Footnotes: guard against invalid post meta values

[ramonjd]

What

meta.footnotes is parsed without a guard:

const footnotes = meta.footnotes ? JSON.parse( meta.footnotes ) : [];
const currentOrder = footnotes.map( ( fn ) => fn.id );

If the value is malformed, or is valid JSON that is not an array, this throws. It runs inside a registry.subscribe callback, so no React error boundary catches it, and editEntityRecord on the next line never runs.

The result: the keystroke appears on screen, the post never becomes dirty, Save stays inert, and the edit is gone. No notice, no crash screen. Every keystroke after that throws too, so the post cannot be edited.

Reachable because the meta is registered as a plain string with no sanitize_callback, and the filter that would coerce it is only registered for users without unfiltered_html. Administrators are unprotected.

How

Treat anything unreadable as no footnotes, and warn in development. This matches what PHP already does:_gutenberg_filter_post_meta_footnotes returns '' for a non-array on save, and the block's index.php checks ! is_array() on render.

This PR adds the same check in JS.

	$footnotes_decoded = json_decode( $footnotes, true );
	if ( ! is_array( $footnotes_decoded ) ) {
		return '';
	}

Manual testing

The best way I could find to test this was, as an administrator (so unfiltered_html is on) on single-site:

1. Open a post. Options → Preferences → Panels → Custom fields → Enable, and let the page reload.
2. In the Custom Fields box, add a field named footnotes with the value null.
3. Update, then reload the editor.
4. Type a character in any block.

On trunk: the character appears, the post never becomes dirty, and the console shows Uncaught TypeError: Cannot read properties of null (reading 'map'). With this change: the post edits and saves normally.

Other values that reproduce it: 5, {"a":1}, {. [] is the valid empty case.

Use of AI

Claude discovered this while testing something else 😄 It drove, I steered.

[github-actions]

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Co-authored-by: ramonjd <ramonopoly@git.wordpress.org>
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[github-actions]

Size Change: +83 B (0%)

Total Size: 7.81 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-library/index.min.js	356 kB	+25 B (+0.01%)
build/scripts/core-data/index.min.js	36.9 kB	+58 B (+0.16%)

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[ellatrix]

Nit: I usually prefer to limit what's in the try block so it's clear what may error

[ramonjd]

Good call! I'll update before merge.

[ellatrix]

Why not an e2e test instead to test what the users sees?

[ramonjd]

No reason - unit tests just run faster and this was covering an unreported edge case. 😄 Happy to add one in a follow up.

Thank you!

[ramonjd]

Actually, I'll just add it here. The unit test covers a few malformed shapes pretty cheaply, but agree it'd be helpful to cover the user symptom. Cheers for the nudge