XML-RPC _insert_post error when updating a post with a thumbnail.

[picklepete]

Versions affected: 3.4+
Summary: Updating a post with wp_editPost is throwing an "Invalid attachment ID" error due to a ​change to set_post_thumbnail in 3.4. In ​3.3.3 and earlier "true" was always returned. Now, when ​the post_thumbnail is ​compared against the existing post_thumbnail the "false" which is returned bubbles up to set_post_thumbnail and an "Invalid attachment ID" is thrown.

The attached patch satisfies two needs:

1. That the attachment's existence is tested, and if it's invalid, an "Invalid attachment ID" error is thrown.
2. That it should be possible for set_post_thumbnail to execute and return false, which it will if the post thumbnail hasn't been changed.

[picklepete]

How to replicate:

<?php
    require 'wp-config.php';
    require 'wp-includes/class-IXR.php';
    require 'wp-includes/class-wp-xmlrpc-server.php';

    $post_ID = 4117;
    $post_data = array('ID' => $post_ID, 'post_thumbnail' => '3964');  // Real attachment ID

    $server = new wp_xmlrpc_server();
    var_dump($server->wp_editPost(array(
        1,
        'username',
        'password',
        $post_ID,
        $post_data
    )));

[nacin]

set_post_thumbnail() can indeed return false if the row wasn't updated because the thumbnail was already set to that ID.

[ryan]

In [22277]:

In wp_xmlrpc_server::_insert_post(), don't return an error if set_post_thumbnail() returns false when the attachment ID doesn't change.

Props picklepete
fixes #22204