Merging account with trustlines

Start by reviewing the accountMerge, payment, and change trust operations described in the issue, then read the comment thread for discussion of the two proposed solutions. Done would require a settled protocol design for handling non-native trustline assets during account merging, rather than an isolated code change.

In order to merge an account that has a trustline first the following operations need to be executed:

• a payment operation that transfers the remaining balance of the trusted asset to some other account
• a change trust operation to remove the trustline

Certain smart contract schemes (such as payment channels) involve refund transactions for escrow accounts that are created and signed well before they are submitted on chain. If such a scheme deals with an assets different from the native asset, then these refund transactions need to contain the following operations (as stated above):

• a payment operation to clear the balance of that asset
• remove the trustline of that asset
• merge account

The refund transaction will fail if the payment operation does not contain the correct balance of the trusted asset at the time the transaction is submitted. Since the refund transaction is created and signed a long time before, this would allow an attacker to invalidate the transaction by sending a single stroop of the trusted asset to the escrow account before the refund transaction is submitted.

A workaround would be to always set the trust limit of the asset to the current balance on the account. However, this has some downsides – e.g., topping up a payment channel would not be possible.

We propose any of the following solutions:

1. Extend the accountMerge operation: if the account to be merged has trustlines and the account to be merged into has the same trustlines, then merge all assets into the latter account.
2. Add a "merge asset" operation that behaves like a payment operation but does not specify an amount – it will transfer the complete remaining amount of the specified asset on the source account instead.