Fee integration

[karim-en]

Description

This PR contains Fee Implementation for Deposit and Withdrawal flows for Ether transfer using Rainbow Bridge (Business requirement).

Specification:

• Deposit and withdrawal can have separate fee percentages.
• By default, the fee collected is locked in the same contract that the owner could claim using the function claim_fee.
• The fee is transferred to the fee owner account instead of the current contract account if the set_fee_owner was called.
• All the fee setters are only callable by the owner.
• The fee_amount has lower and upper bounds.
• If the the calculated fee_amount is higher than the withdraw/deposit amount then charge the full amount as a fee and don't transfer anything to the recipient.
• The fee is set in 6 decimal precision 10% = 0.1 * 10e6 = 100_000
• It is possible to set a particular fee for silos on deposit and withdrawal. The silo is just a NEAR account, so as a side effect, it is possible to set a specific fee to any NEAR account.

Added public function:

fn calculate_fee_amount(&self, amount: U128, fee_type: FeeType, silo: Option<AccountId>) -> U128;
fn set_deposit_fee(&mut self, fee: Option<Fee>);
fn set_withdraw_fee(&mut self, fee: Option<Fee>);
fn set_deposit_fee_per_silo(&mut self, silo: AccountId, fee: Option<Fee>);
fn set_withdraw_fee_per_silo(&mut self, silo: AccountId, fee: Option<Fee>);
fn set_fee_owner(&mut self, owner: Option<AccountId>);
fn claim_fee(&mut self, amount: U128, receiver_id: Option<AccountId>);
fn migrate_fee_storage() -> Self;
fn get_deposit_fee(&self) -> Option<Fee>;
fn get_withdraw_fee(&self) -> Option<Fee>;
fn get_deposit_fee_per_silo(&self, silo: AccountId) -> Option<Fee>;
fn get_withdraw_fee_per_silo(&self, silo: AccountId) -> Option<Fee>;
fn get_fee_owner(&self) -> AccountId;

Migration:
The migration is needed due to the added field fee to the contract structure. To migrate the storage we need to call:
pub fn migrate_fee_storage() -> Self

Performance / NEAR gas cost considerations

The gas for withdrawal and deposit transactions will be increased a little bit due to additional fee calculations and reads from the storage.

Testing

Unit tests have been added to cover multiple fee cases.

How should this be reviewed

The review should verify the functions withdraw, engine_withdraw, and finish_deposit and make sure that the burn/mint/transfer logic is correct.

Additional information

[sept-en]

Please describe all the fields and the logic of how it's expected to work here, e.g. if deposit and withdrawals fee settings override per silo configurations (some may think it is, but it's actually the opposite), what happens if either of these settings are set, etc.

[karim-en]

c4aec91

[sept-en]

Please describe all the fields.

[karim-en]

c4aec91

[sept-en]

This comment needs to be updated.
Also, why don't we mint the whole amount to the current_account_id in the first place?

[karim-en]

We mint to the current_account_id just in case there is an ft-transfer-call when depositing to eth account, but on deposit to near account, we can mint directly to the new_owner_id.
If are asking how this worked with one call, then it worked because the new_owner_id == current_account_id on this ft-transfer-call.
additional to that it is not clear why actually the env::predecessor_account_id(), was used instead of env::current_account_id()

[karim-en]

10d1ab3

[mrLSD]

@karim-en please cover also with integration tests: eth-connector-tests crate.

[mrLSD]

@karim-en and I believe AIP also should be extended.