StaticCallProxy

ZEIP-39

The StaticCallProxy was first proposed in ZEIP-39. Please refer to the ZEIP for information and discussion about how this contract works at a higher level.

Conditional transfers

The StaticCallProxy itself cannot perform any transfers. Instead, it is primarily intended to be used in conjunction with the MultiAssetProxy to validate some on-chain state before or after any other transfers are performed. As implied in its name, the StaticCallProxy uses the STATICCALL opcode described in EIP-214 to ensure that it cannot modify any on-chain state. Therefore, this contract does not require any user approvals to be used.

transferFrom

This contract may perform a STATICCALL and verify its result if its transferFrom method is called.

/// @dev Transfers assets. Either succeeds or throws.
/// @param assetData Byte array encoded for the respective asset proxy.
/// @param from Address to transfer asset from.
/// @param to Address to transfer asset to.
/// @param amount Amount of asset to transfer.
function transferFrom(
    bytes assetData,
    address from,
    address to,
    uint256 amount
)
    external;

Logic

Calling StaticCallProxy.transferFrom will perform the following steps:

1. Decode staticCallTargetAddress, staticCallData, and expectedReturnDataHash from assetData
2. Call staticCallTargetAddress.staticcall(staticCallData)
3. Revert if the call was unsuccessful
4. Calculate the Keccak-256 hash of the return data, resulting in returnDataHash
5. Revert if the returnDataHash does not exactly match the expectedReturnDataHash

Errors

The transferFrom method may revert with the following errors:

Error	Condition
StandardError("UNEXPECTED_STATIC_CALL_RESULT")	The hash of the return data does not match the expectedReturnDataHash
*	This contract will rethrow any revert data received from an unsuccessful call of staticCallTargetAddress.staticcall(staticCallData)

Encoding assetData

This contract expects StaticCall assetData to be encoded using ABIv2 with the following function signature. The id of this contract is 0xc339d10a, which can be calculated as the 4 byte function selector of the same signature.

/// @dev Function signature for encoding StaticCall assetData.
/// @param staticCallTargetAddress Target address of staticcall.
/// @param staticCallData Data that will be passed to staticCallTargetAddress in the staticcall.
/// @param expectedReturnDataHash Expected Keccak-256 hash of the staticcall return data.
function StaticCall(
    address staticCallTargetAddress,
    bytes calldata staticCallData,
    bytes32 expectedReturnDataHash
)
    external;

In Solidity, this data can be encoded with:

bytes memory data = abi.encodeWithSelector(
    0xc339d10a,
    staticCallTargetAddress,
    staticCallData,
    expectedReturnDataHash
);

Authorizations

Since this contract can only read from state, no authorizations are required. This contract's transferFrom method can be called by any address.