fix: return error response for genesis block in debug_traceBlockByNumber

Genesis block (block 0) has no transactions to trace. Instead of
returning null, return a JSON-RPC error matching the debug trace spec.

Relates to #10115

Signed-off-by: Vivek Singh Solanki <viveksolanki0509@gmail.com>

Author: vivek-0509

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

@@ -24,6 +24,7 @@
 import org.hyperledger.besu.ethereum.api.jsonrpc.internal.parameters.TransactionTraceParams;
 import org.hyperledger.besu.ethereum.api.jsonrpc.internal.processor.Tracer;
 import org.hyperledger.besu.ethereum.api.jsonrpc.internal.processor.TransactionTrace;
+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.jsonrpc.internal.results.DebugTraceTransactionResult;
 import org.hyperledger.besu.ethereum.api.query.BlockchainQueries;
@@ -88,6 +89,11 @@ protected BlockParameter blockParameter(final JsonRpcRequestContext request) {
   protected Object resultByBlockNumber(
       final JsonRpcRequestContext request, final long blockNumber) {
 
+    if (blockNumber == 0L) {
+      return new JsonRpcErrorResponse(
+          request.getRequest().getId(), RpcErrorType.GENESIS_BLOCK_NOT_TRACEABLE);
+    }
+
     final TraceOptions traceOptions;
     try {
       traceOptions =

ethereum/api/src/main/java/org/hyperledger/besu/ethereum/api/jsonrpc/internal/response/RpcErrorType.java

@@ -210,6 +210,7 @@ public enum RpcErrorType implements RpcMethodError {
   // Debug failures
   BLOCK_NOT_FOUND(-32000, "Block not found"),
   PARENT_BLOCK_NOT_FOUND(-32000, "Parent block not found"),
+  GENESIS_BLOCK_NOT_TRACEABLE(-32000, "genesis is not traceable"),
 
   // Permissioning/Account allowlist errors
   ACCOUNT_ALLOWLIST_NOT_ENABLED(-32000, "Account allowlist has not been enabled"),

ethereum/api/src/test/java/org/hyperledger/besu/ethereum/api/jsonrpc/internal/methods/DebugTraceBlockByNumberTest.java

@@ -26,8 +26,10 @@
 import org.hyperledger.besu.ethereum.api.jsonrpc.internal.JsonRpcRequestContext;
 import org.hyperledger.besu.ethereum.api.jsonrpc.internal.exception.InvalidJsonRpcParameters;
 import org.hyperledger.besu.ethereum.api.jsonrpc.internal.processor.Tracer;
+import org.hyperledger.besu.ethereum.api.jsonrpc.internal.response.JsonRpcErrorResponse;
 import org.hyperledger.besu.ethereum.api.jsonrpc.internal.response.JsonRpcResponse;
 import org.hyperledger.besu.ethereum.api.jsonrpc.internal.response.JsonRpcSuccessResponse;
+import org.hyperledger.besu.ethereum.api.jsonrpc.internal.response.RpcErrorType;
 import org.hyperledger.besu.ethereum.api.jsonrpc.internal.results.OpCodeLoggerTracerResult;
 import org.hyperledger.besu.ethereum.api.query.BlockchainQueries;
 import org.hyperledger.besu.ethereum.chain.Blockchain;
@@ -117,6 +119,19 @@ private Collection<OpCodeLoggerTracerResult> getResult(final JsonRpcSuccessRespo
     return (Collection<OpCodeLoggerTracerResult>) response.getResult();
   }
 
+  @Test
+  public void shouldReturnErrorForGenesisBlock() {
+    final Object[] params = new Object[] {"0x0"};
+    final JsonRpcRequestContext request =
+        new JsonRpcRequestContext(new JsonRpcRequest("2.0", "debug_traceBlockByNumber", params));
+
+    final JsonRpcResponse jsonRpcResponse = debugTraceBlockByNumber.response(request);
+    assertThat(jsonRpcResponse).isInstanceOf(JsonRpcErrorResponse.class);
+    final JsonRpcErrorResponse errorResponse = (JsonRpcErrorResponse) jsonRpcResponse;
+    assertThat(errorResponse.getErrorType())
+        .isEqualByComparingTo(RpcErrorType.GENESIS_BLOCK_NOT_TRACEABLE);
+  }
+
   @Test
   public void shouldHandleInvalidParametersGracefully() {
     final Object[] invalidParams = new Object[] {"invalid-block-number"};