feat(protocol): add lastProposedIn to slotB

packages/protocol/contracts/layer1/based/ITaikoL1.sol

@@ -87,6 +87,10 @@ interface ITaikoL1 {
         view
         returns (TaikoData.TransitionState memory);
 
+    /// @notice Retrieves the ID of the L1 block where the most recent L2 block was proposed.
+    /// @return The ID of the Li block where the most recent block was proposed.
+    function lastProposedIn() external view returns (uint56);
+
     /// @notice Gets the configuration of the TaikoL1 contract.
     /// @return Config struct containing configuration parameters.
     function getConfig() external pure returns (TaikoData.Config memory);

packages/protocol/contracts/layer1/based/LibProposing.sol

@@ -278,6 +278,7 @@ library LibProposing {
         unchecked {
             ++_state.slotB.numBlocks;
         }
+        _state.slotB.lastProposedIn = uint56(block.number);
 
         LibBonds.debitBond(_state, _resolver, local.params.proposer, meta_.id, _config.livenessBond);
 

packages/protocol/contracts/layer1/based/TaikoData.sol

@@ -191,9 +191,7 @@ library TaikoData {
         uint64 numBlocks;
         uint64 lastVerifiedBlockId;
         bool provingPaused;
-        uint8 __reservedB1;
-        uint16 __reservedB2;
-        uint32 __reservedB3;
+        uint56 lastProposedIn;
         uint64 lastUnpausedAt;
     }
 

packages/protocol/contracts/layer1/based/TaikoL1.sol

@@ -58,10 +58,6 @@ contract TaikoL1 is EssentialContract, ITaikoL1, TaikoEvents {
 
     /// @notice This function shall be called by previously deployed contracts.
     function init2() external onlyOwner reinitializer(2) {
-        // reset some previously used slots for future reuse
-        state.slotB.__reservedB1 = 0;
-        state.slotB.__reservedB2 = 0;
-        state.slotB.__reservedB3 = 0;
         state.__reserve1 = 0;
     }
 
@@ -287,6 +283,12 @@ contract TaikoL1 is EssentialContract, ITaikoL1, TaikoEvents {
         return state.slotB.lastUnpausedAt;
     }
 
+    /// @notice Retrieves the ID of the L1 block where the most recent L2 block was proposed.
+    /// @return The ID of the Li block where the most recent block was proposed.
+    function lastProposedIn() external view returns (uint56) {
+        return state.slotB.lastProposedIn;
+    }
+
     /// @inheritdoc ITaikoL1
     function getConfig() public pure virtual returns (TaikoData.Config memory) {
         return TaikoData.Config({

packages/protocol/contracts/layer1/provers/ProverSet.sol

@@ -30,6 +30,7 @@ contract ProverSet is EssentialContract, IERC1271 {
 
     error INVALID_STATUS();
     error PERMISSION_DENIED();
+    error NOT_FIRST_PROPOSAL();
 
     modifier onlyAuthorized() {
         require(
@@ -81,6 +82,20 @@ contract ProverSet is EssentialContract, IERC1271 {
         LibAddress.sendEtherAndVerify(admin, _amount);
     }
 
+    /// @notice Proposes a block only when it is the first block proposal in the current L1 block.
+    function proposeBlockV2Conditionally(
+        bytes calldata _params,
+        bytes calldata _txList
+    )
+        external
+        onlyProver
+    {
+        ITaikoL1 taiko = ITaikoL1(taikoL1());
+        // Ensure this block is the first block proposed in the current L1 block.
+        require(taiko.lastProposedIn() != block.number, NOT_FIRST_PROPOSAL());
+        taiko.proposeBlockV2(_params, _txList);
+    }
+
     /// @notice Propose a Taiko block.
     function proposeBlockV2(bytes calldata _params, bytes calldata _txList) external onlyProver {
         ITaikoL1(taikoL1()).proposeBlockV2(_params, _txList);

packages/protocol/test/layer1/based/MockTaikoL1.sol

@@ -62,5 +62,7 @@ contract MockTaikoL1 is ITaikoL1 {
         returns (TaikoData.TransitionState memory)
     { }
 
+    function lastProposedIn() external view returns (uint56) { }
+
     function getConfig() external pure virtual returns (TaikoData.Config memory) { }
 }