OpStack 各个角色

op-node 负责和op-geth交易打包落块，交易状态推导，数据传输同步的客户端
batcher 将数据同步到L1的EOA账户
op-processer 提交区块状态到 L1 的 L2OutputOracle 合约
crossDomainMessagener 跨链信使合约，负责L1->L2,L2->L1的通信
OptimismPortal 是 op-stack 的充值提现纽带合约
Bridges 桥合约，主要功能是承载充值提现
L2OutputOracle 在L1层接收L2过来的状态根的合约

L2->L1 提现逻辑

提现的核心步骤

第一步用户在L2层调用withdraw给自己地址提币
第二步业务逻辑在L2合约层进行处理，中间会经过以下几个合约和步骤
1.首先会在L2StandradBridge上面执行call_initiateWithdrawal。根据ETH/ERC20
2.如果提现的是ETH，则会调用CrossDomainMessenger的sendMessage方法，将msgNonce+1，并在方法体内部调用L2CrossDomainMessenger的_sendMessage方法
3.L2CrossDomainMessenger的_sendMessage 会调用L2ToL1MessagePasser的initateWithdrawal。构造出withdrawalHash，并维护msgNonce自增为1。完事发送事件
第三步 sequencer中的op-node 监听到交易事件，将事件打包成交易（此步在链下处理）
第四步 Op-batch负责发打包好的交易rollup到L1里面，Op-proposer负责将这批次的状态根stateroot提交到L1
第五步用户在L1提取资金（但是要注意的是，需要在挑战期过后才能提取），可以使用op-stack-SDK。它内部的逻辑会调用L1层的OptimismPortal来提取资金。

L2链层源码

function _initiateWithdrawal(
    address _l2Token,
    address _from,
    address _to,
    uint256 _amount,
    uint32 _minGasLimit,
    bytes memory _extraData
)
    internal
{
    if (_l2Token == Predeploys.LEGACY_ERC20_ETH) {  // 判断是否是ETH
        _initiateBridgeETH(_from, _to, _amount, _minGasLimit, _extraData);
    } else {
        address l1Token = OptimismMintableERC20(_l2Token).l1Token();  //属于ERC20
        _initiateBridgeERC20(_l2Token, l1Token, _from, _to, _amount, _minGasLimit, _extraData);
    }
}

执行父类的方法，_initiateBridgeETH

function _initiateBridgeETH(
    address _from,
    address _to,
    uint256 _amount,
    uint32 _minGasLimit,
    bytes memory _extraData
)
    internal
{
    require(isCustomGasToken() == false, "StandardBridge: cannot bridge ETH with custom gas token");
    require(msg.value == _amount, "StandardBridge: bridging ETH must include sufficient ETH value");

    _emitETHBridgeInitiated(_from, _to, _amount, _extraData);

    messenger.sendMessage{ value: _amount }({
        _target: address(otherBridge),
        _message: abi.encodeWithSelector(this.finalizeBridgeETH.selector, _from, _to, _amount, _extraData),
        _minGasLimit: _minGasLimit
    });
}

此时，方法进入到CrossDomainMessenger

function sendMessage(address _target, bytes calldata _message, uint32 _minGasLimit) external payable {
    if (isCustomGasToken()) {
        require(msg.value == 0, "CrossDomainMessenger: cannot send value with custom gas token");
    }

    _sendMessage({
        _to: address(otherMessenger),
        _gasLimit: baseGas(_message, _minGasLimit),
        _value: msg.value,
        _data: abi.encodeWithSelector(
            this.relayMessage.selector, messageNonce(), msg.sender, _target, msg.value, _minGasLimit, _message
        )
    });

    emit SentMessage(_target, msg.sender, _message, messageNonce(), _minGasLimit);
    emit SentMessageExtension1(msg.sender, msg.value);

    unchecked {
        ++msgNonce;
    }
}

在调用_sendMessage 的时候，执行的是子类的_sendMessage

function _sendMessage(address _to, uint64 _gasLimit, uint256 _value, bytes memory _data) internal override {
    IL2ToL1MessagePasser(payable(Predeploys.L2_TO_L1_MESSAGE_PASSER)).initiateWithdrawal{ value: _value }(
        _to, _gasLimit, _data
    );
}

最终执行的是L2ToL1MessagePasser，逻辑是将执行交易的参数打包成hash，并发送事件，到这里，L2层的逻辑已经执行完毕。

function initiateWithdrawal(address _target, uint256 _gasLimit, bytes memory _data) public payable {
    bytes32 withdrawalHash = Hashing.hashWithdrawal(
        Types.WithdrawalTransaction({
            nonce: messageNonce(),
            sender: msg.sender,
            target: _target,
            value: msg.value,
            gasLimit: _gasLimit,
            data: _data
        })
    );

    sentMessages[withdrawalHash] = true;

    emit MessagePassed(messageNonce(), msg.sender, _target, msg.value, _gasLimit, _data, withdrawalHash);

    unchecked {
        ++msgNonce;
    }
}

链下执行逻辑

func (l *BatchSubmitter) loadBlocksIntoState(ctx context.Context) error {
    // 获取并判断需要提交的最新 L2 的 start 和 end 块号
    start, end, err := l.calculateL2BlockRangeToStore(ctx)
    if err != nil {
       l.Log.Warn("Error calculating L2 block range", "err", err)
       return err
    } else if start.Number >= end.Number {
       return errors.New("start number is >= end number")
    }

    var latestBlock *types.Block
    // 从起始区块开始获取区块信息,并将区块加到 channelManager 的 blocks
    for i := start.Number + 1; i < end.Number+1; i++ {
       //核心逻辑就是 l.loadBlockIntoState
       block, err := l.loadBlockIntoState(ctx, i)
       if errors.Is(err, ErrReorg) {
          l.Log.Warn("Found L2 reorg", "block_number", i)
          l.lastStoredBlock = eth.BlockID{}
          return err
       } else if err != nil {
          l.Log.Warn("Failed to load block into state", "err", err)
          return err
       }
       l.lastStoredBlock = eth.ToBlockID(block)
       latestBlock = block
    }
    // 提取基本的 L2BlockRef 信息
    l2ref, err := derive.L2BlockToBlockRef(l.RollupConfig, latestBlock)
    if err != nil {
       l.Log.Warn("Invalid L2 block loaded into state", "err", err)
       return err
    }
    // 将L2BlockRef 加载到当前状态根中
    l.Metr.RecordL2BlocksLoaded(l2ref)
    return nil
}

func (l *BatchSubmitter) publishTxToL1(ctx context.Context, queue *txmgr.Queue[txRef], receiptsCh chan txmgr.TxReceipt[txRef], daGroup *errgroup.Group) error {
    // 获取当前 Layer 1 的最新区块（tip）
    l1tip, err := l.l1Tip(ctx)
    if err != nil {
       l.Log.Error("Failed to query L1 tip", "err", err)
       return err
    }
    // 记录当前的 l1tip
    l.recordL1Tip(l1tip)

    // 从状态中获取与当前 L1 tip 相关的交易数据。这一步比较关键，来看一下逻辑
    txdata, err := l.state.TxData(l1tip.ID())

    if err == io.EOF {
       l.Log.Trace("No transaction data available")
       return err
    } else if err != nil {
       l.Log.Error("Unable to get tx data", "err", err)
       return err
    }

    // 发送交易数据到L1  
    if err = l.sendTransaction(txdata, queue, receiptsCh, daGroup); err != nil {
       return fmt.Errorf("BatchSubmitter.sendTransaction failed: %w", err)
    }
    return nil
}

获取与当前L1 tip 相关的交易数据

func (s *channelManager) TxData(l1Head eth.BlockID) (txData, error) {
    s.mu.Lock()
    defer s.mu.Unlock()
    // 上面的代码逻辑是设置互斥锁
    var firstWithTxData *channel
    // 寻找第一个包含交易数据的通道
    for _, ch := range s.channelQueue {
       if ch.HasTxData() {
          firstWithTxData = ch
          break
       }
    }

    dataPending := firstWithTxData != nil && firstWithTxData.HasTxData()
    s.log.Debug("Requested tx data", "l1Head", l1Head, "txdata_pending", dataPending, "blocks_pending", len(s.blocks))

    // 存在待处理数据或达成短路条件，则调用 nextTxData(firstWithTxData) 返回该通道的交易数据
    if dataPending || s.closed {
       return s.nextTxData(firstWithTxData)
    }

    // 没有待处理数据，我们可以添加一个新块到channel，同时返回一个EOF
    if len(s.blocks) == 0 {
       return txData{}, io.EOF
    }

    // 确保当前有足够的空间处理新块
    if err := s.ensureChannelWithSpace(l1Head); err != nil {
       return txData{}, err
    }

    // 处理待处理的块
    if err := s.processBlocks(); err != nil {
       return txData{}, err
    }

    // 处理完所有待处理的块后，注册当前的 L1 头
    s.registerL1Block(l1Head)

    // 将处理后的数据输出
    if err := s.outputFrames(); err != nil {
       return txData{}, err
    }
    // 返回当前通道的交易数据
    return s.nextTxData(s.currentChannel)
}

op-proposer 逻辑，发送状态根

func (l *L2OutputSubmitter) FetchL2OOOutput(ctx context.Context) (*eth.OutputResponse, bool, error) {
    if l.l2ooContract == nil {
       return nil, false, fmt.Errorf("L2OutputOracle contract not set, cannot fetch next output info")
    }

    cCtx, cancel := context.WithTimeout(ctx, l.Cfg.NetworkTimeout)
    defer cancel()
    callOpts := &bind.CallOpts{
       From:    l.Txmgr.From(),
       Context: cCtx,
    }
    // 获取下一个检查点的区块号
    nextCheckpointBlockBig, err := l.l2ooContract.NextBlockNumber(callOpts)
    if err != nil {
       return nil, false, fmt.Errorf("querying next block number: %w", err)
    }
    nextCheckpointBlock := nextCheckpointBlockBig.Uint64()
    // 方法获取当前区块号
    currentBlockNumber, err := l.FetchCurrentBlockNumber(ctx)
    if err != nil {
       return nil, false, err
    }

    // 对比当前区块号和下一个检查点的区块号，确保不会在未来的时间提交区块
    if currentBlockNumber < nextCheckpointBlock {
       l.Log.Debug("Proposer submission interval has not elapsed", "currentBlockNumber", currentBlockNumber, "nextBlockNumber", nextCheckpointBlock)
       return nil, false, nil
    }
    //使用下一个检查点的区块号来获取输出信息
    output, err := l.FetchOutput(ctx, nextCheckpointBlock)
    if err != nil {
       return nil, false, fmt.Errorf("fetching output: %w", err)
    }

    // 检查输出信息的区块引用是否大于最终化的 L2 状态的区块号，且是否允许非最终化的状态
    if output.BlockRef.Number > output.Status.FinalizedL2.Number && (!l.Cfg.AllowNonFinalized || output.BlockRef.Number > output.Status.SafeL2.Number) {
       l.Log.Debug("Not proposing yet, L2 block is not ready for proposal",
          "l2_proposal", output.BlockRef,
          "l2_safe", output.Status.SafeL2,
          "l2_finalized", output.Status.FinalizedL2,
          "allow_non_finalized", l.Cfg.AllowNonFinalized)
       return output, false, nil
    }
    return output, true, nil
}

func (l *L2OutputSubmitter) proposeOutput(ctx context.Context, output *eth.OutputResponse) {
    cCtx, cancel := context.WithTimeout(ctx, 10*time.Minute)
    defer cancel()
    //  如果上述的检查结果为true，则直接提交状态根transaction
    if err := l.sendTransaction(cCtx, output); err != nil {
       l.Log.Error("Failed to send proposal transaction",
          "err", err,
          "l1blocknum", output.Status.CurrentL1.Number,
          "l1blockhash", output.Status.CurrentL1.Hash,
          "l1head", output.Status.HeadL1.Number)
       return
    }
    l.Metr.RecordL2BlocksProposed(output.BlockRef)
}

至此，链下部分也已经处理完成

L1层的处理

在L2OutputOracle.proposeL2Output方法中

function proposeL2Output(
    bytes32 _outputRoot,
    uint256 _l2BlockNumber,
    bytes32 _l1BlockHash,
    uint256 _l1BlockNumber
)
    external
    payable
{
    // 校验发送人
    require(msg.sender == proposer, "L2OutputOracle: only the proposer address can propose new outputs");
    // 校验下一区块号
    require(
        _l2BlockNumber == nextBlockNumber(),
        "L2OutputOracle: block number must be equal to next expected block number"
    );
    // 校验区块时间
    require(
        computeL2Timestamp(_l2BlockNumber) < block.timestamp,
        "L2OutputOracle: cannot propose L2 output in the future"
    );

    require(_outputRoot != bytes32(0), "L2OutputOracle: L2 output proposal cannot be the zero hash");

    if (_l1BlockHash != bytes32(0)) {
        require(
            blockhash(_l1BlockNumber) == _l1BlockHash,
            "L2OutputOracle: block hash does not match the hash at the expected height"
        );
    }

    emit OutputProposed(_outputRoot, nextOutputIndex(), _l2BlockNumber, block.timestamp);
    // 将对应的状态根方入到l2Outputs中
    l2Outputs.push(
        Types.OutputProposal({
            outputRoot: _outputRoot,
            timestamp: uint128(block.timestamp),
            l2BlockNumber: uint128(_l2BlockNumber)
        })
    );
}

结合着时序图来看一下

L1-> L2 储值逻辑

储值的核心步骤

第一步用户在L1层发起储值
第二步用户会在L1链上经历几个核心步骤
1.先进入L1StandardBridge，执行_initiateETHDeposit
2.调用 CrossDomainMessenger 合约的 sendMessage
3.在CrossDomainMessenger.sendMessage 方法中，内部调用L1CrossDomainMessenger的_sendMessage方法，同时维护msgNonce
4.L1CrossDomainMessenger._sendMessage 会抛出TransactionDeposited 事件，至此，L1链执行处理完毕
第三步链下，op-node监听到TransactionDeposited，构建交易的参数，并让op-geth调用L2StandardBridge的finalizeDeposit
第四步 finalizeDeposit执行完成之后，整个充值链路就完成了。

储值在L1层的源码

function depositETH(uint32 _minGasLimit, bytes calldata _extraData) external payable onlyEOA {
    _initiateETHDeposit(msg.sender, msg.sender, _minGasLimit, _extraData);
}

调用父类StandardBridge

function _initiateBridgeETH(
    address _from,
    address _to,
    uint256 _amount,
    uint32 _minGasLimit,
    bytes memory _extraData
)
    internal
{
    require(isCustomGasToken() == false, "StandardBridge: cannot bridge ETH with custom gas token");
    require(msg.value == _amount, "StandardBridge: bridging ETH must include sufficient ETH value");

    // Emit the correct events. By default this will be _amount, but child
    // contracts may override this function in order to emit legacy events as well.
    _emitETHBridgeInitiated(_from, _to, _amount, _extraData);
    // 发送message信息,进入的是CrossDomainMessenger合约中
    messenger.sendMessage{ value: _amount }({
        _target: address(otherBridge),
        _message: abi.encodeWithSelector(this.finalizeBridgeETH.selector, _from, _to, _amount, _extraData),
        _minGasLimit: _minGasLimit
    });
}

逻辑和提现一致，调用_sendMessage方法，此方法是执行子类L1CrossDomainMessenger的重写方法

function sendMessage(address _target, bytes calldata _message, uint32 _minGasLimit) external payable {
    if (isCustomGasToken()) {
        require(msg.value == 0, "CrossDomainMessenger: cannot send value with custom gas token");
    }

    // Triggers a message to the other messenger. Note that the amount of gas provided to the
    // message is the amount of gas requested by the user PLUS the base gas value. We want to
    // guarantee the property that the call to the target contract will always have at least
    // the minimum gas limit specified by the user.
    _sendMessage({
        _to: address(otherMessenger),
        _gasLimit: baseGas(_message, _minGasLimit),
        _value: msg.value,
        _data: abi.encodeWithSelector(
            this.relayMessage.selector, messageNonce(), msg.sender, _target, msg.value, _minGasLimit, _message
        )
    });

    emit SentMessage(_target, msg.sender, _message, messageNonce(), _minGasLimit);
    emit SentMessageExtension1(msg.sender, msg.value);

    unchecked {
        ++msgNonce;
    }
}

function _sendMessage(address _to, uint64 _gasLimit, uint256 _value, bytes memory _data) internal override {
    portal.depositTransaction{ value: _value }({
        _to: _to,
        _value: _value,
        _gasLimit: _gasLimit,
        _isCreation: false,
        _data: _data
    });
}

进入到OptimismPortal._depositTransaction方法

function _depositTransaction(
    address _to,
    uint256 _mint,
    uint256 _value,
    uint64 _gasLimit,
    bool _isCreation,
    bytes memory _data
)
    internal
{
    if (_isCreation && _to != address(0)) revert BadTarget();

    if (_gasLimit < minimumGasLimit(uint64(_data.length))) revert SmallGasLimit();

    if (_data.length > 120_000) revert LargeCalldata();

    // Transform the from-address to its alias if the caller is a contract.
    address from = msg.sender;
    if (msg.sender != tx.origin) {
        from = AddressAliasHelper.applyL1ToL2Alias(msg.sender);
    }

    // 对交易参数进行打包
    bytes memory opaqueData = abi.encodePacked(_mint, _value, _gasLimit, _isCreation, _data);

    // 发送存款事件
    emit TransactionDeposited(from, _to, DEPOSIT_VERSION, opaqueData);
}

以上，在L1层的存款逻辑处理完毕

链下处理

负责整合来自 L1 的信息、处理存款事务以及确保所有数据在时间和逻辑上的一致性。它确保生成的 L2 区块能够正确反映 L1 的状态

func (ba *FetchingAttributesBuilder) PreparePayloadAttributes(ctx context.Context, l2Parent eth.L2BlockRef, epoch eth.BlockID) (attrs *eth.PayloadAttributes, err error) {
    var l1Info eth.BlockInfo
    var depositTxs []hexutil.Bytes
    var seqNumber uint64

    sysConfig, err := ba.l2.SystemConfigByL2Hash(ctx, l2Parent.Hash)
    if err != nil {
       return nil, NewTemporaryError(fmt.Errorf("failed to retrieve L2 parent block: %w", err))
    }

    // If the L1 origin changed in this block, then we are in the first block of the epoch. In this
    // case we need to fetch all transaction receipts from the L1 origin block so we can scan for
    // user deposits.
    if l2Parent.L1Origin.Number != epoch.Number {
       info, receipts, err := ba.l1.FetchReceipts(ctx, epoch.Hash)
       if err != nil {
          return nil, NewTemporaryError(fmt.Errorf("failed to fetch L1 block info and receipts: %w", err))
       }
       if l2Parent.L1Origin.Hash != info.ParentHash() {
          return nil, NewResetError(
             fmt.Errorf("cannot create new block with L1 origin %s (parent %s) on top of L1 origin %s",
                epoch, info.ParentHash(), l2Parent.L1Origin))
       }

       deposits, err := DeriveDeposits(receipts, ba.rollupCfg.DepositContractAddress)
       if err != nil {
          // deposits may never be ignored. Failing to process them is a critical error.
          return nil, NewCriticalError(fmt.Errorf("failed to derive some deposits: %w", err))
       }
       // apply sysCfg changes
       if err := UpdateSystemConfigWithL1Receipts(&sysConfig, receipts, ba.rollupCfg, info.Time()); err != nil {
          return nil, NewCriticalError(fmt.Errorf("failed to apply derived L1 sysCfg updates: %w", err))
       }

       l1Info = info
       depositTxs = deposits
       seqNumber = 0
    } else {
       if l2Parent.L1Origin.Hash != epoch.Hash {
          return nil, NewResetError(fmt.Errorf("cannot create new block with L1 origin %s in conflict with L1 origin %s", epoch, l2Parent.L1Origin))
       }
       info, err := ba.l1.InfoByHash(ctx, epoch.Hash)
       if err != nil {
          return nil, NewTemporaryError(fmt.Errorf("failed to fetch L1 block info: %w", err))
       }
       l1Info = info
       depositTxs = nil
       seqNumber = l2Parent.SequenceNumber + 1
    }

    // Sanity check the L1 origin was correctly selected to maintain the time invariant between L1 and L2
    nextL2Time := l2Parent.Time + ba.rollupCfg.BlockTime
    if nextL2Time < l1Info.Time() {
       return nil, NewResetError(fmt.Errorf("cannot build L2 block on top %s for time %d before L1 origin %s at time %d",
          l2Parent, nextL2Time, eth.ToBlockID(l1Info), l1Info.Time()))
    }

    var upgradeTxs []hexutil.Bytes
    if ba.rollupCfg.IsEcotoneActivationBlock(nextL2Time) {
       upgradeTxs, err = EcotoneNetworkUpgradeTransactions()
       if err != nil {
          return nil, NewCriticalError(fmt.Errorf("failed to build ecotone network upgrade txs: %w", err))
       }
    }

    if ba.rollupCfg.IsFjordActivationBlock(nextL2Time) {
       fjord, err := FjordNetworkUpgradeTransactions()
       if err != nil {
          return nil, NewCriticalError(fmt.Errorf("failed to build fjord network upgrade txs: %w", err))
       }
       upgradeTxs = append(upgradeTxs, fjord...)
    }

    l1InfoTx, err := L1InfoDepositBytes(ba.rollupCfg, sysConfig, seqNumber, l1Info, nextL2Time)
    if err != nil {
       return nil, NewCriticalError(fmt.Errorf("failed to create l1InfoTx: %w", err))
    }

    var afterForceIncludeTxs []hexutil.Bytes
    if ba.rollupCfg.IsInterop(nextL2Time) {
       depositsCompleteTx, err := DepositsCompleteBytes(seqNumber, l1Info)
       if err != nil {
          return nil, NewCriticalError(fmt.Errorf("failed to create depositsCompleteTx: %w", err))
       }
       afterForceIncludeTxs = append(afterForceIncludeTxs, depositsCompleteTx)
    }

    txs := make([]hexutil.Bytes, 0, 1+len(depositTxs)+len(afterForceIncludeTxs)+len(upgradeTxs))
    txs = append(txs, l1InfoTx)
    txs = append(txs, depositTxs...)
    txs = append(txs, afterForceIncludeTxs...)
    txs = append(txs, upgradeTxs...)

    var withdrawals *types.Withdrawals
    if ba.rollupCfg.IsCanyon(nextL2Time) {
       withdrawals = &types.Withdrawals{}
    }

    var parentBeaconRoot *common.Hash
    if ba.rollupCfg.IsEcotone(nextL2Time) {
       parentBeaconRoot = l1Info.ParentBeaconRoot()
       if parentBeaconRoot == nil { // default to zero hash if there is no beacon-block-root available
          parentBeaconRoot = new(common.Hash)
       }
    }

    return &eth.PayloadAttributes{
       Timestamp:             hexutil.Uint64(nextL2Time),
       PrevRandao:            eth.Bytes32(l1Info.MixDigest()),
       SuggestedFeeRecipient: predeploys.SequencerFeeVaultAddr,
       Transactions:          txs,
       NoTxPool:              true,
       GasLimit:              (*eth.Uint64Quantity)(&sysConfig.GasLimit),
       Withdrawals:           withdrawals,
       ParentBeaconBlockRoot: parentBeaconRoot,
    }, nil
}

L2层的最终处理逻辑,进行消息的转发

function relayMessage(
    uint256 _nonce,
    address _sender,
    address _target,
    uint256 _value,
    uint256 _minGasLimit,
    bytes calldata _message
)
    external
    payable
{
    // 确保状态不是暂停
    require(paused() == false, "CrossDomainMessenger: paused");

    // 确保版本正确
    (, uint16 version) = Encoding.decodeVersionedNonce(_nonce);
    require(version < 2, "CrossDomainMessenger: only version 0 or 1 messages are supported at this time");

    // 检查该消息是否已经被转发，防止重复转发
    if (version == 0) {
        bytes32 oldHash = Hashing.hashCrossDomainMessageV0(_target, _sender, _message, _nonce);
        require(successfulMessages[oldHash] == false, "CrossDomainMessenger: legacy withdrawal already relayed");
    }

    // 使用版本 1 的哈希作为消息的唯一标识符.
    bytes32 versionedHash =
        Hashing.hashCrossDomainMessageV1(_nonce, _sender, _target, _value, _minGasLimit, _message);

    if (_isOtherMessenger()) {
        // 确保 msg.value 与 _value 匹配
        assert(msg.value == _value);
        assert(!failedMessages[versionedHash]);
    } else {
        require(msg.value == 0, "CrossDomainMessenger: value must be zero unless message is from a system address");

        require(failedMessages[versionedHash], "CrossDomainMessenger: message cannot be replayed");
    }
    // 确保地址安全
    require(
        _isUnsafeTarget(_target) == false, "CrossDomainMessenger: cannot send message to blocked system address"
    );

    require(successfulMessages[versionedHash] == false, "CrossDomainMessenger: message has already been relayed");

    //确保有足够的燃气执行外部调用和完成执行，若不够，则将消息标记为失败
    if (
        !SafeCall.hasMinGas(_minGasLimit, RELAY_RESERVED_GAS + RELAY_GAS_CHECK_BUFFER)
            || xDomainMsgSender != Constants.DEFAULT_L2_SENDER
    ) {
        failedMessages[versionedHash] = true;
        emit FailedRelayedMessage(versionedHash);

        // Revert in this case if the transaction was triggered by the estimation address. This
        // should only be possible during gas estimation or we have bigger problems. Reverting
        // here will make the behavior of gas estimation change such that the gas limit
        // computed will be the amount required to relay the message, even if that amount is
        // greater than the minimum gas limit specified by the user.
        if (tx.origin == Constants.ESTIMATION_ADDRESS) {
            revert("CrossDomainMessenger: failed to relay message");
        }

        return;
    }
    // 最核心的逻辑，执行SafeCall.call来转发执行逻辑
    xDomainMsgSender = _sender;
    bool success = SafeCall.call(_target, gasleft() - RELAY_RESERVED_GAS, _value, _message);
    xDomainMsgSender = Constants.DEFAULT_L2_SENDER;

    // 根据执行结果处理最终的逻辑
    if (success) {
        assert(successfulMessages[versionedHash] == false);
        successfulMessages[versionedHash] = true;
        emit RelayedMessage(versionedHash);
    } else {
        failedMessages[versionedHash] = true;
        emit FailedRelayedMessage(versionedHash);

        if (tx.origin == Constants.ESTIMATION_ADDRESS) {
            revert("CrossDomainMessenger: failed to relay message");
        }
    }
}