Relax allowed path restrictions for block metadata collections and make the allowed root list filterable

[felixarntz]

This PR implements a solution to allow block metadata collections in additional directories, based on the ideas from the Trac ticket:

• By default, it allows block metadata collections within the wp-includes directory, as well as the root directories for plugins, must-use plugins, and themes.
• The list of allowed root directories can be expanded via a new filter wp_allowed_block_metadata_collection_roots.
• Note: Block metadata collections paths must never exactly match one of the root directories or their parent directories. Otherwise it would be possible to e.g. register a collection for the entire wp-content/plugins directory (or wp-content, or the root directory of WordPress etc.), which then would conflict with other collections within that directory.

Trac ticket: https://core.trac.wordpress.org/ticket/62140

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[swissspidy]

I dislike that symlinked plugins only work when using this filter.

That means developers will have to add some extra code to make their plugins work properly during local development or in other more sophisticated server setups.

What stops plugins and themes from just shipping something like add_filter( 'wp_allowed_block_metadata_collection_roots', static function( $roots ) { $roots[] = __DIR__; return $roots; } );?

I bet we'll see plugins in the directory with such code, intentionally or not.

[gziolo]

An existing utility function get_block_asset_url is used with all block assets. It covers all the same edge cases discussed, so maybe it would be possible to mirror its functionality with the specific requirements of where the collection roots can live.

[felixarntz]

I dislike that symlinked plugins only work when using this filter.

I have an idea for an alternative solution where this list is used more like a denylist than a mix of denylist and allowlist. That should address the limitation. Currently working on implementing it.

What stops plugins and themes from just shipping something like add_filter( 'wp_allowed_block_metadata_collection_roots', static function( $roots ) { $roots[] = __DIR__; return $roots; } );?

Nothing, but nothing in WordPress Core has ever stopped you from _doing_it_wrong - doesn't mean you should. There are other filters for allowlists that anyone could use to loosen the restrictions in a dangerous way, e.g. allowed_redirect_hosts.

Anyway, I think by using this as purely a denylist we should be able to simplify the usage, while still preventing (by default) the key problem this is supposed to prevent (see https://core.trac.wordpress.org/ticket/62140#comment:25).

[felixarntz]

Ping @mreishus, since you worked on the original code. Would be great to get your review.