Upgrading to 1.34.0 Layout Plugin Missing "boxton" for Default Administrative Layout

[quicksketch]

Description of the bug

When updating to Backdrop 1.34.0, users are hitting an error after replacing the "core" directory with new code. This probably occurs when running the update via the UI as well:

Reported by @findlabnet in Zulip#Backdrop > Backdrop 1.34.0 @ 💬.

This is probably related to the changes in #6808.

Steps To Reproduce

To reproduce the behavior:

1. Install Backdrop 1.33.2 (or use git checkout 1.33.x)
2. Clear caches.
3. Update to Backdrop 1.34.0 (or use git checkout 1.34.x)
4. View /admin.
5. Get the above errors.
6. Clear caches. The error is cleared.

Expected behavior

Backdrop 1.34.0 should be able to find layout templates without clearing the caches. This may be done by a fallback or clearing an invalid cache.

Additional information

Add any other information that could help, such as:

• Backdrop CMS version: 1.34.0
• Web server and its version: DDEV standard install
• PHP version: 8.3
• Database sever (MySQL or MariaDB?) and its version:
• Operating System and its version: Linux
• Browser(s) and their versions:

[izmeez]

Yes, I encountered this also. Saw the message and then it went away and I didn't think anymore about it, until now.

[izmeez]

Yes, I did clear caches. Thinking maybe I do that more often than necessary.

[laryn]

I wonder if I should have set the default to layout instead of flexible in the case of a NULL value: #7107

[quicksketch]

I've seen at least 3 additional users (by Zulip and email) that have reported this problem. I think this is urgent that we address this in 1.34.1 to help users upgrading.

[laryn]

@quicksketch What do you think of my suggestion above? It's most likely that people will be using standard layout templates (as opposed to flexible layout templates) on admin pages, so maybe that would be a simple and effective change?

[quicksketch]

@laryn Can you post a PR with the suggestion? I took a brief look last night but could not identify the root of this problem. As far as I can see, layout_get_layout_template_info() is already defaulting to layout.

It's clear that flushing the caches "fixes" the problem, so I think our issue has to do with the structure of the cache having changed.

[laryn]

@quicksketch I will try to test the theory out and post a PR.

[laryn]

@quicksketch PR added here, slightly different than my earlier idea. It defaults to layout unless it's sent a null value explicitly!

backdrop/backdrop#5275