195 lines
6.4 KiB
Markdown
195 lines
6.4 KiB
Markdown
<!--
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order: 6
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-->
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# Hooks
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The `x/evm` module implements an `EvmHooks` interface that extend and customize the `Tx` processing logic externally.
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This supports EVM contracts to call native cosmos modules by
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1. defining a log signature and emitting the specific log from the smart contract,
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2. recognizing those logs in the native tx processing code, and
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3. converting them to native module calls.
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To do this, the interface includes a `PostTxProcessing` hook that registers custom `Tx` hooks in the `EvmKeeper`. These `Tx` hooks are processed after the EVM state transition is finalized and doesn't fail. Note that there are no default hooks implemented in the EVM module.
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```go
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type EvmHooks interface {
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// Must be called after tx is processed successfully, if return an error, the whole transaction is reverted.
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PostTxProcessing(ctx sdk.Context, msg core.Message, receipt *ethtypes.Receipt) error
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}
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```
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## `PostTxProcessing`
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`PostTxProcessing` is only called after a EVM transaction finished successfully and delegates the call to underlying hooks. If no hook has been registered, this function returns with a `nil` error.
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```go
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func (k *Keeper) PostTxProcessing(ctx sdk.Context, msg core.Message, receipt *ethtypes.Receipt) error {
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if k.hooks == nil {
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return nil
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}
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return k.hooks.PostTxProcessing(k.Ctx(), msg, receipt)
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}
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```
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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, they can always return `nil` instead.
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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.
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## Use Case: Call Native ERC20 Module on Evmos
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Here is an example taken from the Evmos [erc20 module](https://evmos.dev/modules/erc20/) that shows how the `EVMHooks` supports a contract calling a native module to convert ERC-20 Tokens into Cosmos native Coins. Following the steps from above.
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You can define and emit a `Transfer` log signature in the smart contract like this:
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```solidity
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event Transfer(address indexed from, address indexed to, uint256 value);
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function _transfer(address sender, address recipient, uint256 amount) internal virtual {
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require(sender != address(0), "ERC20: transfer from the zero address");
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require(recipient != address(0), "ERC20: transfer to the zero address");
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_beforeTokenTransfer(sender, recipient, amount);
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_balances[sender] = _balances[sender].sub(amount, "ERC20: transfer amount exceeds balance");
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_balances[recipient] = _balances[recipient].add(amount);
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emit Transfer(sender, recipient, amount);
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}
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```
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The application will register a `BankSendHook` to the `EvmKeeper`. It recognizes the ethereum tx `Log` and converts it to a call to the bank module's `SendCoinsFromAccountToAccount` method:
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```go
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const ERC20EventTransfer = "Transfer"
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// PostTxProcessing implements EvmHooks.PostTxProcessing
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func (k Keeper) PostTxProcessing(
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ctx sdk.Context,
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msg core.Message,
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receipt *ethtypes.Receipt,
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) error {
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params := h.k.GetParams(ctx)
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if !params.EnableErc20 || !params.EnableEVMHook {
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// no error is returned to allow for other post processing txs
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// to pass
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return nil
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}
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erc20 := contracts.ERC20BurnableContract.ABI
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for i, log := range receipt.Logs {
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if len(log.Topics) < 3 {
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continue
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}
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eventID := log.Topics[0] // event ID
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event, err := erc20.EventByID(eventID)
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if err != nil {
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// invalid event for ERC20
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continue
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}
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if event.Name != types.ERC20EventTransfer {
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h.k.Logger(ctx).Info("emitted event", "name", event.Name, "signature", event.Sig)
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continue
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}
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transferEvent, err := erc20.Unpack(event.Name, log.Data)
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if err != nil {
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h.k.Logger(ctx).Error("failed to unpack transfer event", "error", err.Error())
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continue
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}
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if len(transferEvent) == 0 {
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continue
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}
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tokens, ok := transferEvent[0].(*big.Int)
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// safety check and ignore if amount not positive
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if !ok || tokens == nil || tokens.Sign() != 1 {
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continue
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}
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// check that the contract is a registered token pair
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contractAddr := log.Address
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id := h.k.GetERC20Map(ctx, contractAddr)
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if len(id) == 0 {
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// no token is registered for the caller contract
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continue
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}
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pair, found := h.k.GetTokenPair(ctx, id)
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if !found {
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continue
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}
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// check that conversion for the pair is enabled
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if !pair.Enabled {
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// continue to allow transfers for the ERC20 in case the token pair is disabled
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h.k.Logger(ctx).Debug(
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"ERC20 token -> Cosmos coin conversion is disabled for pair",
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"coin", pair.Denom, "contract", pair.Erc20Address,
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)
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continue
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}
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// ignore as the burning always transfers to the zero address
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to := common.BytesToAddress(log.Topics[2].Bytes())
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if !bytes.Equal(to.Bytes(), types.ModuleAddress.Bytes()) {
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continue
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}
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// check that the event is Burn from the ERC20Burnable interface
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// NOTE: assume that if they are burning the token that has been registered as a pair, they want to mint a Cosmos coin
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// create the corresponding sdk.Coin that is paired with ERC20
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coins := sdk.Coins{{Denom: pair.Denom, Amount: sdk.NewIntFromBigInt(tokens)}}
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// Mint the coin only if ERC20 is external
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switch pair.ContractOwner {
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case types.OWNER_MODULE:
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_, err = h.k.CallEVM(ctx, erc20, types.ModuleAddress, contractAddr, true, "burn", tokens)
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case types.OWNER_EXTERNAL:
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err = h.k.bankKeeper.MintCoins(ctx, types.ModuleName, coins)
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default:
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err = types.ErrUndefinedOwner
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}
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if err != nil {
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h.k.Logger(ctx).Debug(
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"failed to process EVM hook for ER20 -> coin conversion",
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"coin", pair.Denom, "contract", pair.Erc20Address, "error", err.Error(),
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)
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continue
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}
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// Only need last 20 bytes from log.topics
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from := common.BytesToAddress(log.Topics[1].Bytes())
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recipient := sdk.AccAddress(from.Bytes())
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// transfer the tokens from ModuleAccount to sender address
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if err := h.k.bankKeeper.SendCoinsFromModuleToAccount(ctx, types.ModuleName, recipient, coins); err != nil {
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h.k.Logger(ctx).Debug(
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"failed to process EVM hook for ER20 -> coin conversion",
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"tx-hash", receipt.TxHash.Hex(), "log-idx", i,
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"coin", pair.Denom, "contract", pair.Erc20Address, "error", err.Error(),
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)
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continue
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}
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}
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return nil
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```
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Lastly, register the hook in `app.go`:
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```go
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app.EvmKeeper = app.EvmKeeper.SetHooks(app.Erc20Keeper)
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```
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