Handling Failed xCalls

There are a few failure conditions to watch out for when using xcall.

High Slippage Conditions

When tokens are bridged through Connext, slippage can impact the xcall during the swap on origin or destination. If slippage is too high on the origin swap, the xcall will just revert. If slippage is too high on the destination swap (after it has already gone through the origin swap), then there are a couple options to consider.

  • Cancel the transfer and bridge back to origin (sender will lose any funds related to the origin slippage) [not available yet].

  • Wait it out until slippage conditions improve (relayers will continuously re-attempt the transfer execution).

  • Increase the slippage tolerance.

Increasing Slippage Tolerance

The _delegate parameter of xcall is an address that has rights to update the original slippage tolerance by calling Connext's forceUpdateSlippage function with the following signature:

function forceUpdateSlippage(TransferInfo calldata _params, uint256 _slippage) external;

The TransferInfo struct that must be supplied:

struct TransferInfo {
  uint32 originDomain;
  uint32 destinationDomain;
  uint32 canonicalDomain;
  address to;
  address delegate;
  bool receiveLocal;
  bytes callData;
  uint256 slippage;
  address originSender;
  uint256 bridgedAmt;
  uint256 normalizedIn;
  uint256 nonce;
  bytes32 canonicalId;

The parameters in TransferInfo must match the same parameters used in the original xcall. It's possible to obtain the original parameters by querying the subgraph (origin or destination) with the transferId associated with the xcall.

The Connext SDK also exposes an updateSlippage method for this.

Low Relayer Fee

If the estimated relayer fee paid was too low, then users may have to increase the relayer fee after the xcall has been sent.

Reverts on Receiver Contract

If the call on the receiver contract (also referred to as "target" contract) reverts, funds sent in with the call will end up on the receiver contract. To avoid situations where user funds get stuck on the receivers, developers should build any contract implementing IXReceive defensively.

Ultimately, the goal should be to handle any revert-susceptible code and ensure that the logical owner of funds always maintains agency over them.

Try/Catch with External Calls

One way to guard against unexpected reverts is to use try/catch statements which allow contracts to handle errors on external function calls.

contract TargetContract {
  function xReceive(
    bytes32 _transferId,
    uint256 _amount,
    address _asset,
    address _originSender,
    uint32 _origin,
    bytes memory _callData
  ) external returns (bytes memory) {
    try {
    } catch { 
      // Make sure funds are delivered to logical owner on failing external calls

Options for Funds on Receiver

We recommend that xApp developers consider recovery options in case of reverting calls on the receiver. For example, there could be an internal accounting structure to record transferIds and allow rightful originSenders to rescue their funds from the receiver contract. Note that this approach requires authentication and would cause xcalls to go through the slow path.

Alternatively, the protocol can implement an allowlist for addresses that are able to rescue funds and redirect them to users.

Connext is actively researching standards and best practices for receiver contracts. Reach out to us if you questions!

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