Generate permalinks

I want the permalinks to be like youtube generated from letters and numbers in 9 digits I modified this code

add_filter( 'wp_unique_post_slug', 'unique_slug_108286', 10, 4 );
function unique_slug_108286( $slug) { 
    $n=4;
    $slug = bin2hex(random_bytes($n));  //just an example
    return $slug;
}

so it’s worked and gave me a random slug , but the problem is that this slug is changed every time I entered the post at the backend , so I need it to be generated once and unique

I also found this solution it also worked but the same problem It’s changed every time I enter the post

add_filter( 'wp_unique_post_slug', 'unique_slug_so_11762070', 10, 6 );

function unique_slug_so_11762070( $slug, $post_ID, $post_status, $post_type, $post_parent, $original_slug ) {
    $new_slug = so_11762070_unique_post_slug('guid');
    return $new_slug;
}

# From: https://stackoverflow.com/a/11762698
function so_11762070_unique_post_slug($col,$table="wp_posts"){
     global $wpdb;

     $alphabet = array_merge( range(0, 9), range('a','z') );

     $already_exists = true;
     do {

         $guidchr = array();
         for ($i=0; $i<32; $i++)
         $guidchr[] = $alphabet[array_rand( $alphabet )];


         $guid = sprintf( "%s", implode("", array_slice($guidchr, 0, 12, true)) );

       // check that GUID is unique
       $already_exists = (boolean) $wpdb->get_var("
       SELECT COUNT($col) as the_amount FROM $table WHERE $col="$guid"
       ");

      } while (true == $already_exists);

     return $guid;
}

The second argument of wp_unique_post_slug filter is post id. You can utilize that to generate the slug only once while creating the post for the first time.

Method 1:

add_filter( 'wp_unique_post_slug', 'unique_slug_108286', 10, 2 );
function unique_slug_108286( $slug, $postId ) {
    if ( ! $postId ) {
        $n = 4;
        $slug = bin2hex( random_bytes( $n ) );  //just an example
    }
    return $slug;
}

Other method would be using post meta to indicate whether a slug has been generated.

Method 2:

add_filter( 'wp_unique_post_slug', 'unique_slug_108286', 10, 2 );
function unique_slug_108286( $slug, $postId ) {
    if ( $postId && ! get_post_meta( $postId, 'slug_generated', true ) ) {
        $n = 4;
        $slug = bin2hex( random_bytes( $n ) );  //just an example
        update_post_meta( $postId, 'slug_generated', true );
    }
    return $slug;
}