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Overview

The Polygon_SpokePool is deployed on Polygon PoS (formerly Matic). It implements Polygon’s unique FxPortal cross-chain messaging system and integrates with the custom PolygonTokenBridger contract due to special constraints in Polygon’s native bridge. Contract: contracts/Polygon_SpokePool.sol

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Key Characteristics

  • FxPortal messaging: Uses Polygon’s FxChild contract for cross-chain admin verification
  • Custom token bridger: Uses PolygonTokenBridger instead of direct bridge calls
  • Delegatecall validation: Admin functions executed via delegatecall from processMessageFromRoot()
  • CCTP support: Integrates Circle CCTP for USDC transfers
  • OFT support: Supports LayerZero OFT tokens
  • Native token wrapping: Handles MATIC→WMATIC conversions
  • EOA-only restrictions: Enforces EOA callers for certain functions to prevent griefing

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Inheritance

contract Polygon_SpokePool is
    IFxMessageProcessor,
    SpokePool,
    CircleCCTPAdapter
  • Implements IFxMessageProcessor to receive FxPortal messages
  • Inherits base SpokePool functionality
  • Inherits CircleCCTPAdapter for USDC bridging

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Constructor

constructor(
    address _wrappedNativeTokenAddress,
    uint32 _depositQuoteTimeBuffer,
    uint32 _fillDeadlineBuffer,
    IERC20 _l2Usdc,
    ITokenMessenger _cctpTokenMessenger,
    uint32 _oftDstEid,
    uint256 _oftFeeCap
)
    SpokePool(
        _wrappedNativeTokenAddress,
        _depositQuoteTimeBuffer,
        _fillDeadlineBuffer,
        _oftDstEid,
        _oftFeeCap
    )
    CircleCCTPAdapter(
        _l2Usdc,
        _cctpTokenMessenger,
        CircleDomainIds.Ethereum
    )
Parameters:
  • _wrappedNativeTokenAddress: Address of WMATIC on Polygon
  • _depositQuoteTimeBuffer: Max age for deposit quote timestamps
  • _fillDeadlineBuffer: Max future offset for fill deadlines
  • _l2Usdc: Circle USDC address on Polygon (or 0x0 to disable CCTP)
  • _cctpTokenMessenger: Circle TokenMessenger contract for CCTP bridging
  • _oftDstEid: LayerZero endpoint ID for OFT messaging
  • _oftFeeCap: Maximum fee for OFT transfers

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Initialization

function initialize(
    uint32 _initialDepositId,
    PolygonTokenBridger _polygonTokenBridger,
    address _crossDomainAdmin,
    address _withdrawalRecipient,
    address _fxChild
) public initializer
Parameters:
  • _initialDepositId: Starting deposit nonce
  • _polygonTokenBridger: Address of PolygonTokenBridger contract
  • _crossDomainAdmin: L1 HubPool address
  • _withdrawalRecipient: Address receiving bridged tokens on L1 (typically HubPool)
  • _fxChild: Address of Polygon’s FxChild contract
Implementation: 
callValidated = false;
__SpokePool_init(_initialDepositId, _crossDomainAdmin, _withdrawalRecipient);
_setPolygonTokenBridger(payable(_polygonTokenBridger));
_setFxChild(_fxChild);

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Admin Verification

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_requireAdminSender()

function _requireAdminSender() internal view override {
    if (!callValidated) revert CallValidatedNotSet();
}
Polygon-specific pattern: Admin verification requires that the function is called within the processMessageFromRoot() flow, validated by the callValidated flag.

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FxPortal Message Processing

function processMessageFromRoot(
    uint256 /*stateId*/,
    address rootMessageSender,
    bytes calldata data
) public validateInternalCalls {
    // Validation logic
    if (msg.sender != fxChild) revert NotFxChild();
    if (rootMessageSender != crossDomainAdmin) revert NotHubPool();

    // Delegatecall to execute admin function
    (bool success, ) = address(this).delegatecall(data);
    if (!success) revert DelegateCallFailed();

    emit ReceivedMessageFromL1(msg.sender, rootMessageSender);
}
Flow:
  1. HubPool on L1 sends message via FxPortal
  2. FxChild on Polygon calls processMessageFromRoot() on this contract
  3. Validates msg.sender == fxChild and rootMessageSender == crossDomainAdmin
  4. Uses delegatecall to execute the admin function on this contract
  5. The delegatecalled function checks callValidated == true in its onlyAdmin modifier

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validateInternalCalls Modifier

modifier validateInternalCalls() {
    if (callValidated) revert CallValidatedAlreadySet();
    
    callValidated = true;  // Allow admin calls
    _;
    callValidated = false; // Reset after execution
}
Purpose: Temporarily sets callValidated flag to allow admin functions to pass _requireAdminSender() check.

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Token Bridging

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_bridgeTokensToHubPool()

function _bridgeTokensToHubPool(
    uint256 amountToReturn,
    address l2TokenAddress
) internal override {
    address oftMessenger = _getOftMessenger(l2TokenAddress);

    // If the token is USDC, use CCTP bridge
    if (_isCCTPEnabled() && l2TokenAddress == address(usdcToken)) {
        _transferUsdc(withdrawalRecipient, amountToReturn);
    }
    // If token has OFT messenger, use LayerZero OFT
    else if (oftMessenger != address(0)) {
        _fundedTransferViaOft(
            IERC20(l2TokenAddress),
            IOFT(oftMessenger),
            withdrawalRecipient,
            amountToReturn
        );
    }
    // Otherwise use PolygonTokenBridger
    else {
        PolygonIERC20Upgradeable(l2TokenAddress).safeIncreaseAllowance(
            address(polygonTokenBridger),
            amountToReturn
        );
        polygonTokenBridger.send(
            PolygonIERC20Upgradeable(l2TokenAddress),
            amountToReturn
        );
    }
}
Three bridging mechanisms:
  1. CCTP for USDC: Uses Circle’s Cross-Chain Transfer Protocol
  2. LayerZero OFT: For tokens with configured OFT messengers
  3. PolygonTokenBridger: Default mechanism for standard ERC20s

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PolygonTokenBridger

Why needed: Polygon’s bridge has special constraints:
  • Complex token mapping system
  • Requires specific burn/lock patterns
  • Special handling for native MATIC vs WMATIC
  • The PolygonTokenBridger contract encapsulates this complexity
Warning in code: 
// WARNING: Withdrawing MATIC can result in the L1 PolygonTokenBridger.startExitWithBurntTokens()
// failing due to a MAX_LOGS constraint imposed by the ERC20Predicate, so if this SpokePool
// will be used to withdraw MATIC then additional constraints need to be imposed to limit
// the # of logs produced by the L2 withdrawal transaction.

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MATIC Handling

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Native Token Wrapping

function wrap() public nonReentrant {
    _wrap();
}

function _wrap() internal {
    uint256 balance = address(this).balance;
    if (balance > 0) wrappedNativeToken.deposit{ value: balance }();
}
Why needed: MATIC transfers from L1→L2 bridging don’t trigger contract calls, so wrapping must be done explicitly.

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_preExecuteLeafHook()

function _preExecuteLeafHook(address l2TokenAddress) internal override {
    // Wrap MATIC → WMATIC before distributing tokens.
    // Only wrap if the token is not an OFT token
    if (_getOftMessenger(l2TokenAddress) == address(0)) {
        _wrap();
    }
}
Ensures any MATIC is wrapped to WMATIC before executing relayer refunds or slow fills.

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Multicall Protection

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_validateMulticallData()

function _validateMulticallData(bytes[] calldata data) internal pure override {
    bool hasOtherPublicFunctionCall = false;
    bool hasExecutedLeafCall = false;
    
    for (uint256 i = 0; i < data.length; i++) {
        bytes4 selector = bytes4(data[i][:4]);
        
        // Block nested multicalls
        if (selector == MultiCallerUpgradeable.multicall.selector) {
            revert MulticallExecuteLeaf();
        }
        // Block mixing executeRelayerRefundLeaf with other calls
        else if (selector == SpokePoolInterface.executeRelayerRefundLeaf.selector) {
            if (hasOtherPublicFunctionCall) revert MulticallExecuteLeaf();
            hasExecutedLeafCall = true;
        }
        else {
            if (hasExecutedLeafCall) revert MulticallExecuteLeaf();
            hasOtherPublicFunctionCall = true;
        }
    }
}
Restrictions:
  1. No nested multicalls
  2. Cannot mix executeRelayerRefundLeaf() with other function calls
  3. Can multicall multiple executeRelayerRefundLeaf() calls together
  4. Can multicall other public functions together (but not with execute functions)
Reason: Prevents griefing attacks that could create oversized L2→L1 messages exceeding L1 calldata limits.

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EOA Enforcement

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executeRelayerRefundLeaf() Override

function executeRelayerRefundLeaf(
    uint32 rootBundleId,
    SpokePoolInterface.RelayerRefundLeaf memory relayerRefundLeaf,
    bytes32[] memory proof
) public payable override {
    // Check if caller is EOA (not contract or EIP7702 delegated wallet)
    if (relayerRefundLeaf.amountToReturn > 0 &&
        (msg.sender != tx.origin || msg.sender.code.length > 0))
        revert NotEOA();
    
    super.executeRelayerRefundLeaf(rootBundleId, relayerRefundLeaf, proof);
}
Why needed: Prevents contracts from batching executeRelayerRefundLeaf() with other calls that could produce excessive logs, which would cause the L2→L1 message to fail. Checks:
  • msg.sender != tx.origin: Rejects calls from contracts
  • msg.sender.code.length > 0: Rejects EIP-7702 delegated wallets

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Admin Functions

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setFxChild()

function setFxChild(address newFxChild) public onlyAdmin nonReentrant {
    _setFxChild(newFxChild);
}

function _setFxChild(address _fxChild) internal {
    fxChild = _fxChild;
    emit SetFxChild(_fxChild);
}

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setPolygonTokenBridger()

function setPolygonTokenBridger(
    address payable newPolygonTokenBridger
) public onlyAdmin nonReentrant {
    _setPolygonTokenBridger(newPolygonTokenBridger);
}

function _setPolygonTokenBridger(address payable _polygonTokenBridger) internal {
    polygonTokenBridger = PolygonTokenBridger(_polygonTokenBridger);
    emit SetPolygonTokenBridger(address(_polygonTokenBridger));
}

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State Variables

// Address of FxChild which sends and receives messages to and from L1
address public fxChild;

// Contract that processes all cross-chain transfers between this contract and HubPool
PolygonTokenBridger public polygonTokenBridger;

// Internal variable that only flips temporarily to true upon receiving messages from L1
bool private callValidated;

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Events

event SetFxChild(address indexed newFxChild);
event SetPolygonTokenBridger(address indexed polygonTokenBridger);
event ReceivedMessageFromL1(address indexed caller, address indexed rootMessageSender);

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Errors

error MulticallExecuteLeaf();
error CallValidatedAlreadySet();
error CallValidatedNotSet();
error DelegateCallFailed();
error NotHubPool();
error NotFxChild();

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Unique Features

  1. FxPortal integration: Unique cross-chain messaging system specific to Polygon
  2. Delegatecall pattern: Admin functions executed via delegatecall for validation
  3. PolygonTokenBridger: Custom contract to handle Polygon bridge complexity
  4. MATIC wrapping: Explicit wrapping needed due to Polygon’s bridging behavior
  5. Griefing protection: EOA enforcement and multicall restrictions to prevent L2→L1 message failures
  6. Multi-bridge support: CCTP, OFT, and native Polygon bridge

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Architecture Notes

  • HubPool sends messages via FxPortal’s StateSender on L1
  • FxChild contract on Polygon receives and forwards messages
  • Admin calls go through processMessageFromRoot() → delegatecall → actual function
  • The callValidated flag ensures functions are only callable via this flow
  • Token bridging is delegated to PolygonTokenBridger to handle Polygon-specific requirements
  • Special care taken to prevent excessive log production in L2→L1 messages
  • SpokePool - Base contract
  • Polygon Adapter - L1→L2 message relay via FxPortal from HubPool
  • PolygonTokenBridger - Token bridging helper (see source code)

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