fix(medusa): Transaction lock issues on create/update cart items

[adrien2p]

what

The fixes will be done in multiple steps

• (This PR) Add idempotency support on the create-line-item handler and few performance improvements along the way
• (later) Ensure the create line item only handler line item creation and not update
• (later) Refactor cart service add line item method to multiple smaller method with each of them having their responsibility

FIXES CORE-801

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Latest commit: 75b9822

The changes in this PR will be included in the next version bump.

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@medusajs/medusa	Patch

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[srindom]

This fix will require the caller of addLineItem to always bring a cart's line items to a valid state. I don't think we should force the caller to hold this responsibility.

Could we instead update the isolation level in the controller to be serializable and throw a 409 if there are read/write dependencies?

[srindom]

If we make the endpoint idempotent, we can add retry functionality in the js client that automatically retries the request if we reach a 409.

[adrien2p]

Yes sure, i should add idempotency support in that case but yeah if we explicitly explain to the consumer that in some cases it needs to be retried i am ok with that 👍 i thought about it but wasn’t sure as it transform the endpoint management 🥹

[srindom]

Wdyt of avoiding joining all items in addLineItem like I have done here: develop...fix/serialized-add-to-cart#diff-b1b00df3e31608f5099498d5dc7dfc5f92032809e1e16cbd7b1e9a85304c8edbR578

Curious if this would be more performant in cases where the cart holds many items, despite the extra db queries?

[olivermrbl]

LGTM, will let @srindom take a final stab at it as he had a couple of suggestions.

[srindom]

LGTM!