[ANCHOR-525] Introduce NotifyAmountsAssetsUpdated RPC

[philipliu]

Description

This introduces the NotifyAmountsAssetsUpdated RPC which allows SEP-6 implementations to update all amounts/assets when it calculates fees.

Context

The current NotifyAmountsUpdated RPC was previously created for the SEP-24 withdrawal flow where only the amount values for amount_out and amount_fee need to be updated. In SEP-6, the asset values need to be updated as well since in the case of a deposit, the amount_in asset and amount are not known at transaction creation time. In the SEP-6 withdrawal flow, the amount_out asset and amount are unknown.

The current reference implementation works around this by updating the amount_in value and asset when it requests off-chain funds which really could be down when it updates the fees initially (Sep6EventProcessor).

Additionally, by not using this RPC, the SEP-6 implementation would not need to purposely set wrong amount values when creating the transaction (Sep6Service).

Testing

• ./gradlew test

Documentation

TODO: make the stellar-docs change if this is ok.

Known limitations

N/A

[JakeUrban]

I'm not sure I fully understand the problem this is addressing and what the changes are.

The explanation of the notify_amounts_updated is as follows:

If onchain funds were received, but for some reason the amount_in differs from specified in the interactive flow (amount_expected), you can update amount_out and amount_fee to make them correspond to the actual amount_in. The status of the transaction in this case won't be changed and will be equal to pending_anchor.

This method is really only for an edge case, which is when the amount the client said they would send is not what was actually sent.

For 99% of cases, amounts and assets should be set when requesting and receiving funds using the following methods:

• request_offchain_funds
• notify_offchain_funds_received
• request_onchain_funds
• notify_onchain_funds_received

Why can't SEP-6 also use these methods, or why aren't these methods sufficient?

[philipliu]

@JakeUrban Ah, this was a misunderstanding on my part. This all stems from the fact that I was using notify_amounts_updated to update the fees before requesting KYC. This RPC requires amount_fee and amount_out to be previously set, which is not the case for SEP-6 when transactions are created. The SEP-6 reference implementation does use those methods you listed so there's no need to introduce this new PR.

I'll close this PR.