Trigger variable product sync hook after save

Because only a product ID is passed to callbacks on 'woocommerce_variable_product_sync',
they will need to instantiate their own instance of the product to do anything useful
with it. This creates a lot of potential for problems given that the product has at that
stage had its data sync'd but not all of it saved. For example, a callback may modify some
meta data on its own instance of the product, then save that, only to immediately have
that overridden if the instance of $product in WC_Product_Variable::sync() also had changes
to that meta data.

Instead, trigger 'woocommerce_variable_product_sync' only after the product has been
saved. This is backward compatible because the hook was triggered at the very end of
the process in WC < 2.7.

includes/class-wc-product-variable.php

@@ -455,11 +455,11 @@ class WC_Product_Variable extends WC_Product {
 			$data_store->sync_stock_status( $product );
 			self::sync_attributes( $product ); // Legacy update of attributes.
 
-			do_action( 'woocommerce_variable_product_sync', $product->get_id(), $product->get_visible_children( 'edit' ), $save );
-
 			if ( $save ) {
 				$product->save();
 			}
+
+			do_action( 'woocommerce_variable_product_sync', $product->get_id(), $product->get_visible_children( 'edit' ), $save );
 		}
 		return $product;
 	}