Blank page when `templates/index.html` exists in classic themes.

[costdev]

Description

See the related Trac ticket: https://core.trac.wordpress.org/ticket/54910

If a classic theme has templates/index.html, which may be empty or have non-block HTML markup in it, it can be incorrectly identified as a block theme. For example, some developers and/or hosts, whether good practice or not, can place an index.html file in every directory to prevent directory listing. Alternatively, some classic themes may have other uses for templates/index.html, and the classic theme may not be actively, or regularly, maintained. This has resulted in some users receiving a white screen upon updating to 5.9.

An idea from @carolinan in the Trac ticket above suggested checking templates/index.html for block markup. The patch aims to do exactly that. However, this needs insights and evaluation from the editor team regarding block theme architecture and we also need to consider performance. We've implemented caching using WP_Theme::cache_get()/add()/delete() to help with this.

Step-by-step reproduction instructions

1. Create a classic theme which happens to include an empty templates/index.html.
2. Activate the theme.

• The Editor item will be available and the frontend will (likely) output Empty template: Index.

3. Add some non-block HTML to templates/index.html.

• The Editor item will be available.
• Viewing the Editor will (likely) show This block has encountered an error and cannot be previewed..
• The frontend will output the non-block HTML inside .wp-site-blocks.

Screenshots, screen recording, code snippet

No response

Environment info

No response

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

[Trapsta]

@costdev guessing this should be currently status blocked pending discussion on the trac ticket on how best to implement the patch but I think in the meantime we could be testing for any issues that might come up in the editor and also check for any performance hits/concern from the patch. correct?

[costdev]

@Trapsta At the moment it looks that way yes. Testing is much appreciated! 🙂