Handle the BackwardLET in the L2 bridge syncer.

Function signature

function backwardLET(
        uint256 newDepositCount,
        bytes32[_DEPOSIT_CONTRACT_TREE_DEPTH] calldata newFrontier,
        bytes32 nextLeaf,
        bytes32[_DEPOSIT_CONTRACT_TREE_DEPTH] calldata proof
    ) external virtual onlyGlobalExitRootRemover ifEmergencyState

Event signature

event BackwardLET(
        uint256 previousDepositCount,
        bytes32 previousRoot,
        uint256 newDepositCount,
        bytes32 newRoot
    );

Behavior

Once we index the event:

• removing all the bridges that have the depositCount field greater than newDepositCount from the event in the bridge table, and moving them to a separate table.
  • TBD reorgs handling, in a way that if certain bridge that was affected by BackwardLET later on the BackwardLET gets reorged,
• figure out how to update the exitTree, which is AppendOnlyTree

  aggkit/bridgesync/processor.go

  Line 569 in 9ba13e3

  exitTree *tree.AppendOnlyTree

  . Should it become the UpdateableTree? 🤔

Open questions

1. (L2) reorgs handling, because it seems that bridges should be removed from bridge table,
2. exitTree should be UpdatableTree, since now we should be able to update its content (namely remove leafs)?