The $context argument passed to filters such as hooked_block_types, hooked_block, and hooked_block_{$hooked_block_type} allows them to conditionally insert a hooked block. If the anchor block is contained in a template or template part, $context will be set to a WP_Block_Template object reflecting that template or part.

The aforementioned filters are applied when hooked block insertion is run upon reading a template (or part) from the DB (and before sending the template/part content with hooked blocks inserted over the REST API to the client), but also upon writing to the DB, as that's when the ignoredHookedBlocks metadata attribute is set.

Prior to this changeset, the $context passed to Block Hooks related filters in the latter case reflected the template/part that was already stored in the database (if any), which is a bug; instead, it needs to reflect the template/part that will result from the incoming POST network request that will trigger a database update.

Those incoming changes are encapsulated in the $changes argument passed to the reset_pre_insert_template and reset_pre_insert_template_part filters, respectively, and thus to the inject_ignored_hooked_blocks_metadata_attributes function that is hooked to them. $changes is of type stdClass and only contains the fields that need to be updated. That means that in order to create a WP_Block_Template object, a two-step process is needed:

• Emulate what the updated wp_template or wp_template_part post object in the database will look like by merging $changes on top of the existing $post object fetched from the DB, or from the theme's block template (part) file, if any.
• Create a WP_Block_Template from the resulting object.

To achieve the latter, a new helper method (_build_block_template_object_from_post_object) is extracted from the existing _build_block_template_result_from_post function. (The latter cannot be used directly as it includes a few database calls that will fail if no post object for the template has existed yet in the database.)

While somewhat complicated to implement, the overall change allows for better separation of concerns and isolation of entities. This is visible e.g. in the fact that inject_ignored_hooked_blocks_metadata_attributes no longer requires a $request argument, which is reflected by unit tests no longer needing to create a $request object to pass to it, thus decoupling the function from the templates endpoint controller.

Unit tests for inject_ignored_hooked_blocks_metadata_attributes have been moved to a new, separate file. Test coverage has been added such that now, all three relevant scenarios are covered:

• The template doesn't exist in the DB, nor is there a block theme template file for it.
• The template doesn't exist in the DB, but there is a block theme template file for it.
• The template already exists in the DB.

Trac ticket: https://core.trac.wordpress.org/ticket/60754

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.