session data should always contain the expiry date

[Aymeric Augustin]

Currently, session data only contains the expiry date when it has been explicitly set.

As a consequence, session backends are required to store the expiration date as metadata to determine if the session has or hasn't expired:

• the cache backend transmits the expiry age to the cache server
• the database backend store the expiry date next to the data
• the file backend doesn't store the expiry date at all (see #18194) — it can be rebuilt from the file's mtime but that sounds fragile
• the signed_cookies backend implements its own timestamping and signing

SessionBase already signs the session data (to prevent code execution attacks in case the pickled session data was compromised), and it timestamps it when an explicit expiry date is set.

I propose that SessionBase always timestamp and sign session data. This would simplify session expiry handling. The signed_cookies backend would no longer need to perform its own timestamping and signing.

[Aymeric Augustin]

This refactoring seems necessary to enforce signed_cookies expiry when a non-default expiry date is set.

[Aymeric Augustin]

​https://github.com/django/django/pull/453 shares some ideas with this ticket.

[Aymeric Augustin <aymeric.augustin@…>]

In 88393357a6839fb4ce57793523bc25b17dc188a4:

[1.5.x] Marked cookies-based session expiry test as an expected failure.

Refs #19201.

Backport of 58337b3 from master.

[Aymeric Augustin <aymeric.augustin@…>]

In 58337b32236eb57d82bf62ed077add3ec69e37f2:

Marked cookies-based session expiry test as an expected failure.

Refs #19201.

[Aymeric Augustin]

A helpful reference on session expiry implementation: https://code.djangoproject.com/ticket/18194#comment:12