Use synchronized call to access the chain head block in estimateGas()

Signed-off-by: Matthew Whitehead <[email protected]>

ethereum/api/src/main/java/org/hyperledger/besu/ethereum/api/jsonrpc/internal/methods/AbstractEstimateGas.java

@@ -23,6 +23,7 @@
 import org.hyperledger.besu.ethereum.api.jsonrpc.internal.response.JsonRpcErrorResponse;
 import org.hyperledger.besu.ethereum.api.jsonrpc.internal.response.RpcErrorType;
 import org.hyperledger.besu.ethereum.api.query.BlockchainQueries;
+import org.hyperledger.besu.ethereum.chain.Blockchain;
 import org.hyperledger.besu.ethereum.core.BlockHeader;
 import org.hyperledger.besu.ethereum.mainnet.ValidationResult;
 import org.hyperledger.besu.ethereum.processing.TransactionProcessingResult;
@@ -48,8 +49,16 @@ public AbstractEstimateGas(
   }
 
   protected BlockHeader blockHeader() {
-    final long headBlockNumber = blockchainQueries.headBlockNumber();
-    return blockchainQueries.getBlockchain().getBlockHeader(headBlockNumber).orElse(null);
+    final Blockchain theChain = blockchainQueries.getBlockchain();
+
+    // Optimistically get the block header for the chain head without taking a lock,
+    // but revert to the safe implementation if it returns an empty optional. (It's
+    // possible the chain head has been updated but the block is still being persisted
+    // to storage/cache under the lock).
+    return theChain
+        .getBlockHeader(theChain.getChainHeadHash())
+        .or(() -> theChain.getBlockHeaderSafe(theChain.getChainHeadHash()))
+        .orElse(null);
   }
 
   protected CallParameter overrideGasLimitAndPrice(