I need to have some of my product variations to use the same stock and so i created this function.
Updating the stock works fine but i still have one problem.

This is my product structure :
Product A–> variation 1, variation 2 = group1 ; variation 3, variation 4 = group2 … (same SKU)
Product B–> variation 1, variation 2 = group1 ; variation 3, variation 4 = group2 … (same SKU)
Product C–> variation 1, variation 2 = group1 ; variation 3, variation 4 = group2 … (same SKU)
I suppose it’s not the best way to do it but I already have 50 products and as many variations like this, so I would like not to redo everything from scratch.

That’s why i need to add a SKU check in order to only update products/variations that have the same SKU.

add_action( 'woocommerce_order_status_processing', 'rudr_update_shared_stock' );

function rudr_update_shared_stock( $order_id ) {
    $order = wc_get_order( $order_id );
    $items = $order->get_items();

foreach ( $items as $item ) {
    $product = $item->get_product();
    $product_id = $product->get_id();

    $group = get_post_meta( $product_id, 'custom_field', true );

    if ( ! empty( $group ) ) {
        
        $args = array(
            'post_type' => 'product_variation',
            'post_status' => 'publish',
            'meta_query' => array(
                array(
                    'key' => 'custom_field',
                    'value' => $group,
                    'compare' => '=',
                )
            )
        );

        $products = new WP_Query( $args );

        if ( $products->have_posts() ) {
            $lowest_stock = PHP_INT_MAX;
            $ordered_product_stock = $product->get_stock_quantity();

            while ( $products->have_posts() ) {
                $products->the_post();
                $product_id = get_the_ID();

                $product = wc_get_product( $product_id );
                $current_stock = $product->get_stock_quantity();

                if ( $current_stock < $lowest_stock ) {
                    $lowest_stock = $current_stock;
                }
            }

            wp_reset_postdata();

            $args = array(
                'post_type' => 'product_variation',
                'post_status' => 'publish',
                'meta_query' => array(
                    array(
                        'key' => 'custom_field',
                        'value' => $group,
                        'compare' => '=',
                    ),
                    array(
                        'key' => '_stock',
                        'value' => $lowest_stock,
                        'compare' => '>',
                    )
                )
            );

            $products = new WP_Query( $args );

            if ( $products->have_posts() ) {
                $total_quantity = $item->get_quantity();

                while ( $products->have_posts() ) {
                    $products->the_post();
                    $product_id = get_the_ID();

                    $product = wc_get_product( $product_id );
                    $current_stock = $product->get_stock_quantity();

                    $new_stock = $current_stock - $total_quantity;

                    if ( $product_id == $item->get_variation_id() ) {
                        
                        $new_stock = $ordered_product_stock - $total_quantity;
                    } elseif ( $new_stock < 0 ) {
                        
                        $new_stock = 0;
                    }

                    $product->set_stock_quantity( $new_stock );
                    $product->save();
                }

                wp_reset_postdata();
            }
        }
    }
}

}