502 error on large previews when SESSION_ENGINE is set to signed_cookies

Issue Summary

We are trying to preview a page on wagtail. It seems previews are store in session unlike drafts. And we have a huge description text. So we are getting 502 error on preview.

Cookie length should be less than or equal to 4096 characters.

Drafts and live are working fine with this session engine.

Steps to Reproduce

1. Set SESSION_ENGINE = 'django.contrib.sessions.backends.signed_cookies' in settings.py
2. login to cms
3. go tp pages
4. create a new child page or edit existing.
5. Add big text in description i.e sample text
6. click preview

cc @pdpinch

Screens:

Technical details

• Python version: 3.6
• Django version: 2.0.2
• SESSION_ENGINE = ‘django.contrib.sessions.backends.signed_cookies’
• Wagtail version: 2.0
• Browser version: chrome Version 66.0.3359.139 (Official Build) (64-bit) osx

[gasman]

@blarghmatey By definition, I think a preview has to show the outcome of a proposed change without committing that change to the database (even just as a draft revision). If an editor hits preview, and then changes their mind about their edit, and navigates away without saving, the system should behave as if the edit was never made at all (and thus revisiting the edit view should take you back to the page’s original state).

[loicteixeira]

Thanks for the report @amir-qayyum-khan.

For reference, preview data is stored in the session since #3383. While #3583 reduces preview data size, the issue will essentially still exists.

The fact that it might exceed the cookies max-size was raised at the time but it was agreed to add a warning to the documentation (although it seems to have never happened) given that not using the session probably meant creating an API endpoint which would have been a lot more work. Maybe it will be time to revisit this.

In the meantime, you should be able to configure uwsgi to accept bigger cookies (IRRC, nginx has some sort of auto-scaling but uwsgi doesn’t).