How to validate XML-RPC post creation and cancel when needed?

None of the previous solutions were working for me (maybe because I´m posting using metaWeblog.newPost)

So I made my own:

1-Make a copy of xmlrpc.php and rename to xmlrpc2.php to stay safe from WordPress updates.

2-Paste the code below this part:

/** Include the bootstrap for setting up WordPress environment */
require_once __DIR__ . '/wp-load.php';

Paste this code to prevent duplicate titles:

$mixml = file_get_contents('php://input');
$mixml2 = simplexml_load_string($mixml);
$json = json_encode($mixml2);

if ( ! is_admin() ) {
    require_once( ABSPATH . 'wp-admin/includes/post.php' );
}

$jsonIterator = new RecursiveIteratorIterator(
    new RecursiveArrayIterator(json_decode($json, TRUE)),
    RecursiveIteratorIterator::SELF_FIRST);

$nextok=false;
//Sorry for the noob parsing :)
foreach ($jsonIterator as $key => $val) {   
    if ($val=="title") $nextok=true;    
    if (($key=="string")&&($nextok)) { 
        $nextok=false;      
        $post_id = post_exists($val);
        //already exists, so I break the posting with exit
        if ($post_id) exit;                             
    }                   
}

Explanation:

1. Read the XMLRPC input.
2. Include post.php so we can use post_exists later.
3. Get the post title. Sorry for the dirty parsing :) you will probably want to improve it.
4. If the title already exists (even in recycle bin) we use exit so nothing is posted.

You can modify the original xmlrpc.php file, but you may lose the changes in future updates.

Renaming xmlrpc.php file is recommended since some servers block that file path and some brute force attacks target it.

It looks like the xmlrpc_prepare_post filter is only applied to the output of the wp_getPost and wp_getRevision methods of the wp_xmlrpc_server class.

It would be great if this code line:

do_action( 'xmlrpc_call', 'wp.newPost' );

would be replaced with extra input arguments, for example:

do_action( 'xmlrpc_call', 'wp.newPost', ..., $content_struct );

but that's not going to happen according to this ticket.

So we need to find another way around this.

Possible workarounds:

Here are some untested ideas using the xmlrpc_call and the xmlrpc_wp_insert_post_data filters.

Modify input data before it's inserted with wp_insert_posts():

/**
 * Prevent duplicate posts when doing wp.newPost via XML-RPC
 *
 * @see http://wordpress.stackexchange.com/a/157261/26350
 */

add_action( 'xmlrpc_call', 'wpse_xmlrpc_call' );

function wpse_xmlrpc_call( $method )
{
    if( 'wp.newPost' === $method )
        add_filter( 'xmlrpc_wp_insert_post_data', 'wpse_xmlrpc_wp_insert_post_data' );
}

function wpse_xmlrpc_wp_insert_post_data( $post_data )
{
    // Check if the post title exists:
    $tmp = get_page_by_title( 
        $post_data['post_title'], 
        OBJECT, 
        $post_data['post_type'] 
    );

    // Go from 'insert' to 'update' mode within wp_insert_post():
    if( is_object ( $tmp ) )
        $post_data['ID'] = $tmp->ID; 

    return $post_data;  
}

Here we try to find an existing post with the same title, during wp.newPost calls. If we find one, we add it's ID to the $post_data array, so it will be updated instead.

Notice that we could also have modified the post_status instead with:

    $post_data['post_status'] = 'trash'; 

so all extra inserts are directed to the trash.

You could also try to create your own insert method via the xmlrpc_methods filter.

I hope you can modify this to your needs, assuming this will work ;-)

Update: I've now tested this idea and it works, I can both trash the duplicated posts or update it directly.

Thanks to @DavidPeterson for noticing my silly PHP syntax errors from when I edited the code within the WPSE editor ;-)

In the wp-includes/class-wp-xmlrpc-server.php file

Inside the function mw_newPost() after these lines:

$post_title = isset( $content_struct['title'] ) ? $content_struct['title'] : null;
$post_content = isset( $content_struct['description'] ) ? $content_struct['description'] : null;

Add:

global $wpdb;
$some_post = $wpdb->get_row("
    SELECT ID
    FROM {$wpdb->posts}
    WHERE post_title = '{$post_title}'
");

And create a statement:

if (!empty($some_post->ID) and $some_post->ID > 0) {
    return 0;
} else {
    // the rest of the code that already exists in the function
    /*
    $post_status = $publish ? 'publish' : 'draft';
    ...
    return strval($post_ID);
    */
}