Hooks Trampoline

Hooks are a CoW Protocol feature that allow traders to specify custom Ethereum calls as part of their order to get executed atomically in the same transaction as they trade.

The HooksTrampoline contract protects the protocol from two things:

• Executing calls from the privileged context of the settlement contract. Fees are accrued in the contract, and users could simply specify hooks that ERC20.transfer(user, amount) if the calling context for user hooks were the settlement contract.
• Reverting unnecessary hooks during a settlement. This can cause a two issues:
  • Interactions from the settlement contract are called with all remaining gasleft(). This means, that a revert from an INVALID opcode for example, would consume 63/64ths of the total transaction gas. This means that hooks can make settlements extremely expensive for nothing.
  • Other orders being executed as part of the settlement would also not be included.

As such, the HooksTrampoline contract is designed to execute user-specified hooks:

1. From an unprivileged context (the HooksTrampoline contract instead of the CoW Protocol settlement contract).
2. Specify a gas limit, to cap INVALID opcodes from consuming too much gas.
3. Allow the calls to revert.

In addition, the HooksTrampoline also only allows calls from the settlement contract. This means that hook implementations can add checks that ensure that they are only called from within a settlement:

require(msg.sender == HOOKS_TRAMPOLINE_ADDRESS, "not a settlement");

Settlement Diagram

sequenceDiagram
    participant Solver
    participant Settlement
    participant HooksTrampoline
    participant Hook

    Solver->>Settlement: settle
    activate Settlement

    Settlement->>HooksTrampoline: execute
    activate HooksTrampoline
    loop pre-hooks
        HooksTrampoline->>Hook: call
        activate Hook

        Hook->>HooksTrampoline: return/revert
        deactivate Hook
    end
    HooksTrampoline->>Settlement: return
    deactivate HooksTrampoline

    Settlement->>Settlement: swap

    Settlement->>HooksTrampoline: execute
    activate HooksTrampoline
    loop post-hooks
        HooksTrampoline->>Hook: call
        activate Hook

        Hook->>HooksTrampoline: return/revert
        deactivate Hook
    end
    HooksTrampoline->>Settlement: return
    deactivate HooksTrampoline


    Settlement->>Solver: return
    deactivate Settlement

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Development

Installation

This project uses Foundry.

Additional dependencies can be installed with:

forge install

Test

forge test

Deployment

Copy .env.sample to .env and fill each variable with the necessary information.

You can do a test run of the transaction with the following command:

source .env
forge script script/DeployHooksTrampoline.s.sol -vvvv --rpc-url "$ETH_RPC_URL"

The following command executes the deployment transaction onchain and verifies the contract code on the block explorer.

source .env
forge script script/DeployHooksTrampoline.s.sol -vvvv --rpc-url "$ETH_RPC_URL" --verify --broadcast

This contract uses deterministic deployments. The official deployment addresses for all supported chains can be found in the file networks.json. Entries are manually added to that file after each deployment to a new chain.