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docs: ADR-002 EVM hooks (#429)

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* Add adr-002 evm hooks proposal

* add bank send hook example

* fix typo

* add more details to the adr

* polishes

* add comments and missing sections

* explain the error translation

* update working, add consequences

* add reference to staking module hooks

* add ApplyTransaction snippet
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@ -34,7 +34,5 @@ Please add a entry below in your Pull Request for an ADR.
## ADR Table of Contents ## ADR Table of Contents
<!-- - [ADR 001: State](adr-001-state.md)
Add an entry in the following format: - [ADR 002: EVM Hooks](adr-002-evm-hooks.md)
- [ADR XXX: ADR Title](./adr-XXX-adr-title.md)
-->

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# ADR 002: EVM Hooks
## Changelog
- 2021-08-11: first draft
## Status
PROPOSED
## Abstract
The current ADR proposes a hook interface to the EVM module, to extend the tx processing logic externally,
specifically to support EVM contract calling native modules through logs.
## Context
<!-- > This section describes the forces at play, including technological, political, social, and project local. These forces are probably in tension and should be called out as such. The language in this section is value-neutral. It is simply describing facts. It should clearly explain the problem and motivation that the proposal aims to resolve. -->
Currently, there are no way for EVM smart contracts to call cosmos native modules, one way to do this is by emitting
specific logs from the contract, and recognize those logs in tx processing code and convert them to native module calls.
To do this in an extensible way, we can add a post tx processing hook into the EVM module, which allows third-party to
add custom logic to process transaction logs.
## Decision
<!-- > This section describes our response to these forces. It is stated in full sentences, with an active voice. "We will ..." -->
This ADR proposes to add an `EvmHooks` interface and a method to register hooks in the `EvmKeeper`:
```golang
type EvmHooks interface {
PostTxProcessing(ctx sdk.Context, txHash ethcmn.Hash, logs []*ethtypes.Log) error
}
func (k *EvmKeeper) SetHooks(eh types.EvmHooks) *Keeper;
```
- `PostTxProcessing` is only called after the EVM transaction finished successfully, it's executed in the same cache context
as the EVM transaction, if it returns an error, the whole EVM transaction is reverted, if the hook implementor doesn't
want to revert the tx, he can always return nil instead.
The error returned by the hooks is translated to a VM error `failed to process native logs`, the detailed error
message is stored in the return value.
The message is sent to native modules asynchronously, there's no way for the caller to catch and recover the error.
The EVM state transition method `ApplyTransaction` should be changed like this:
```golang
// Create cached context which convers both the tx processing and post processing
revision := k.Snapshot()
res, err := k.ApplyMessage(evm, msg, ethCfg, false)
if err != nil {
return err
}
...
if !res.Failed() {
// Only call hooks if tx executed successfully.
err = k.hooks.PostTxProcessing(k.ctx, txHash, logs)
if err != nil {
// If hooks return error, revert the whole tx.
k.RevertToSnapshot(revision)
res.VmError = "failed to process native logs"
res.Ret = []byte(err.Error())
}
}
```
There are no default hooks implemented in the EVM module, so the proposal is backward compatible, only opens extra
extensibility for certain use cases.
### Use Case: Call Native Module
To support contract calling native module with this proposal, one can define a log signature and emits the specific log
from the smart contract, native logic registers a `PostTxProcessing` hook which recognizes the log and does the native module
call.
For example, to support smart contract to transfer native tokens, one can define and emit a `__CosmosNativeBankSend` log
signature in the smart contract like this:
```solidity
event __CosmosNativeBankSend(address recipient, uint256 amount, string denom);
function withdraw_to_native_token(amount uint256) public {
_balances[msg.sender] -= amount;
// send native tokens from contract address to msg.sender.
emit __CosmosNativeBankSend(msg.sender, amount, "native_denom");
}
```
And the application registers a `BankSendHook` to `EvmKeeper`, it recognizes the log and converts it to a call to the bank
module's `SendCoinsFromAccountToAccount` method:
```golang
var (
// BankSendEvent represent the signature of
// `event __CosmosNativeBankSend(address recipient, uint256 amount, string denom)`
BankSendEvent abi.Event
)
func init() {
addressType, _ := abi.NewType("address", "", nil)
uint256Type, _ := abi.NewType("uint256", "", nil)
stringType, _ := abi.NewType("string", "", nil)
BankSendEvent = abi.NewEvent(
"__CosmosNativeBankSend",
"__CosmosNativeBankSend",
false,
abi.Arguments{abi.Argument{
Name: "recipient",
Type: addressType,
Indexed: false,
}, abi.Argument{
Name: "amount",
Type: uint256Type,
Indexed: false,
}, abi.Argument{
Name: "denom",
Type: stringType,
Indexed: false,
}},
)
}
type BankSendHook struct {
bankKeeper bankkeeper.Keeper
}
func NewBankSendHook(bankKeeper bankkeeper.Keeper) *BankSendHook {
return &BankSendHook{
bankKeeper: bankKeeper,
}
}
func (h BankSendHook) PostTxProcessing(ctx sdk.Context, txHash ethcmn.Hash, logs []*ethtypes.Log) error {
for _, log := range logs {
if len(log.Topics) == 0 || log.Topics[0] != BankSendEvent.ID {
continue
}
if !ContractAllowed(log.Address) {
// Check the contract whitelist to prevent accidental native call.
continue
}
unpacked, err := BankSendEvent.Inputs.Unpack(log.Data)
if err != nil {
log.Warn("log signature matches but failed to decode")
continue
}
contract := sdk.AccAddress(log.Address.Bytes())
recipient := sdk.AccAddress(unpacked[0].(ethcmn.Address).Bytes())
coins := sdk.NewCoins(sdk.NewCoin(unpacked[2].(string), sdk.NewIntFromBigInt(unpacked[1].(*big.Int))))
err = h.bankKeeper.SendCoins(ctx, contract, recipient, coins)
if err != nil {
return err
}
}
}
return nil
}
```
Register the hook in `app.go`:
```golang
evmKeeper.SetHooks(NewBankSendHook(bankKeeper));
```
## Consequences
<!-- > This section describes the resulting context, after applying the decision. All consequences should be listed here, not just the "positive" ones. A particular decision may have positive, negative, and neutral consequences, but all of them affect the team and project in the future. -->
### Backwards Compatibility
<!-- All ADRs that introduce backward incompatibilities must include a section describing these incompatibilities and their severity. The ADR must explain how the author proposes to deal with these incompatibilities. ADR submissions without a sufficient backward compatibility treatise may be rejected outright. -->
The proposed ADR is backward compatible.
### Positive
- Improve extensibility of EVM module
### Negative
- It's possible that some contracts accidentally define a log with the same signature and cause an unintentional result.
To mitigate this, the implementor could whitelist contracts that are allowed to invoke native calls.
### Neutral
- The contract can only call native modules asynchronously, which means it can neither get the result nor handle the error.
## Further Discussions
<!-- While an ADR is in the DRAFT or PROPOSED stage, this section should contain a summary of issues to be solved in future iterations (usually referencing comments from a pull-request discussion).
Later, this section can optionally list ideas or improvements the author or reviewers found during the analysis of this ADR. -->
## Test Cases [optional]
<!-- Test cases for implementation are mandatory for ADRs that are affecting consensus changes. Other ADRs can choose to include links to test cases if applicable. -->
## References
<!-- - {reference link} -->
- [Hooks in staking module](https://docs.cosmos.network/v0.43/modules/staking/06_hooks.html)