Incorrect error handling when converting classic to block menus

[dlh01]

Description

Some errors that can occur when Gutenberg_Navigation_Fallback::create_classic_menu_fallback() calls Gutenberg_Classic_To_Block_Menu_Converter::convert() aren't handled as accurately as they could be.

1. ::convert() can return a WP_Error object, but this error object won't be detected by the empty() check in ::create_classic_menu_fallback(). The WP_Error return type is also missing from the docs.
2. ::convert() returns an array instead of a string when no menu items are returned by wp_get_nav_menu_items().

See here:

gutenberg/lib/compat/wordpress-6.3/class-gutenberg-navigation-fallback.php

Lines 90 to 118 in 92dd32e

	private static function create_classic_menu_fallback() {
	// See if we have a classic menu.
	$classic_nav_menu = static::get_fallback_classic_menu();
	if ( ! $classic_nav_menu ) {
	return new WP_Error( 'no_classic_menus', __( 'No Classic Menus found.', 'gutenberg' ) );
	}
	// If there is a classic menu then convert it to blocks.
	$classic_nav_menu_blocks = Gutenberg_Classic_To_Block_Menu_Converter::convert( $classic_nav_menu );
	if ( empty( $classic_nav_menu_blocks ) ) {
	return new WP_Error( 'cannot_convert_classic_menu', __( 'Unable to convert Classic Menu to blocks.', 'gutenberg' ) );
	}
	// Create a new navigation menu from the classic menu.
	$classic_menu_fallback = wp_insert_post(
	array(
	'post_content' => $classic_nav_menu_blocks,
	'post_title' => $classic_nav_menu->name,
	'post_name' => $classic_nav_menu->slug,
	'post_status' => 'publish',
	'post_type' => 'wp_navigation',
	),
	true // So that we can check whether the result is an error.
	);
	return $classic_menu_fallback;
	}

and here:

gutenberg/lib/compat/wordpress-6.3/class-gutenberg-classic-to-block-menu-converter.php

Lines 16 to 53 in 92dd32e

	/**
	* Converts a Classic Menu to blocks.
	*
	* @param WP_Term $menu The Menu term object of the menu to convert.
	* @return string the serialized and normalized parsed blocks.
	*/
	public static function convert( $menu ) {
	if ( ! is_nav_menu( $menu ) ) {
	return new WP_Error(
	'invalid_menu',
	__( 'The menu provided is not a valid menu.', 'gutenberg' )
	);
	}
	$menu_items = wp_get_nav_menu_items( $menu->term_id, array( 'update_post_term_cache' => false ) );
	if ( empty( $menu_items ) ) {
	return array();
	}
	// Set up the $menu_item variables.
	// Adds the class property classes for the current context, if applicable.
	_wp_menu_item_classes_by_context( $menu_items );
	$menu_items_by_parent_id = static::group_by_parent_id( $menu_items );
	$first_menu_item = isset( $menu_items_by_parent_id[0] )
	? $menu_items_by_parent_id[0]
	: array();
	$inner_blocks = static::to_blocks(
	$first_menu_item,
	$menu_items_by_parent_id
	);
	return serialize_blocks( $inner_blocks );
	}

Step-by-step reproduction instructions

n/a

Screenshots, screen recording, code snippet

No response

Environment info

• WordPress trunk
• Gutenberg trunk

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

No

[mrfoxtalbot]

Could this cause a fatal error? I had a slightly old version of the Gutenberg plugin on my site (15.8.1) and ran into this error:

An error of type E_COMPILE_ERROR was caused in line 14 of the file /srv/htdocs/wp-content/plugins/gutenberg/lib/experimental/class-wp-classic-to-block-menu-converter.php. Error message: Cannot declare class WP_Classic_To_Block_Menu_Converter, because the name is already in use

[getdave]

The redeclare class warning is likely because both Core and Gutenberg have a class with the same name. @ramonjd did a fix for this I believe so perhaps it needs backporting to older versions of Gutenberg?

[getdave]

This Issue will be resolved by merging WordPress/wordpress-develop#4858.

[github-actions]

Hi,
This issue has gone 30 days without any activity. This means it is time for a check-in to make sure it is still relevant. If you are still experiencing this issue with the latest versions, you can help the project by responding to confirm the problem and by providing any updated reproduction steps.
Thanks for helping out.

[dlh01]

Resolved as described previously.