Cancel Offer

The lifecycle of an offer finishes with its cancellation. In this section, we will cover the process of canceling an offer.

Monochain Cancelling Process

In a monochain scenario, offer cancellation is a straightforward procedure. Essentially, only two actions are required:

• Unlocking source token: The source tokens, that seller have freezed while creating the offer, must be refunded.
• Delete the offer: The offer must be deleted from the offers mapping.

In order to cancel the offer, seller has to invoke method cancelOffer.

function cancelOffer(
    bytes32 _offerId,
    MessagingFee calldata _fee,
    bytes calldata _extraSendOptions
) external payable returns (MessagingReceipt memory msgReceipt);

About the parameters of this method:

• offerId: The ID of the offer to be canceled
• fee: The Layer Zero fee, that would be zero in the monochain offer case
• extraSendOptions: Options required for Layer Zero cross-chain transaction. In monochain case would be empty.

Upon calling the cancelOffer method, the OTC market will:

• Validate data: Verify whether the offer exists, if the cancellation is being initiated from the source chain, and if the signer has the authority to cancel the offer.
• Unlock the assets: The source token from Escrow will be transferred to seller account
• Delete the offer: Offer will be deleted from the offers mapping.

Cross-chain Cancelling Process

Cross-chain offer cancellation is split into 2 cross-chain messages:

• Cancel Offer Order: Message from the offer source chain to destination chain, ordering destination chain OTC to delete the offer.
• Offer Canceled: Message from the destination chain OTC to source chain informing that the offer is successfully deleted in destination chain OTC and now can be deleted on the source chain OTC.

Since the cancellation of a cross-chain offer involves two transactions, the quote method must be called twice — once on the destination chain and once on the source chain.

1. Quote on the destination chain

Firstly, quoteCancelOffer should be invoked on the destination chain OTC, in order to quote sending the Offer Canceled message.

function quoteCancelOffer(bytes32 _offerId) external returns (MessagingFee memory fee);

The quote is a view (read) function, so calling it incurs no cost.

The only required parameter is offerId — ID of the offer to be canceled.

The quoteCancelOffer returns the returnFee — Layer Zero fee, required to send the Offer Canceled message. returnFee will be useful in the following step.

2. Building Extra Send Options

ExtraSendOptions are required in order to invoke the quoteCancelOfferOrder method on the source chain OTC market.

import { OptionsBuilder } from "@layerzerolabs/lz-evm-oapp-v2/contracts/oapp/libs/OptionsBuilder.sol";

extraSendOptions = OptionsBuilder.newOptions().addExecutorLzReceiveOption(0, uint128(returnFee.nativeFee));

To build extraSendOptions the returnFee form the previous step is required.

2. Quote on the source chain

After computing the extraSendOptions, the quoteCancelOfferOrder can be invoked.

function quoteCancelOfferOrder(
    bytes32 _srcSellerAddress,
    bytes32 _offerId,
    bytes calldata _extraSendOptions,
    bool _payInLzToken
) external returns (MessagingFee memory fee);

The required parameters are:

• srcSellerAddress: address of the signer of future cancelOffer transaction.
• offerId: ID of the offer to be canceled
• extraSendOptions: The built earlier extraSendOptions.
• payInLzToken: Decide whether to cover LayerZero fee in native or ZRO token.

The quoteCancelOfferOrder is a view (read) function, so calling it incurs no cost.

quoteCancelOfferOrder returns the fee required for the main cancelOffer method.

3. Cancel

On source chain OTC market the method cancelOffer should be invoked. The interface of this method can be seen here.

The fee from the source chain quote is required in this context, as well as extraSendOptions parameter built earlier.

While canceling the cross-chain offer, the OTC market will:

• Validate data: The OTC market will verify whether the offer exists, if the cancellation is being initiated from the source chain, and if the signer has the authority to cancel the offer.
• Send Cancel Offer Order message Send the CancelOfferOrder cross-chain message to the destination chain OTC market.

Once the Cancel Offer Order message is delivered to the destination chain OTC market, it will:

• Cancel the offer: Delete offer from the offers mapping.
• Emit Event: Log OfferCanceled event notifying offchain workers.
• Send Offer Canceled message: Send the OfferCanceled cross-chain message to the source chain OTC market.

When the Offer Canceled message is delivered, the OTC market will:

• Cancel the offer: Delete offer from the offers mapping.
• Emit Event: Log OfferCanceled event notifying offchain workers.

4. Problematic

Offer is Accepted and Canaled simultaneously

As previously mentioned, the Bakstag Protocol currently maintains a duplicate storage of offers. Therefore, to cancel an offer, it must be deleted in both the source and destination chains OTC markets. Likewise the cancelOffer request should come from source chain OTC market

The simplest way to cancel the offer is:

1. Cancel on source chain
2. Cancel on destination chain

But this implementation has a problem:

Let’s consider a scenario where an offer source chain is A and destination chain is B. The offer is being canceled on chain A while simultaneously being accepted on chain B. Here’s what would happen:

1. • A OTC market: Offer is canceled
   • B OTC market: Offer is accepted

2. • A OTC market: Offer is deleted from the mapping and the assets are refunded to the seller
   • B OTC market: Offer is updated, buyer's destination token transferred to destination seller address and treasury

1. • A OTC market: OfferCanceled cross-chain message send to destination chain OTC market.
   • B OTC market: OfferAccepted cross-chain message send to source chain OTC market.

1. • A OTC market: OfferAccepted message received, but offer no longer exist on this OTC marked, so source token can not be transferred to source buyer address
   • B OTC market: Offer Canceled message received, the offer deleted from mapping.

The example above illustrates a simple cross-chain transaction is insufficient for canceling an offer.

Now, let’s consider the same scenario, but using the Cancel Offer implementation currently employed in the Bakstag Protocol:

1. • A OTC market: Offer is canceled
   • B OTC market: Offer is accepted

2. • A OTC market: CancelOfferOrder cross-chain message is sent to destination chain OTC market.
   • B OTC market: Offer is updated, destination token transferred from the buyer account to destination seller address and treasury. OfferAccepted cross-chain message send to source chain OTC market.

3. • A OTC market: OfferAccepted message received, source token transferred to source buyer address
   • B OTC market: CancelOfferOrder message received, offer deleted from the mapping, OfferCanceled cross-chain message sent back to source chain OTC market.

4. • A OTC market: OfferCanceled message received the offer deleted from the mapping, source token transferred to source seller address

Not enough gas for sending OfferCanceled message

The ABA-type transaction introduces another potential issue. The seller specifies the amount of gas available for executing the transaction on the destination chain by providing the extraSendOptions parameter. What happens if the seller specifies an insufficient amount of gas to process the CancelOfferOrder message on the destination OTC market?

The answer is straightforward — nothing critically wrong would occur. If there isn't enough gas to process the CancelOfferOrder and send the OfferCanceled message back to the source chain OTC market, the transaction would revert on destination chian. As a result, the offer would not be deleted from either the source chain or the destination chain OTC markets.