Filter to check XML-RPC data before any DB insertion

[enrico.sorcinelli]

After searching into XML-RPC server class code, I realized that it seems that there isn't way to check XML-RPC input data before starting to insert/update any rows to database nor to return an IXR custom error message.
For example for new post, in order to check custom fields, a possible workaround is to use wp_insert_post_empty_content filter, but we are unable to customize the error message. Moreover at this point some DB rows are inserted, so inside the filter above we have to call wp_delete_post manually in order to clean DB (taking care to check auto-draft post status).
In the case of editing post, things get a bit more complicated, so we could use transactions with the help of xmlrpc_call/wp_insert_post actions.

So, the patch aims to add a new filter named xmlrpc_before_insert_post that allows to do this check in a more robust manner (for wp.newPost and wp.editPost XML-RCP methods).

Typical usage:

<?php

if ( defined( 'XMLRPC_REQUEST' ) ) {
   add_filter( 'xmlrpc_before_insert_post', 'my_filter_xmlrpc_before_insert_post', 10, 3 );
}
function my_filter_xmlrpc_before_insert_post ( $post_data, $content_struct, $user  ) {
   // do checks with $post_data, i.e:
   if ( title_contains_stop_words( $post_data['post_title'] ) )
      return new IXR_Error( 500, 'Post title contains invalid words' );
   return $post_data,
}

The filter is placed inside _insert_post helper function before get_default_post_to_edit() that isthe first statement that adds a new DB row.

Regards

[jbpaul17]

Punting to Future Release per today's 4.8 bug scrub.