How can I fix this possible race condition in Magento 1?

Question

Background

I have an entity that's directly linked to Magento shipments with three fields:

• entity_id
• data
• shipment_id

Problem

This entity is created based on a condition in an observer on the sales_order_shipment_save_before event. Something that looks like this:

if (condition) {
    // Create my entity and keep going
}
else {
    // Throw an exception and stop the Magento shipment creation
}

As you can see, if the condition is not matched, it must stop generating the Magento shipment. However, using this event, I cannot get the Magento shipment id using $shipment->getIncrementId(), because it's a before event.

If I try to use the sales_order_shipment_save_after event, I am able to get the increment id. However, I cannot stop the creation of the Magento shipment as it already has been created.

Temporary fix

A hacky way of fixing my issue is to get the next increment id by calling some code like this:

$sql = sprintf("SELECT AUTO_INCREMENT FROM information_schema.tables WHERE table_name = 'sales_flat_shipment' and table_schema = '%s'",
               Mage::getConfig()->getNode('global/resources/default_setup/connection/dbname'));
$readConn = Mage::getSingleton('core/resource')->getConnection('core_read');
$results = $readConn->fetchAll($sql);
$nextIncrementId = $results[0]['AUTO_INCREMENT'];

But this could possibly run into a race condition.

So how can I fix my problem without running into a race condition?

Answer 1

I don't think any of the both events can do the trick by itself. As long as you don't need that entity_id for the condition before the Magento shipping kicks in, you can use both events to make it work:

• sales_order_shipment_save_before: Do you checks in the condition and if they succeed, let them go through. Save all information within the observer.

• sales_order_shipment_save_after: Save the id that was generated in the Magento shipping and save all additional information you have saved before within the observer.

To make that work, you have to make sure the observer is loaded as a Singleton object, because otherwise the information will be gone as soon as it's recreated during the event. To me this solution is much better since it doesn't use any hacks to make it work, even though it comes with that additional complexity of another event.

Answer 2

I didn't look really deep, but I saw this in save():

        $this->_beforeSave();
        if ($this->_dataSaveAllowed) {
            $this->_getResource()->save($this);
            $this->_afterSave();
        }

There's no setter for $_dataSaveAllowed.

Maybe extend the shipment model to add a setter for this attribute and set it to false in your observer?