add_action( 'save_post_product', 'auto_add_product_attributes', 50, 3 );
function auto_add_product_attributes( $post_id, $post, $update  ) {

    ## --- Checking --- ##

    // Exit if it's an autosave
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
        return $post_id;

    // Exit if it's an update
    if( $update ){
        update_post_meta( $post_id, '_updating', 'YES' );
        return $post_id;
    } else {
        update_post_meta( $post_id, '_updating', 'NO' );
    }

    // Exit if user is not allowed
    if ( ! current_user_can( 'edit_product', $post_id ) )
        return $post_id;

    ## --- The code --- ##

    global $wpdb;

    // Get all existing product attributes
    $attributes = $wpdb->get_results( "SELECT * FROM {$wpdb->prefix}woocommerce_attribute_taxonomies" );

    $position  = 0;  // Auto incremented position value starting at '0'
    $visible   = ''; // can be: '' or '1'
    $variation = ''; // can be: '' or '1'
    $data      = array(); // initialising (empty array)

    foreach( $attributes as $attribute ){
        // Get the correct taxonomy for product attributes
        $taxonomy = 'pa_'.$attribute->attribute_name;
        $attribute_id = $attribute->attribute_id;

        // Get all term Ids values for the current product attribute (array)
        $term_ids = get_terms(array('taxonomy' => $taxonomy, 'fields' => 'ids'));

        // Get an empty instance of the WC_Product_Attribute object
        $product_attribute = new WC_Product_Attribute();

        // Set the related data in the WC_Product_Attribute object
        $product_attribute->set_id( $attribute_id );
        $product_attribute->set_name( $taxonomy );
        $product_attribute->set_options( $term_ids );
        $product_attribute->set_position( $position );
        $product_attribute->set_visible( $visible );
        $product_attribute->set_variation( $variation );

        // Add the product WC_Product_Attribute object in the data array
        $data[$taxonomy] = $product_attribute;

        $position++; // Incrementing position
    }
    // Get an instance of the WC_Product object
    $product = wc_get_product( $post_id );

    // Set the array of WC_Product_Attribute objects in the product
    $product->set_attributes( $data );

    $product->save(); // Save the product
}