[1.5.x] Fixed #20922 -- Allowed customizing the serializer used by contrib.sessions

Added settings.SESSION_SERIALIZER which is the import path of a serializer
to use for sessions.

Thanks apollo13, carljm, shaib, akaariai, charettes, and dstufft for reviews.

Backport of b0ce6fe656 from master

11 files changed, 104 insertions, 52 deletions

diff --git a/django/conf/global_settings.py b/django/conf/global_settings.py
index d4d4732abe..4cf2ba73c1 100644
--- a/django/conf/global_settings.py
+++ b/django/conf/global_settings.py
@@ -467,6 +467,7 @@ SESSION_SAVE_EVERY_REQUEST = False                      # Whether to save the se
 SESSION_EXPIRE_AT_BROWSER_CLOSE = False                 # Whether a user's session cookie expires when the Web browser is closed.
 SESSION_ENGINE = 'django.contrib.sessions.backends.db'  # The module to store session data
 SESSION_FILE_PATH = None                                # Directory to store session files if using the file session module. If None, the backend will use a sensible default.
+SESSION_SERIALIZER = 'django.contrib.sessions.serializers.PickleSerializer'  # class to serialize session data
 
 #########
 # CACHE #
diff --git a/django/conf/project_template/project_name/settings.py b/django/conf/project_template/project_name/settings.py
index 35117ddc61..331be43833 100644
--- a/django/conf/project_template/project_name/settings.py
+++ b/django/conf/project_template/project_name/settings.py
@@ -126,6 +126,8 @@ INSTALLED_APPS = (
     # 'django.contrib.admindocs',
 )
 
+SESSION_SERIALIZER = 'django.contrib.sessions.serializers.JSONSerializer'
+
 # A sample logging configuration. The only tangible logging
 # performed by this configuration is to send an email to
 # the site admins on every HTTP 500 error when DEBUG=False.
diff --git a/django/contrib/messages/storage/session.py b/django/contrib/messages/storage/session.py
index 225dfda289..c3e293c22e 100644
--- a/django/contrib/messages/storage/session.py
+++ b/django/contrib/messages/storage/session.py
@@ -1,4 +1,8 @@
+import json
+
 from django.contrib.messages.storage.base import BaseStorage
+from django.contrib.messages.storage.cookie import MessageEncoder, MessageDecoder
+from django.utils import six
 
 
 class SessionStorage(BaseStorage):
@@ -20,14 +24,23 @@ class SessionStorage(BaseStorage):
         always stores everything it is given, so return True for the
         all_retrieved flag.
         """
-        return self.request.session.get(self.session_key), True
+        return self.deserialize_messages(self.request.session.get(self.session_key)), True
 
     def _store(self, messages, response, *args, **kwargs):
         """
         Stores a list of messages to the request's session.
         """
         if messages:
-            self.request.session[self.session_key] = messages
+            self.request.session[self.session_key] = self.serialize_messages(messages)
         else:
             self.request.session.pop(self.session_key, None)
         return []
+
+    def serialize_messages(self, messages):
+        encoder = MessageEncoder(separators=(',', ':'))
+        return encoder.encode(messages)
+
+    def deserialize_messages(self, data):
+        if data and isinstance(data, six.string_types):
+            return json.loads(data, cls=MessageDecoder)
+        return data
diff --git a/django/contrib/messages/tests/base.py b/django/contrib/messages/tests/base.py
index b3ced12773..7719d662aa 100644
--- a/django/contrib/messages/tests/base.py
+++ b/django/contrib/messages/tests/base.py
@@ -61,6 +61,7 @@ class BaseTest(TestCase):
             MESSAGE_TAGS    = '',
             MESSAGE_STORAGE = '%s.%s' % (self.storage_class.__module__,
                                          self.storage_class.__name__),
+            SESSION_SERIALIZER = 'django.contrib.sessions.serializers.JSONSerializer',
         )
         self.settings_override.enable()
 
diff --git a/django/contrib/messages/tests/session.py b/django/contrib/messages/tests/session.py
index e162f49fc2..0d2daaa197 100644
--- a/django/contrib/messages/tests/session.py
+++ b/django/contrib/messages/tests/session.py
@@ -10,13 +10,13 @@ def set_session_data(storage, messages):
     Sets the messages into the backend request's session and remove the
     backend's loaded data cache.
     """
-    storage.request.session[storage.session_key] = messages
+    storage.request.session[storage.session_key] = storage.serialize_messages(messages)
     if hasattr(storage, '_loaded_data'):
         del storage._loaded_data
 
 
 def stored_session_messages_count(storage):
-    data = storage.request.session.get(storage.session_key, [])
+    data = storage.deserialize_messages(storage.request.session.get(storage.session_key, []))
     return len(data)
 
 
diff --git a/django/contrib/sessions/backends/base.py b/django/contrib/sessions/backends/base.py
index f79a264500..44b06d6441 100644
--- a/django/contrib/sessions/backends/base.py
+++ b/django/contrib/sessions/backends/base.py
@@ -2,10 +2,6 @@ from __future__ import unicode_literals
 
 import base64
 from datetime import datetime, timedelta
-try:
-    from django.utils.six.moves import cPickle as pickle
-except ImportError:
-    import pickle
 import string
 
 from django.conf import settings
@@ -15,6 +11,7 @@ from django.utils.crypto import get_random_string
 from django.utils.crypto import salted_hmac
 from django.utils import timezone
 from django.utils.encoding import force_bytes
+from django.utils.module_loading import import_by_path
 
 # session_key should not be case sensitive because some backends can store it
 # on case insensitive file systems.
@@ -38,6 +35,7 @@ class SessionBase(object):
         self._session_key = session_key
         self.accessed = False
         self.modified = False
+        self.serializer = import_by_path(settings.SESSION_SERIALIZER)
 
     def __contains__(self, key):
         return key in self._session
@@ -82,24 +80,25 @@ class SessionBase(object):
         return salted_hmac(key_salt, value).hexdigest()
 
     def encode(self, session_dict):
-        "Returns the given session dictionary pickled and encoded as a string."
-        pickled = pickle.dumps(session_dict, pickle.HIGHEST_PROTOCOL)
-        hash = self._hash(pickled)
-        return base64.b64encode(hash.encode() + b":" + pickled).decode('ascii')
+        "Returns the given session dictionary serialized and encoded as a string."
+        serialized = self.serializer().dumps(session_dict)
+        hash = self._hash(serialized)
+        return base64.b64encode(hash.encode() + b":" + serialized).decode('ascii')
 
     def decode(self, session_data):
         encoded_data = base64.b64decode(force_bytes(session_data))
         try:
             # could produce ValueError if there is no ':'
-            hash, pickled = encoded_data.split(b':', 1)
-            expected_hash = self._hash(pickled)
+            hash, serialized = encoded_data.split(b':', 1)
+            expected_hash = self._hash(serialized)
             if not constant_time_compare(hash.decode(), expected_hash):
                 raise SuspiciousOperation("Session data corrupted")
             else:
-                return pickle.loads(pickled)
+                return self.serializer().loads(serialized)
         except Exception:
-            # ValueError, SuspiciousOperation, unpickling exceptions. If any of
-            # these happen, just return an empty dictionary (an empty session).
+            # ValueError, SuspiciousOperation, deserialization exceptions. If
+            # any of these happen, just return an empty dictionary (an empty
+            # session).
             return {}
 
     def update(self, dict_):
diff --git a/django/contrib/sessions/backends/signed_cookies.py b/django/contrib/sessions/backends/signed_cookies.py
index c2b7a3123f..77a6750ce4 100644
--- a/django/contrib/sessions/backends/signed_cookies.py
+++ b/django/contrib/sessions/backends/signed_cookies.py
@@ -1,26 +1,9 @@
-try:
-    from django.utils.six.moves import cPickle as pickle
-except ImportError:
-    import pickle
-
 from django.conf import settings
 from django.core import signing
 
 from django.contrib.sessions.backends.base import SessionBase
 
 
-class PickleSerializer(object):
-    """
-    Simple wrapper around pickle to be used in signing.dumps and
-    signing.loads.
-    """
-    def dumps(self, obj):
-        return pickle.dumps(obj, pickle.HIGHEST_PROTOCOL)
-
-    def loads(self, data):
-        return pickle.loads(data)
-
-
 class SessionStore(SessionBase):
 
     def load(self):
@@ -31,7 +14,7 @@ class SessionStore(SessionBase):
         """
         try:
             return signing.loads(self.session_key,
-                serializer=PickleSerializer,
+                serializer=self.serializer,
                 # This doesn't handle non-default expiry dates, see #19201
                 max_age=settings.SESSION_COOKIE_AGE,
                 salt='django.contrib.sessions.backends.signed_cookies')
@@ -91,7 +74,7 @@ class SessionStore(SessionBase):
         session_cache = getattr(self, '_session_cache', {})
         return signing.dumps(session_cache, compress=True,
             salt='django.contrib.sessions.backends.signed_cookies',
-            serializer=PickleSerializer)
+            serializer=self.serializer)
 
     @classmethod
     def clear_expired(cls):
diff --git a/django/contrib/sessions/models.py b/django/contrib/sessions/models.py
index 0179c358b3..3a6e31152f 100644
--- a/django/contrib/sessions/models.py
+++ b/django/contrib/sessions/models.py
@@ -5,7 +5,7 @@ from django.utils.translation import ugettext_lazy as _
 class SessionManager(models.Manager):
     def encode(self, session_dict):
         """
-        Returns the given session dictionary pickled and encoded as a string.
+        Returns the given session dictionary serialized and encoded as a string.
         """
         return SessionStore().encode(session_dict)
 
diff --git a/django/contrib/sessions/serializers.py b/django/contrib/sessions/serializers.py
new file mode 100644
index 0000000000..92a31c054b
--- /dev/null
+++ b/django/contrib/sessions/serializers.py
@@ -0,0 +1,20 @@
+from django.core.signing import JSONSerializer as BaseJSONSerializer
+try:
+    from django.utils.six.moves import cPickle as pickle
+except ImportError:
+    import pickle
+
+
+class PickleSerializer(object):
+    """
+    Simple wrapper around pickle to be used in signing.dumps and
+    signing.loads.
+    """
+    def dumps(self, obj):
+        return pickle.dumps(obj, pickle.HIGHEST_PROTOCOL)
+
+    def loads(self, data):
+        return pickle.loads(data)
+
+
+JSONSerializer = BaseJSONSerializer
diff --git a/django/contrib/sessions/tests.py b/django/contrib/sessions/tests.py
index 8bcc505ee6..829eb82107 100644
--- a/django/contrib/sessions/tests.py
+++ b/django/contrib/sessions/tests.py
@@ -273,21 +273,25 @@ class SessionTestsMixin(object):
         self.assertEqual(self.session.decode(encoded), data)
 
     def test_actual_expiry(self):
-        # Regression test for #19200
-        old_session_key = None
-        new_session_key = None
-        try:
-            self.session['foo'] = 'bar'
-            self.session.set_expiry(-timedelta(seconds=10))
-            self.session.save()
-            old_session_key = self.session.session_key
-            # With an expiry date in the past, the session expires instantly.
-            new_session = self.backend(self.session.session_key)
-            new_session_key = new_session.session_key
-            self.assertNotIn('foo', new_session)
-        finally:
-            self.session.delete(old_session_key)
-            self.session.delete(new_session_key)
+        # this doesn't work with JSONSerializer (serializing timedelta)
+        with override_settings(SESSION_SERIALIZER='django.contrib.sessions.serializers.PickleSerializer'):
+            self.session = self.backend()  # reinitialize after overriding settings
+
+            # Regression test for #19200
+            old_session_key = None
+            new_session_key = None
+            try:
+                self.session['foo'] = 'bar'
+                self.session.set_expiry(-timedelta(seconds=10))
+                self.session.save()
+                old_session_key = self.session.session_key
+                # With an expiry date in the past, the session expires instantly.
+                new_session = self.backend(self.session.session_key)
+                new_session_key = new_session.session_key
+                self.assertNotIn('foo', new_session)
+            finally:
+                self.session.delete(old_session_key)
+                self.session.delete(new_session_key)
 
 
 class DatabaseSessionTests(SessionTestsMixin, TestCase):
diff --git a/django/utils/module_loading.py b/django/utils/module_loading.py
index f8aadb34df..ede585e562 100644
--- a/django/utils/module_loading.py
+++ b/django/utils/module_loading.py
@@ -2,6 +2,35 @@ import imp
 import os
 import sys
 
+from django.core.exceptions import ImproperlyConfigured
+from django.utils import six
+from django.utils.importlib import import_module
+
+
+def import_by_path(dotted_path, error_prefix=''):
+    """
+    Import a dotted module path and return the attribute/class designated by the
+    last name in the path. Raise ImproperlyConfigured if something goes wrong.
+    """
+    try:
+        module_path, class_name = dotted_path.rsplit('.', 1)
+    except ValueError:
+        raise ImproperlyConfigured("%s%s doesn't look like a module path" % (
+            error_prefix, dotted_path))
+    try:
+        module = import_module(module_path)
+    except ImportError as e:
+        msg = '%sError importing module %s: "%s"' % (
+            error_prefix, module_path, e)
+        six.reraise(ImproperlyConfigured, ImproperlyConfigured(msg),
+                    sys.exc_info()[2])
+    try:
+        attr = getattr(module, class_name)
+    except AttributeError:
+        raise ImproperlyConfigured('%sModule "%s" does not define a "%s" attribute/class' % (
+            error_prefix, module_path, class_name))
+    return attr
+
 
 def module_has_submodule(package, module_name):
     """See if 'module' is in 'package'."""