revert some changes

packages/protocol/contracts/L1/TaikoL1.sol

@@ -167,8 +167,13 @@ 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() public view override returns (ICrossChainSync.Snippet memory) {
-        return LibUtils.getSyncedSnippet(state, getConfig());
+    function getSyncedSnippet(uint64 blockId)
+        public
+        view
+        override
+        returns (ICrossChainSync.Snippet memory)
+    {
+        return LibUtils.getSyncedSnippet(state, getConfig(), blockId);
     }
 
     /// @notice Gets the state variables of the TaikoL1 contract.

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

@@ -55,25 +55,24 @@ library LibUtils {
 
     function getSyncedSnippet(
         TaikoData.State storage state,
-        TaikoData.Config memory config
+        TaikoData.Config memory config,
+        uint64 blockId
     )
         external
         view
         returns (ICrossChainSync.Snippet memory)
     {
-        uint64 _blockId = state.slotB.lastVerifiedBlockId;
+        uint64 _blockId = blockId == 0 ? state.slotB.lastVerifiedBlockId : blockId;
         uint64 slot = _blockId % config.blockRingBufferSize;
 
         TaikoData.Block storage blk = state.blocks[slot];
-
         if (blk.blockId != _blockId) revert L1_BLOCK_MISMATCH();
         if (blk.verifiedTransitionId == 0) revert L1_TRANSITION_NOT_FOUND();
-
         TaikoData.TransitionState storage transition =
             state.transitions[slot][blk.verifiedTransitionId];
 
         return ICrossChainSync.Snippet({
-            blockId: _blockId,
+            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; // slot 1
+    mapping(uint256 blockId => bytes32 blockHash) public l2Hashes;
+    mapping(uint256 l1height => ICrossChainSync.Snippet) public snippets;
 
     // A hash to check the integrity of public inputs.
-    bytes32 public publicInputHash; // slot 2
-    uint64 public gasExcess; // slot 3
+    bytes32 public publicInputHash; // slot 3
+    uint64 public gasExcess; // slot 4
+    uint64 public latestSyncedL1Height;
 
-    Snippet private _l1Snippet; // slot 4, 5, 6
-
-    uint256[144] private __gap;
+    uint256[146] private __gap;
 
     event Anchored(bytes32 parentHash, uint64 gasExcess);
 
@@ -147,15 +147,17 @@ contract TaikoL2 is CrossChainOwned, ICrossChainSync {
             ownerChainId, "state_root", l1StateRoot
         );
 
+        emit CrossChainSynced(l1Height, l1BlockHash, l1StateRoot);
+
         // Update state variables
         l2Hashes[parentId] = blockhash(parentId);
+        snippets[l1Height] = ICrossChainSync.Snippet({
+            blockId: l1Height,
+            blockHash: l1BlockHash,
+            stateRoot: l1StateRoot
+        });
         publicInputHash = publicInputHashNew;
-
-        _l1Snippet.blockId = l1Height;
-        _l1Snippet.blockHash = l1BlockHash;
-        _l1Snippet.stateRoot = l1StateRoot;
-        emit CrossChainSynced(l1Height, l1BlockHash, l1StateRoot);
-
+        latestSyncedL1Height = l1Height;
         emit Anchored(blockhash(parentId), gasExcess);
     }
 
@@ -170,8 +172,14 @@ contract TaikoL2 is CrossChainOwned, ICrossChainSync {
     }
 
     /// @inheritdoc ICrossChainSync
-    function getSyncedSnippet() public view returns (ICrossChainSync.Snippet memory) {
-        return _l1Snippet;
+    function getSyncedSnippet(uint64 blockId)
+        public
+        view
+        override
+        returns (ICrossChainSync.Snippet memory)
+    {
+        uint256 id = blockId == 0 ? latestSyncedL1Height : blockId;
+        return snippets[id];
     }
 
     /// @notice Gets the basefee and gas excess using EIP-1559 configuration for
@@ -263,10 +271,10 @@ 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 _l1Snippet.blockId is zero, we skip this step.
+            // Note that when latestSyncedL1Height is zero, we skip this step.
             uint256 numL1Blocks;
-            if (_l1Snippet.blockId > 0 && l1Height > _l1Snippet.blockId) {
-                numL1Blocks = l1Height - _l1Snippet.blockId;
+            if (latestSyncedL1Height > 0 && l1Height > latestSyncedL1Height) {
+                numL1Blocks = l1Height - latestSyncedL1Height;
             }
 
             if (numL1Blocks > 0) {

packages/protocol/contracts/common/ICrossChainSync.sol

@@ -34,6 +34,8 @@ interface ICrossChainSync {
     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() external view returns (Snippet memory snippet);
+    function getSyncedSnippet(uint64 blockId) external view returns (Snippet memory snippet);
 }

packages/protocol/test/HelperContracts.sol

@@ -61,7 +61,7 @@ contract DummyCrossChainSync is EssentialContract, ICrossChainSync {
         _snippet.stateRoot = stateRoot;
     }
 
-    function getSyncedSnippet() public view returns (Snippet memory) {
+    function getSyncedSnippet(uint64 /*blockId*/ ) public view returns (Snippet memory) {
         return _snippet;
     }
 }

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

@@ -192,11 +192,13 @@ contract TaikoL1Test is TaikoL1TestBase {
     }
 
     /// @dev getCrossChainBlockHash tests
-    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);
+    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);
     }
 
     /// @dev getSyncedSnippet tests
@@ -233,13 +235,26 @@ contract TaikoL1Test is TaikoL1TestBase {
 
             verifyBlock(Carol, 1);
 
-            assertEq(L1.getSyncedSnippet().blockId, blockId);
-            assertEq(L1.getSyncedSnippet().blockHash, blockHash);
-            assertEq(L1.getSyncedSnippet().stateRoot, stateRoot);
+            // Querying written blockhash
+            assertEq(L1.getSyncedSnippet(blockId).blockHash, blockHash);
 
             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 {