session.set_expiry() causes a TypeError when using JSONSerializer

[René Puls]

I am using django.contrib.sessions.backends.cached_db as the SESSION_ENGINE and django.contrib.sessions.serializers.JSONSerializer as the SESSION_SERIALIZER.

Setting a session expiration date using session.set_expiry() results in a TypeError when the session is saved:

Traceback:
File "/Users/pwr/Library/Python/2.7/lib/python/site-packages/django/core/handlers/base.py" in get_response
  201.                 response = middleware_method(request, response)
File "/Users/pwr/Library/Python/2.7/lib/python/site-packages/django/contrib/sessions/middleware.py" in process_response
  38.                     request.session.save()
File "/Users/pwr/Library/Python/2.7/lib/python/site-packages/django/contrib/sessions/backends/cached_db.py" in save
  62.         super(SessionStore, self).save(must_create)
File "/Users/pwr/Library/Python/2.7/lib/python/site-packages/django/contrib/sessions/backends/db.py" in save
  57.             session_data=self.encode(self._get_session(no_load=must_create)),
File "/Users/pwr/Library/Python/2.7/lib/python/site-packages/django/contrib/sessions/backends/base.py" in encode
  87.         serialized = self.serializer().dumps(session_dict)
File "/Users/pwr/Library/Python/2.7/lib/python/site-packages/django/core/signing.py" in dumps
  88.         return json.dumps(obj, separators=(',', ':')).encode('latin-1')
File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/__init__.py" in dumps
  250.         sort_keys=sort_keys, **kw).encode(obj)
File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/encoder.py" in encode
  207.         chunks = self.iterencode(o, _one_shot=True)
File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/encoder.py" in iterencode
  270.         return _iterencode(o, 0)
File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/encoder.py" in default
  184.         raise TypeError(repr(o) + " is not JSON serializable")

Exception Type: TypeError at /users/login/
Exception Value: datetime.datetime(2013, 11, 9, 5, 0, tzinfo=<DstTzInfo 'Europe/Berlin' CET+1:00:00 STD>) is not JSON serializable

The documentation already notes that ​JSONSerializer cannot handle datetime values, which is working as intended according to #21124. The documentation for ​set_expiry also notes that "datetime and timedelta values are only serializable if you are using the PickleSerializer".

If this is indeed intentional, I suggest that the 1.6 release notes should at least mention that session expiration times can no longer be set in the default configuration.

[Tim Graham]

Accepting as a documentation addition.

[G.Z.]

I can confirm that this seems like a relevant issue - there should be at least clear warning about a possibility of such errors coming up just from using default settings and methods.

[Tim Graham]

Attached is a proposed patch. Please let me know if you would add anything else.

[René Puls]

Looks good to me. Thank you!

[Tim Graham <timograham@…>]

In ffdae5b66d681145d6e9288fd98718e6c7b263e4:

Fixed #21404 -- Added session.set_expiry() note to 1.6 release notes.

Thanks pwr for the suggestion.

[Tim Graham <timograham@…>]

In a776e3f42088a5a82bd0ec9c5353981a1a23e10c:

[1.6.x] Fixed #21404 -- Added session.set_expiry() note to 1.6 release notes.

Thanks pwr for the suggestion.

Backport of ffdae5b66d from master