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| # ADR 010: Custom precompiled for app-chain | ||
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| ## Changelog | ||
| * 2023-10-02: first draft | ||
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| ## Context | ||
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| Cronos is an EVM-compatible built on Cosmos SDK. The "ethermint" module allows a full compatibility and interoperability with Ethereum. However, it comes with the sacrifice of cosmos native features or cosmos interchain features (IBC) | ||
| as end-users are not able to access them through an ethereum transaction. The solution is to provide precompile, allowing users to access to cosmos features through smart contracts. | ||
| A precompiled behave like a smart contract, it is designed by an address and can be invoked by other smart contracts. The difference is that its execution is not inside a smart contract. | ||
| App-chain specific logics can be embedded to the pre-compiled so that it could be used to expose cosmos native features at the EVM layer. | ||
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| ## Decision | ||
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| ### Fork go-ethereum for stateful precompiled support | ||
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| One of the major difficulties in supporting pre-compiled is the lack of flexibility in geth for defining custom precompiled and its non-ability to modify the state. | ||
| We made the choice of forking geth and implement additional interfaces and logics so that app-chain can define its own precompiled. | ||
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| The PR [#7](https://github.com/evmos/go-ethereum/pull/7) implements the ability to set custom precompiled to the EVM. | ||
| while this PR [#10](https://github.com/evmos/go-ethereum/pull/10) allows the precompiled to modify the state (for stateful precompiled) | ||
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| In the future, we are planning to maintain the following fork permanently | ||
| `github.com/evmos/go-ethereum` | ||
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| ### Implement pre-compiled for cosmos native module | ||
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| We provide pre-compile for cosmos native module such as bank or staking. | ||
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| The input and output of methods are mapped with those defined cosmos native transactions | ||
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| ```solidity | ||
| interface IBankModule { | ||
| function mint(address,uint256) external payable returns (bool); | ||
| function balanceOf(address,address) external view returns (uint256); | ||
| function burn(address,uint256) external payable returns (bool); | ||
| function transfer(address,address,uint256) external payable returns (bool); | ||
| } | ||
| interface IStakingModule { | ||
| # to be completed | ||
| } | ||
| ``` | ||
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| To call a precompiled, create an instance of the module interface with the precompiled address. | ||
| The methods are ABI encoded, and can be called directly as calling the function through the interface: | ||
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| ```solidity | ||
| address constant bankContract = 0x0000000000000000000000000000000000000064; | ||
| IBankModule bank = IBankModule(bankContract); | ||
| bank.transfer(msg.sender, recipient, amount); | ||
| ``` | ||
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| ### Implement pre-compiled for cosmos interchain communication | ||
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| #### Relayer | ||
| One of the benefits of supporting precompiled for IBC is that it would be possible to track the change of state triggered by the ibc transaction with ethereum receipts. | ||
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| For that reason we are exposing all relayer related actions to the evm layer. An action is defined by a 4 bytes prefix. | ||
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| ```go | ||
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| const ( | ||
| prefixSize4Bytes = 4 | ||
| // Client | ||
| prefixCreateClient = iota + 1 | ||
| prefixUpdateClient | ||
| prefixUpgradeClient | ||
| prefixSubmitMisbehaviour | ||
| // Connection | ||
| prefixConnectionOpenInit | ||
| prefixConnectionOpenTry | ||
| prefixConnectionOpenAck | ||
| prefixConnectionOpenConfirm | ||
| // Channel | ||
| prefixChannelOpenInit | ||
| prefixChannelOpenTry | ||
| prefixChannelOpenAck | ||
| prefixChannelOpenConfirm | ||
| prefixChannelCloseInit | ||
| prefixChannelCloseConfirm | ||
| prefixRecvPacket | ||
| prefixAcknowledgement | ||
| prefixTimeout | ||
| prefixTimeoutOnClose | ||
| ) | ||
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| ``` | ||
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| -> TODO describe the change in go-relayer | ||
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| #### ICA | ||
| For ICA, we are following the same standard as for the cosmos module. | ||
| The ICA precompiled interface is defined by: | ||
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| ```solidity | ||
| interface IICAModule { | ||
| event SubmitMsgsResult(uint64 seq); | ||
| function registerAccount(string calldata connectionID, string calldata version) external payable returns (bool); | ||
| function queryAccount(string calldata connectionID, address addr) external view returns (string memory); | ||
| function submitMsgs(string calldata connectionID, bytes calldata data, uint256 timeout) external payable returns (uint64); | ||
| } | ||
| ``` | ||
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| The calldata in `submitMsgs` represents the cosmos proto-encoded msg to be sent to the destination app-chain. Only one msg is supported at the moment. | ||
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| For example to call a bank transaction to another app-chain | ||
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| ```solidity | ||
| #todo | ||
| ``` | ||
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| In case of ICA, it is also important to be able to track the result of the transaction `submitMsgs` so that the user or the smart contract can respond accordingly. | ||
| The function `submitMsgs` returns a sequence number that can be used by the user or smart contract to track the state of the msg using the precompiled `ICACallback`. | ||
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| ```solidity | ||
| interface IICACallback { | ||
| function onPacketResultCallback(uint64 seq, bool ack) external payable returns (bool); | ||
| } | ||
| ``` | ||
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| ### Precompiled gas cost | ||
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| A precompiled is basically an op-code at the evm layer thus, it has a constant cost. Because a stateful precompiled can change the state, it is much harder to define a specific cost. | ||
| To be fair, we are evaluating the cost of a pre-compile based on the average cost of the cosmos transaction in the past history. | ||
| Future work aim to provide a more accurate cost on the pre-compiled | ||
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| ## Further discussions | ||
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| - Disabling cosmos native modules in cronos | ||
| - Establishing a standard for ibc for evm based chain (relayer) | ||
| - Extending the usage of pre-compile for broader use cases | ||
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| ## References | ||
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| * https://github.com/crypto-org-chain/cronos/pull/837 | ||
| * https://github.com/crypto-org-chain/cronos/pull/1163 | ||
| * https://github.com/crypto-org-chain/cronos/pull/1014 |