WordPress/wp-includes/blocks/footnotes.php

<?php
/**
 * Server-side rendering of the `core/footnotes` block.
 *
 * @package WordPress
 */

/**
 * Renders the `core/footnotes` block on the server.
 *
 * @since 6.3.0
 *
 * @param array    $attributes Block attributes.
 * @param string   $content    Block default content.
 * @param WP_Block $block      Block instance.
 *
 * @return string Returns the HTML representing the footnotes.
 */
function render_block_core_footnotes( $attributes, $content, $block ) {
	// Bail out early if the post ID is not set for some reason.
	if ( empty( $block->context['postId'] ) ) {
		return '';
	}

	if ( post_password_required( $block->context['postId'] ) ) {
		return;
	}

	$footnotes = get_post_meta( $block->context['postId'], 'footnotes', true );

	if ( ! $footnotes ) {
		return;
	}

	$footnotes = json_decode( $footnotes, true );

	if ( count( $footnotes ) === 0 ) {
		return '';
	}

	$wrapper_attributes = get_block_wrapper_attributes();

	$block_content = '';

	foreach ( $footnotes as $footnote ) {
		$block_content .= sprintf(
			'<li id="%1$s">%2$s <a href="#%1$s-link">↩︎</a></li>',
			$footnote['id'],
			$footnote['content']
		);
	}

	return sprintf(
		'<ol %1$s>%2$s</ol>',
		$wrapper_attributes,
		$block_content
	);
}

/**
 * Registers the `core/footnotes` block on the server.
 *
 * @since 6.3.0
 */
function register_block_core_footnotes() {
	foreach ( array( 'post', 'page' ) as $post_type ) {
		register_post_meta(
			$post_type,
			'footnotes',
			array(
				'show_in_rest' => true,
				'single'       => true,
				'type'         => 'string',
			)
		);
	}
	register_block_type_from_metadata(
		__DIR__ . '/footnotes',
		array(
			'render_callback' => 'render_block_core_footnotes',
		)
	);
}
add_action( 'init', 'register_block_core_footnotes' );

/**
 * Saves the footnotes meta value to the revision.
 *
 * @since 6.3.0
 *
 * @param int $revision_id The revision ID.
 */
function wp_save_footnotes_meta( $revision_id ) {
	$post_id = wp_is_post_revision( $revision_id );

	if ( $post_id ) {
		$footnotes = get_post_meta( $post_id, 'footnotes', true );

		if ( $footnotes ) {
			// Can't use update_post_meta() because it doesn't allow revisions.
			update_metadata( 'post', $revision_id, 'footnotes', $footnotes );
		}
	}
}
add_action( 'wp_after_insert_post', 'wp_save_footnotes_meta' );

/**
 * Keeps track of the revision ID for "rest_after_insert_{$post_type}".
 *
 * @since 6.3.0
 *
 * @global int $wp_temporary_footnote_revision_id The footnote revision ID.
 *
 * @param int $revision_id The revision ID.
 */
function wp_keep_footnotes_revision_id( $revision_id ) {
	global $wp_temporary_footnote_revision_id;
	$wp_temporary_footnote_revision_id = $revision_id;
}
add_action( '_wp_put_post_revision', 'wp_keep_footnotes_revision_id' );

/**
 * This is a specific fix for the REST API. The REST API doesn't update
 * the post and post meta in one go (through `meta_input`). While it
 * does fix the `wp_after_insert_post` hook to be called correctly after
 * updating meta, it does NOT fix hooks such as post_updated and
 * save_post, which are normally also fired after post meta is updated
 * in `wp_insert_post()`. Unfortunately, `wp_save_post_revision` is
 * added to the `post_updated` action, which means the meta is not
 * available at the time, so we have to add it afterwards through the
 * `"rest_after_insert_{$post_type}"` action.
 *
 * @since 6.3.0
 *
 * @global int $wp_temporary_footnote_revision_id The footnote revision ID.
 *
 * @param WP_Post $post The post object.
 */
function wp_add_footnotes_revisions_to_post_meta( $post ) {
	global $wp_temporary_footnote_revision_id;

	if ( $wp_temporary_footnote_revision_id ) {
		$revision = get_post( $wp_temporary_footnote_revision_id );

		if ( ! $revision ) {
			return;
		}

		$post_id = $revision->post_parent;

		// Just making sure we're updating the right revision.
		if ( $post->ID === $post_id ) {
			$footnotes = get_post_meta( $post_id, 'footnotes', true );

			if ( $footnotes ) {
				// Can't use update_post_meta() because it doesn't allow revisions.
				update_metadata( 'post', $wp_temporary_footnote_revision_id, 'footnotes', $footnotes );
			}
		}
	}
}

foreach ( array( 'post', 'page' ) as $post_type ) {
	add_action( "rest_after_insert_{$post_type}", 'wp_add_footnotes_revisions_to_post_meta' );
}

/**
 * Restores the footnotes meta value from the revision.
 *
 * @since 6.3.0
 *
 * @param int $post_id     The post ID.
 * @param int $revision_id The revision ID.
 */
function wp_restore_footnotes_from_revision( $post_id, $revision_id ) {
	$footnotes = get_post_meta( $revision_id, 'footnotes', true );

	if ( $footnotes ) {
		update_post_meta( $post_id, 'footnotes', $footnotes );
	} else {
		delete_post_meta( $post_id, 'footnotes' );
	}
}
add_action( 'wp_restore_post_revision', 'wp_restore_footnotes_from_revision', 10, 2 );

/**
 * Adds the footnotes field to the revision.
 *
 * @since 6.3.0
 *
 * @param array $fields The revision fields.
 * @return array The revision fields.
 */
function wp_add_footnotes_to_revision( $fields ) {
	$fields['footnotes'] = __( 'Footnotes' );
	return $fields;
}
add_filter( '_wp_post_revision_fields', 'wp_add_footnotes_to_revision' );

/**
 * Gets the footnotes field from the revision.
 *
 * @since 6.3.0
 *
 * @param string $revision_field The field value, but $revision->$field
 *                               (footnotes) does not exist.
 * @param string $field          The field name, in this case "footnotes".
 * @param object $revision       The revision object to compare against.
 * @return string The field value.
 */
function wp_get_footnotes_from_revision( $revision_field, $field, $revision ) {
	return get_metadata( 'post', $revision->ID, $field, true );
}
add_filter( 'wp_post_revision_field_footnotes', 'wp_get_footnotes_from_revision', 10, 3 );