Expose deposit amounts in AbstractDepositorContract

[nkuba]

In this PR we introduce two changes to the AbstractDepositorContract around the deposit amounts.

Expose minimum deposit amount

Integrators may need a minimum deposit amount to introduce validation if the amount the user wants to deposit meets the Bridge requirement of a deposit to exceed the dust threshold.
Originally proposed in thesis/acre#253 (comment).

Return initial deposit amount from _initializeDeposit function

The amount may be useful for integrators for validations of the deposit amount. Originally proposed in thesis/acre#253 (comment).

It turns out that this version where we read from the storage:

        initialDepositAmount =
            bridge.deposits(depositKey).amount *
            SATOSHI_MULTIPLIER;

costs 117 366 gas.
Which is less than an alternative approach:

            initialDepositAmount =
                fundingTx
                    .outputVector
                    .extractOutputAtIndex(reveal.fundingOutputIndex)
                    .extractValue() *
                SATOSHI_MULTIPLIER;

which costs 117 601 gas.

We return initialDepositAmount from two functions: _initializeDeposit and _finalizeDeposit. I tried to introduce a getter function:

    function _getInitialDepositAmount(
        uint256 depositKey
    ) internal view returns (uint256) {
        IBridgeTypes.DepositRequest memory deposit = bridge.deposits(
            depositKey
        );
        require(deposit.revealedAt != 0, "Deposit not initialized");

        return deposit.amount * SATOSHI_MULTIPLIER;
    }

and removed initialDepositAmount return from _initializeDeposit and _finalizeDeposit functions.
Unfortunately, the overall cost in the reference BitcoinDepositor implementation from thesis/acre#253 wasn't improved:
BEFORE:

+ initializeStake --> 143 004
+ finalizeStake --> 195 178
= total --> 338 182

AFTER:

+ initializeStake --> 142 912
+ finalizeStake --> 197 960
= total --> 340 872

[github-actions]

Solidity API documentation preview available in the artifacts of the https://github.com/keep-network/tbtc-v2/actions/runs/8063943343 check.

[github-actions]

Solidity API documentation preview available in the artifacts of the https://github.com/keep-network/tbtc-v2/actions/runs/8064155396 check.

[lukasz-zimnoch]

Regarding _getInitialDepositAmount, did you try to use the storage pointer:

IBridgeTypes.DepositRequest storage deposit = bridge.deposits(
 depositKey
);

instead of the presented:

IBridgeTypes.DepositRequest memory deposit = bridge.deposits(
 depositKey
);

?

The storage pointer requires 2 storage reads while the version with memory reads all 7 storage fields of the DepositRequest. This can explain the increased gas consumption. The current version works for me but if we prefer a separate function, we can give it a shot.

[nkuba]

Regarding _getInitialDepositAmount, did you try to use the storage pointer:

IBridgeTypes.DepositRequest storage deposit = bridge.deposits(
 depositKey
);

instead of the presented:

IBridgeTypes.DepositRequest memory deposit = bridge.deposits(
 depositKey
);

?

The storage pointer requires 2 storage reads while the version with memory reads all 7 storage fields of the DepositRequest. This can explain the increased gas consumption. The current version works for me but if we prefer a separate function, we can give it a shot.

Unfortunately, I cannot do this as bridge.deposits returns memory variable:

TypeError: Type struct IBridgeTypes.DepositRequest memory is not implicitly convertible to expected type struct IBridgeTypes.DepositRequest storage pointer.
   --> @keep-network/tbtc-v2/contracts/integrator/AbstractTBTCDepositor.sol:295:9:
    |
295 |         IBridgeTypes.DepositRequest storage deposit = bridge.deposits(
    |         ^ (Relevant source part starts here and spans across multiple lines).


Error HH600: Compilation failed

Do you see another way to get through it?

[github-actions]

Solidity API documentation preview available in the artifacts of the https://github.com/keep-network/tbtc-v2/actions/runs/8079203442 check.

[lukasz-zimnoch]

Unfortunately, I cannot do this as bridge.deposits returns memory variable:

Oh yeah, that's a call to the Bridge so an alternative would be exposing a dedicated function on the Bridge's side. Not worth bothering about it in that case.