Fix a condition which would lead to MySQL deadlock

[gmacdougall]

Running a delete by an index condition, for example:

DELETE FROM spree_adjustments
  WHERE adjustable_id = 1
  AND adjustable_type = 'Spree::Shipment'

will require MySQL to acquire a gap lock on the index which indexes
adjustable_id and adjustable_type.

Running this in a heavily loaded environment can lead to deadlocks in
MySQL as multiple threads need an exclusive gap lock on the index in
question.

By deleting records by the primary key instead, MySQL is able to deal
with this without requiring an exclusive lock on the primary key index,
avoiding the deadlock situation.

[athal7]

👍

[cbrunsdon]

👍

[Senjai]

👍

[jhawthorn]

I don't think the where clause is necessary, I think the fix here is just using destroy_all, which will issue one DELETE for each record.

Old:
adjustments.shipping.delete_all

DELETE FROM spree_adjustments
  WHERE adjustable_id = 1
  AND adjustable_type = 'Spree::Shipment'

This PR:

Spree::Adjustment.where(id: adjustments.shipping.pluck(:id)).destroy_all

SELECT * FROM spree_adjustments
  WHERE adjustable_id = 1
  AND adjustable_type = 'Spree::Shipment';
SELECT * spree_adjustments
  WHERE id IN (1,2);
DELETE FROM spree_adjustments WHERE id = 1;
(callback queries)
DELETE FROM spree_adjustments WHERE id = 2;
(callback queries)

Proposed:

adjustments.shipping.destroy_all

SELECT * FROM spree_adjustments
  WHERE adjustable_id = 1
  AND adjustable_type = 'Spree::Shipment';
DELETE FROM spree_adjustments WHERE id = 1;
(callback queries)
DELETE FROM spree_adjustments WHERE id = 2;
(callback queries)

[gmacdougall]

Thanks for that catch John. I think you're correct. I will make that modification.