Loans: transfer_debt implementation

[lemunozm]

Description

Fixes #1378

Changes

• Base implementation
• Tests
• Benchmarks

[lemunozm]

This is ready for a final review

[hieronx]

Nice! Maybe good if @mustermeiszer also reviews but looks good to me

[lemunozm]

Just to double-check @hieronx, could it make sense to allow transferring debt to the same loan? i.e. for paying the interest with the amount borrowed from the principal.

Maybe I'm being too strict with this restriction

[mustermeiszer]

Regarding the logic here.

Is it okay if

• transfer_debt_action returns Ok(()) during proposal - borrower has enough funds to repay
• transfer_debt_action returns Err(..) during execution - borrower has NOT enough funds to repay

Otherwise it looks good.

[lemunozm]

I would say yes.

Although the borrower account is not considered here, the borrower repays with the amount they borrow, so the account funds are not moved. But it's also true that the borrower can propose a transfer_debt, later do borrow()/repay(), and when the transfer_debt is applied, it will fail because maybe the loan is already fully repaid or even closed.

How to remove later that "proposed action" that can never be applied is another topic that should be solved.

[mustermeiszer]

It can also fail if in the meantime the pool itself has not enough funds to borrow. So it can fail for various reasons. But I can not see any misbehaviour that could occur if this action has been released but never executed. Do you see some?

[lemunozm]

It can also fail if in the meantime the pool itself has not enough funds to borrow

Actually, this can not happen, because trasnfer_debt() does not transfer funds to/from the pool. It's one of the main reasons to implement this.

Do you see some?

If not executed, it does not change/mutate anything, so it takes no effect. Nevertheless from a game theory point of view, some investors could have redeemed/invested money because they knew about some "transfer_debt" actions that were supposed to occur but did not. So false positives could affect how investors behave, but not sure how really it affects the system or if it could be an attack vector. By now, I think there's no issue with this

[mustermeiszer]

Re-approve