Validate header timestamp in engine_forkchoiceUpdated

nimbus/beacon/api_handler/api_forkchoice.nim

@@ -51,6 +51,24 @@ template validateVersion(attr, com, apiVersion) =
         raise invalidParams("if timestamp is earlier than Shanghai," &
           " payloadAttributes must be PayloadAttributesV1")
 
+template validateHeaderTimestamp(header, com, apiVersion) =
+  # See fCUV3 specification No.2 bullet iii
+  # https://github.com/ethereum/execution-apis/blob/v1.0.0-beta.4/src/engine/cancun.md#specification-1
+  if com.isCancunOrLater(header.timestamp):
+    if apiVersion != Version.V3:
+      raise invalidAttr("forkChoiceUpdated" & $apiVersion &
+          " doesn't support head block with Cancun timestamp")
+  # See fCUV2 specification No.2 bullet 1
+  # https://github.com/ethereum/execution-apis/blob/v1.0.0-beta.4/src/engine/shanghai.md#specification-1
+  elif com.isShanghaiOrLater(header.timestamp):
+    if apiVersion != Version.V2:
+      raise invalidAttr("forkChoiceUpdated" & $apiVersion &
+          " doesn't support head block with Shanghai timestamp")
+  else:
+    if apiVersion != Version.V1:
+      raise invalidAttr("forkChoiceUpdated" & $apiVersion &
+          " doesn't support head block with timestamp earlier than Shanghai")
+
 proc forkchoiceUpdated*(ben: BeaconEngineRef,
                         apiVersion: Version,
                         update: ForkchoiceStateV1,
@@ -100,6 +118,8 @@ proc forkchoiceUpdated*(ben: BeaconEngineRef,
     com.syncReqNewHead(header)
     return simpleFCU(PayloadExecutionStatus.syncing)
 
+  validateHeaderTimestamp(header, com, apiVersion)
+
   # Block is known locally, just sanity check that the beacon client does not
   # attempt to push us back to before the merge.
   #