[BUG] Editing a page sometimes fails

[jrief]

Description

In very special occasions, attempting to edit a page results in a database integrity error. This is caused by a Placeholder-object holding plugins in different languages. In our project we have 3 placeholders (out of 83k) where placeholder.cmsplugin_set.all() delivers plugins in German as well as in English.

This presumably is a data inconsistency, but I'm unsure of it.

Steps to reproduce

• Open that special page (a page with a placeholder holding plugins in different languages) – unable to reproduce this in the first place.
• Click on the button named "New page content".

This raises the following exception:

IntegrityError at /en/admin/djangocms_versioning/pagecontentversion/4562/edit-redirect/
duplicate key value violates unique constraint "cms_cmsplugin_placeholder_id_language_position_bad60ebe_uniq"
DETAIL:  Key (placeholder_id, language, "position")=(213316, de, 2) already exists.

Additional information (CMS/Python/Django versions)

Happens in Django-CMS 4.1

Do you want to help fix this issue?

In order to prevent this, we could change this line:

https://github.com/django-cms/djangocms-versioning/blob/f726bc203c06ccbb8ba79dc9defb98d1b4e06a9d/djangocms_versioning/cms_config.py#L216

to

placeholder.copy_plugins(new_placeholder, language=placeholder.source.language)

but this just fixes the symptoms but not the root cause.

Instead, I wrote a small script to reduce the contents of those 3 placeholders to just their proper language.

• Yes, I want to help fix this issue and I will join #workgroup-pr-review on Slack to confirm with the community that a PR is welcome.
• No, I only want to report the issue.

[fsbraun]

It might be possible that this is the aftermath of another bug: #7841.

Hard to tell, though.