Empty array for non-single post meta breaks post save through REST API

[BrechtVds]

The change in #50790 causes an "Updating failed. Could not delete meta value from database." error when saving post meta in Gutenberg.

The reason seems to be that Gutenberg uses default values for any post meta that has been registered for the REST API. For non-single meta that's an empty array. Now when someone makes a meta change and tries to save it in Gutenberg, that empty array gets passed along. Because of the change, the WP_REST_Meta_Fields::update_value() sees an empty array and wants to delete that non-existing meta, causing the error.

Can be replicated by adding this sample plugin file:

<?php
/*
Plugin Name: Test Bug Plugin
Description: Testing bug.
Version: 1.0
License: GPLv2 or later
*/

function test_bug_plugin_register_meta() {
    register_meta( 'post', 'example-post-meta-field', array(
		'show_in_rest' => true,
		'single' => false, // Is the default value. Single = true doesn't cause the problem.
	) );
}
add_action( 'init', 'test_bug_plugin_register_meta');

Then use a simple plugin like ​https://wordpress.org/plugins/conditionally-display-featured-image-on-singular-pages/ which adds a checkbox in the "Featured Image" section when creating a post, which gets saved as post meta.

This video shows the error message: ​https://www.loom.com/share/025fe7b6f2f44955b303daf3317f88d0

Wasn't sure if this should be logged as a Gutenberg or core bug. But Gutenberg defaulting to an empty array for non-single meta seems logical to me, so I feel like this should get fixed in the REST API.

First Trac ticket. Let me know if I need to add/change anything!

[BrechtVds]

Forgot to include the exact error message the REST API returns when trying to save:

{"code":"rest_meta_database_error","message":"Could not delete meta value from database.","data":{"key":"example-post-meta-field","status":500}}

[TimothyBlynJacobs]

Thanks for the report @BrechtVds and welcome to trac!

Yeah, this is definitely something to be fixed on the REST API side. Gutenberg isn't doing anything special to cause this behavior. The issue is that delete_metadata errors if there is nothing to delete.

I think we should probably add a safe guard in \WP_REST_Meta_Fields::delete_meta_value to check that get_metadata_raw returns at least one value. If it doesn't, IMO, we should return true and treat it as a no-op the same way we do for updating metadata to an identical value.

[TimothyBlynJacobs]

#52793 was marked as a duplicate.

[goaroundagain]

I've updated the unit test in my (duplicate) ticket to reproduce the issue. Maybe it can help https://core.trac.wordpress.org/attachment/ticket/52793/52793.diff

[TimothyBlynJacobs]

Thanks for the test @goaroundagain! Could you separate it out into a different test case? Something like test_delete_does_not_trigger_error_if_no_meta_values?

@BrechtVds, @goaroundagain Would either of you be interested in working on a patch for this issue?

[BrechtVds]

A bit strapped for time and I'd have to educate myself about the process and best practices first. I will follow along to see how it's supposed to be handled and hopefully have some more time to contribute to core in the future.

[goaroundagain]

I patched the error by checking in \WP_REST_Meta_Fields::delete_meta_value if get_metadata_raw is null and then return true

[peterwilsoncc]

This looks good to me, tests well locally too.

In 52787.2.diff​:

• No functional change to 52787.diff​
• Added a duplicate test for deleting non-existent data with null as well as when using an empty array

[peterwilsoncc]

In 50567:

REST API: Prevent database error when deleting meta data.

Add a check to WP_REST_Meta_Fields::delete_meta_value() ensuring meta data is set before attempting to delete it from the database. If the data does not exist, the delete is considered successful as the data matches the desired state.

Props BrechtVds, goaroundagain, TimothyBlynJacobs.
Fixes #52787.

[peterwilsoncc]

Reopening for commit to the 5.7 branch following second a committer's sign-off.

[TimothyBlynJacobs]

Patch looks great to me.

[peterwilsoncc]

In 50573:

REST API: Prevent database error when deleting meta data.

Add a check to WP_REST_Meta_Fields::delete_meta_value() ensuring meta data is set before attempting to delete it from the database. If the data does not exist, the delete is considered successful as the data matches the desired state.

Props BrechtVds, goaroundagain, TimothyBlynJacobs.
Merges [50567] to the 5.7 branch.
Fixes #52787.