_KEY] = user.backend
     if hasattr(request, 'user'):
diff --git a/django/contrib/sessions/backends/base.py b/django/contrib/sessions/backends/base.py
index b553fa4d58..b4cdeadd81 100644
--- a/django/contrib/sessions/backends/base.py
+++ b/django/contrib/sessions/backends/base.py
@@ -239,6 +239,16 @@ class SessionBase(object):
         self.delete()
         self.create()
 
+    def cycle_key(self):
+        """
+        Creates a new session key, whilst retaining the current session data.
+        """
+        data = self._session_cache
+        key = self.session_key
+        self.create()
+        self._session_cache = data
+        self.delete(key)
+
     # Methods that child classes must implement.
 
     def exists(self, session_key):
diff --git a/django/contrib/sessions/tests.py b/django/contrib/sessions/tests.py
index ffa32d6ae1..5efc17df00 100644
--- a/django/contrib/sessions/tests.py
+++ b/django/contrib/sessions/tests.py
@@ -37,6 +37,15 @@ False
 False
 >>> db_session.modified, db_session.accessed
 (True, True)
+>>> db_session['a'], db_session['b'] = 'c', 'd'
+>>> db_session.save()
+>>> prev_key = db_session.session_key
+>>> prev_data = db_session.items()
+>>> db_session.cycle_key()
+>>> db_session.session_key == prev_key
+False
+>>> db_session.items() == prev_data
+True
 
 # Submitting an invalid session key (either by guessing, or if the db has
 # removed the key) results in a new key being generated.
@@ -75,6 +84,16 @@ False
 False
 >>> file_session.modified, file_session.accessed
 (True, True)
+>>> file_session['a'], file_session['b'] = 'c', 'd'
+>>> file_session.save()
+>>> prev_key = file_session.session_key
+>>> prev_data = file_session.items()
+>>> file_session.cycle_key()
+>>> file_session.session_key == prev_key
+False
+>>> file_session.items() == prev_data
+True
+
 >>> Session.objects.filter(pk=file_session.session_key).delete()
 >>> file_session = FileSession(file_session.session_key)
 >>> file_session.save()
@@ -112,6 +131,16 @@ False
 False
 >>> cache_session.modified, cache_session.accessed
 (True, True)
+>>> cache_session['a'], cache_session['b'] = 'c', 'd'
+>>> cache_session.save()
+>>> prev_key = cache_session.session_key
+>>> prev_data = cache_session.items()
+>>> cache_session.cycle_key()
+>>> cache_session.session_key == prev_key
+False
+>>> cache_session.items() == prev_data
+True
+
 >>> Session.objects.filter(pk=cache_session.session_key).delete()
 >>> cache_session = CacheSession(cache_session.session_key)
 >>> cache_session.save()