add assert to remove handling noop primary response when 5.0 nodes are not supported

Author: areek

core/src/main/java/org/elasticsearch/action/bulk/TransportShardBulkAction.java

@@ -23,6 +23,7 @@
 import org.apache.logging.log4j.message.ParameterizedMessage;
 import org.apache.logging.log4j.util.Supplier;
 import org.elasticsearch.ExceptionsHelper;
+import org.elasticsearch.Version;
 import org.elasticsearch.action.DocWriteRequest;
 import org.elasticsearch.action.DocWriteResponse;
 import org.elasticsearch.action.delete.DeleteRequest;
@@ -381,6 +382,13 @@ public enum ReplicaItemExecutionMode {
         FAILURE
     }
 
+    static {
+        assert Version.CURRENT.minimumCompatibilityVersion().after(Version.V_5_0_0) == false:
+                "Remove logic handling NoOp result from primary response; see TODO in replicaItemExecutionMode" +
+                        " as the current minimum compatible version [" + 
+                        Version.CURRENT.minimumCompatibilityVersion() + "] is after 5.0";
+    }
+
     /**
      * Determines whether a bulk item request should be executed on the replica.
      * @return {@link ReplicaItemExecutionMode#NORMAL} upon normal primary execution with no failures
@@ -398,6 +406,8 @@ static ReplicaItemExecutionMode replicaItemExecutionMode(final BulkItemRequest r
         } else {
             // NOTE: write requests originating from pre-6.0 nodes can send a no-op operation to
             // the replica; we ignore replication
+            // TODO: remove noOp result check from primary response, when pre-6.0 nodes are not supported
+            // we should return ReplicationItemExecutionMode.NORMAL instead
             return primaryResponse.getResponse().getResult() != DocWriteResponse.Result.NOOP
                     ? ReplicaItemExecutionMode.NORMAL // execution successful on primary
                     : ReplicaItemExecutionMode.NOOP; // ignore replication