feat(protocol): only keep one latest copy of synced snippet

packages/protocol/contracts/L1/TaikoL1.sol

@@ -167,13 +167,8 @@ contract TaikoL1 is
     /// @notice Important: as this contract doesn't send each block's state root as a signal when
     /// the block is verified, bridging developers should subscribe to CrossChainSynced events
     /// to ensure all synced state roots are verifiable using merkle proofs.
-    function getSyncedSnippet(uint64 blockId)
-        public
-        view
-        override
-        returns (ICrossChainSync.Snippet memory)
-    {
-        return LibUtils.getSyncedSnippet(state, getConfig(), blockId);
+    function getSyncedSnippet() public view override returns (ICrossChainSync.Snippet memory) {
+        return LibUtils.getSyncedSnippet(state, getConfig());
     }
 
     /// @notice Gets the state variables of the TaikoL1 contract.

packages/protocol/contracts/L1/libs/LibUtils.sol

@@ -55,14 +55,13 @@ library LibUtils {
 
     function getSyncedSnippet(
         TaikoData.State storage state,
-        TaikoData.Config memory config,
-        uint64 blockId
+        TaikoData.Config memory config
     )
         external
         view
         returns (ICrossChainSync.Snippet memory)
     {
-        uint64 _blockId = blockId == 0 ? state.slotB.lastVerifiedBlockId : blockId;
+        uint64 _blockId = state.slotB.lastVerifiedBlockId;
         uint64 slot = _blockId % config.blockRingBufferSize;
 
         TaikoData.Block storage blk = state.blocks[slot];
@@ -74,8 +73,7 @@ library LibUtils {
             state.transitions[slot][blk.verifiedTransitionId];
 
         return ICrossChainSync.Snippet({
-            remoteBlockId: blockId,
-            syncedInBlock: blk.proposedIn,
+            blockId: _blockId,
             blockHash: transition.blockHash,
             stateRoot: transition.stateRoot
         });

packages/protocol/contracts/L2/TaikoL2.sol

@@ -45,15 +45,15 @@ contract TaikoL2 is CrossChainOwned, ICrossChainSync {
 
     // Mapping from L2 block numbers to their block hashes.
     // All L2 block hashes will be saved in this mapping.
-    mapping(uint256 blockId => bytes32 blockHash) public l2Hashes;
-    mapping(uint256 l1height => ICrossChainSync.Snippet) public snippets;
+    mapping(uint256 blockId => bytes32 blockHash) public l2Hashes; // slot 1
 
     // A hash to check the integrity of public inputs.
-    bytes32 public publicInputHash; // slot 3
-    uint64 public gasExcess; // slot 4
-    uint64 public latestSyncedL1Height;
+    bytes32 public publicInputHash; // slot 2
+    uint64 public gasExcess; // slot 3
 
-    uint256[146] private __gap;
+    Snippet private _l1Snippet; // slot 4, 5, 6
+
+    uint256[144] private __gap;
 
     event Anchored(bytes32 parentHash, uint64 gasExcess);
 
@@ -147,19 +147,14 @@ contract TaikoL2 is CrossChainOwned, ICrossChainSync {
             ownerChainId, "state_root", l1StateRoot
         );
 
-        emit CrossChainSynced(uint64(block.number), l1Height, l1BlockHash, l1StateRoot);
-
         // Update state variables
         l2Hashes[parentId] = blockhash(parentId);
-        snippets[l1Height] = ICrossChainSync.Snippet({
-            remoteBlockId: l1Height,
-            syncedInBlock: uint64(block.number),
-            blockHash: l1BlockHash,
-            stateRoot: l1StateRoot
-        });
-
         publicInputHash = publicInputHashNew;
-        latestSyncedL1Height = l1Height;
+
+        _l1Snippet.blockId = l1Height;
+        _l1Snippet.blockHash = l1BlockHash;
+        _l1Snippet.stateRoot = l1StateRoot;
+        emit CrossChainSynced(l1Height, l1BlockHash, l1StateRoot);
 
         emit Anchored(blockhash(parentId), gasExcess);
     }
@@ -175,14 +170,8 @@ contract TaikoL2 is CrossChainOwned, ICrossChainSync {
     }
 
     /// @inheritdoc ICrossChainSync
-    function getSyncedSnippet(uint64 blockId)
-        public
-        view
-        override
-        returns (ICrossChainSync.Snippet memory)
-    {
-        uint256 id = blockId == 0 ? latestSyncedL1Height : blockId;
-        return snippets[id];
+    function getSyncedSnippet() public view returns (ICrossChainSync.Snippet memory) {
+        return _l1Snippet;
     }
 
     /// @notice Gets the basefee and gas excess using EIP-1559 configuration for
@@ -274,10 +263,10 @@ contract TaikoL2 is CrossChainOwned, ICrossChainSync {
             // Calculate how much more gas to issue to offset gas excess.
             // after each L1 block time, config.gasTarget more gas is issued,
             // the gas excess will be reduced accordingly.
-            // Note that when latestSyncedL1Height is zero, we skip this step.
+            // Note that when _l1Snippet.blockId is zero, we skip this step.
             uint256 numL1Blocks;
-            if (latestSyncedL1Height > 0 && l1Height > latestSyncedL1Height) {
-                numL1Blocks = l1Height - latestSyncedL1Height;
+            if (_l1Snippet.blockId > 0 && l1Height > _l1Snippet.blockId) {
+                numL1Blocks = l1Height - _l1Snippet.blockId;
             }
 
             if (numL1Blocks > 0) {

packages/protocol/contracts/common/ICrossChainSync.sol

@@ -22,25 +22,18 @@ pragma solidity 0.8.24;
 /// both chains remain consistent and can be cross-referenced with integrity.
 interface ICrossChainSync {
     struct Snippet {
-        uint64 remoteBlockId;
-        uint64 syncedInBlock;
+        uint64 blockId;
         bytes32 blockHash;
         bytes32 stateRoot;
     }
 
     /// @dev Emitted when a block has been synced across chains.
-    /// @param syncedInBlock The ID of this chain's block where the sync
-    /// happened.
     /// @param blockId The ID of the remote block whose block hash are synced.
     /// @param blockHash The hash of the synced block.
     /// @param stateRoot The block's state root.
-    event CrossChainSynced(
-        uint64 indexed syncedInBlock, uint64 indexed blockId, bytes32 blockHash, bytes32 stateRoot
-    );
+    event CrossChainSynced(uint64 indexed blockId, bytes32 blockHash, bytes32 stateRoot);
 
     /// @notice Fetches the hash of a block from the opposite chain.
-    /// @param blockId The target block id. Specifying 0 retrieves the hash
-    /// of the latest block.
     /// @return snippet The block hash and signal root synced.
-    function getSyncedSnippet(uint64 blockId) external view returns (Snippet memory snippet);
+    function getSyncedSnippet() external view returns (Snippet memory snippet);
 }

packages/protocol/contracts/signal/SignalService.sol

@@ -45,20 +45,20 @@ contract SignalService is EssentialContract, ISignalService {
     uint256[49] private __gap;
 
     event TrustedRelayUpdated(uint64 indexed hopChainId, uint64 indexed srcChainId, address hop);
-    event ChainDataRelayed(
+    event SnippetRelayed(
         uint64 indexed chainid, bytes32 indexed kind, bytes32 data, bytes32 signal
     );
 
-    error SS_INVALID_PARAMS();
-    error SS_INVALID_PROOF();
     error SS_EMPTY_PROOF();
     error SS_INVALID_APP();
     error SS_INVALID_HOP_PROOF();
     error SS_INVALID_LAST_HOP_CHAINID();
     error SS_INVALID_MID_HOP_CHAINID();
+    error SS_INVALID_PARAMS();
+    error SS_INVALID_PROOF();
     error SS_INVALID_RELAY();
-    error SS_INVALID_SIGNAL();
     error SS_INVALID_STATE_ROOT();
+    error SS_INVALID_SIGNAL();
     error SS_LOCAL_CHAIN_DATA_NOT_FOUND();
     error SS_UNSUPPORTED();
 
@@ -211,7 +211,7 @@ contract SignalService is EssentialContract, ISignalService {
         returns (bytes32 slot)
     {
         bytes32 signal = signalForChainData(chainId, kind, data);
-        emit ChainDataRelayed(chainId, kind, data, signal);
+        emit SnippetRelayed(chainId, kind, data, signal);
         return _sendSignal(address(this), signal);
     }
 

packages/protocol/test/HelperContracts.sol

@@ -55,12 +55,13 @@ contract SkipProofCheckSignal is SignalService {
 contract DummyCrossChainSync is EssentialContract, ICrossChainSync {
     Snippet private _snippet;
 
-    function setSyncedData(bytes32 blockHash, bytes32 stateRoot) external {
+    function setSnippet(uint64 blockId, bytes32 blockHash, bytes32 stateRoot) external {
+        _snippet.blockId = blockId;
         _snippet.blockHash = blockHash;
         _snippet.stateRoot = stateRoot;
     }
 
-    function getSyncedSnippet(uint64) external view returns (Snippet memory) {
+    function getSyncedSnippet() public view returns (Snippet memory) {
         return _snippet;
     }
 }

packages/protocol/test/L1/TaikoL1.t.sol

@@ -192,13 +192,11 @@ contract TaikoL1Test is TaikoL1TestBase {
     }
 
     /// @dev getCrossChainBlockHash tests
-    function test_L1_getCrossChainBlockHash0() external {
-        bytes32 genHash = L1.getSyncedSnippet(0).blockHash;
-        assertEq(GENESIS_BLOCK_HASH, genHash);
-
-        // Reverts if block is not yet verified!
-        vm.expectRevert(TaikoErrors.L1_BLOCK_MISMATCH.selector);
-        L1.getSyncedSnippet(1);
+    function test_L1_getCrossChainSnippet_Genesis() external {
+        ICrossChainSync.Snippet memory snippet = L1.getSyncedSnippet();
+        assertEq(snippet.blockId, 0);
+        assertEq(snippet.blockHash, GENESIS_BLOCK_HASH);
+        assertEq(snippet.stateRoot, 0);
     }
 
     /// @dev getSyncedSnippet tests
@@ -235,26 +233,13 @@ contract TaikoL1Test is TaikoL1TestBase {
 
             verifyBlock(Carol, 1);
 
-            // Querying written blockhash
-            assertEq(L1.getSyncedSnippet(blockId).blockHash, blockHash);
+            assertEq(L1.getSyncedSnippet().blockId, blockId);
+            assertEq(L1.getSyncedSnippet().blockHash, blockHash);
+            assertEq(L1.getSyncedSnippet().stateRoot, stateRoot);
 
             mine(5);
             parentHashes[blockId] = blockHash;
         }
-
-        uint64 queriedBlockId = 1;
-        bytes32 expectedSR = bytes32(1e9 + uint256(queriedBlockId));
-
-        assertEq(expectedSR, L1.getSyncedSnippet(queriedBlockId).stateRoot);
-
-        // 2nd
-        queriedBlockId = 2;
-        expectedSR = bytes32(1e9 + uint256(queriedBlockId));
-        assertEq(expectedSR, L1.getSyncedSnippet(queriedBlockId).stateRoot);
-
-        // Not found -> reverts
-        vm.expectRevert(TaikoErrors.L1_BLOCK_MISMATCH.selector);
-        L1.getSyncedSnippet((count + 1));
     }
 
     function test_L1_deposit_hash_creation() external {

packages/protocol/test/bridge/Bridge.t.sol

@@ -643,7 +643,8 @@ contract BridgeTest is TaikoTest {
 
         addressManager.setAddress(dest, "signal_service", address(mockProofSignalService));
 
-        crossChainSync.setSyncedData(
+        crossChainSync.setSnippet(
+            123,
             0xd5f5d8ac6bc37139c97389b00e9cf53e89c153ad8a5fc765ffe9f44ea9f3d31e,
             0x631b214fb030d82847224f0b3d3b906a6764dded176ad3c7262630204867ba85
         );

packages/protocol/test/signal/SignalService.t.sol

@@ -12,6 +12,7 @@ contract MockSignalService is SignalService {
         address /*relay*/
     )
         internal
+        pure
         override
         returns (bytes32)
     {