Tech Doc: Orders and Adjustments updates

Updating adjustments can be a bit tricky in Spree due to a web of callbacks. Adjustments can be in three different states (simplified): open, closed and finalised. An adjustment can be updated either open or closed.

But when an adjustment is associated to an order, and update triggers the after_save callback update_adjustable which calls update! on the order and that eventually triggers a reset of the adjustment unless it's closed. Here is the rough call trace:

1. adjustment.update_attributes triggers adjustable.update!.
2. Order#update! triggers the Spree::OrderUpdater.
3. The updater updates all shipments and adjustments.
4. Adjustment#update! doesn't do anything if the adjustment is closed.
5. When the adjustment is open, originator.update_adjustment is called.
6. ShippingMethod#Adjustment recalculates the amount and calls adjustment.update_attribute_without_callbacks(:amount, compute_amount(calculable)).

All this is Spree code. It's not something we accidentally caused with OFN overrides. So Spree defines this way:

• Open shipping adjustments are always recalculated -> can't be changed to a custom value.
• Closed shipping adjustments are not recalculated -> can be changed to a custom value.