Block patterns with template type: Index do not load on the index template

[carolinan]

Description

Block patterns have a parameter called template type. A template type is, for example, a page or archive.
Patterns with this parameter are meant to be available when you:

• Create a new template of that type
• Open an existing template of that type that has no content (you can delete all the existing blocks, save and refresh).

But block patterns with the template type index do not show in the pattern selection modal when you are editing the index.

Step-by-step reproduction instructions

You can test this with the theme Twenty Twenty-four, downloadable at https://github.com/WordPress/twentytwentyfour

Delete the home.html template in the theme folder.
Open the Site Editor, and open the index template.
Delete all blocks. Save.
Reload the editor.
Confirm that no pattern selection modal shows.

Expected: There should be two patterns available.

Screenshots, screen recording, code snippet

No response

Environment info

Current 6.4 nightly through the beta tester plugin
Current Gutenberg trunk

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[aaronrobertshaw]

Thanks for submitting this issue @carolinan 👍

I'm struggling to find exactly when the expected behaviour was broken. This isn't an area I'm familiar with unfortunately.

From my digging so far, this doesn't appear to be a patterns issue directly, it seems more related to templates.

The start modal that displays the patterns will only render when there is fallback template content. You can find that check in the code here. The hook that retrieves that fallback content involves an API request, which for me always seems to return an empty string for an index template. It doesn't for other template types.

If I fudge the StartModal code to ignore if there is fallback content or not the modal appears with the expected block patterns.

There are a couple of related PRs that might help further investigations. #51477 apparently broke the template lookup API but that was resolved in #54599. Perhaps there is something else needed in that direction?

cc/ @ntsekouras @scruffian who might have more expertise here 🤞

[kevin940726]

I have a PR up in #54817. I'm not sure if I'm understanding the problem correctly though. Seems like there's a race here which I don't know if it's expected. If we manually delete all the content in a template and save it, then naturally it can't be used as a fallback content. Ironically, this is also when we want to show the start modal for the users.

Perhaps the problem lies in the check where we always expect the fallback content to be present. I don't have any historical knowledge as to why we need that check though. If we could remove it then the fix could be simpler I guess 🤔. I'll leave the decision to someone more knowledgeable than me on this aspect.