[C++] [Compute] Question on "EVENT" macro in ExecNode

[Li Jin]

Hello!

I am trying to implement a new type of join in Arrow Compute engine (asof
join). I have been looking at code of HashJoinNode and found some debug
code that seems to be useful:

e.g.:
    EVENT(span_, "InputReceived", {{"batch.length", batch.length}, {"side",
side}});

But when I try to use similar code in my ExecNode, I got an error:

/home/icexelloss/workspace/arrow/cpp/src/arrow/compute/exec/asof_join_node.cc:67:9:
error: unused variable ‘side’ [-Werror=unused-variable]
   67 |     int side = (input == inputs_[0]) ? 0 : 1;
      |         ^~~~

(here is my code):
  void InputReceived(ExecNode* input, ExecBatch batch) override {
    int side = (input == inputs_[0]) ? 0 : 1;
    EVENT(span_, "InputReceived", {{"batch.length", batch.length}, {"side",
side}});
  }

I wonder:
(1) Is there a special cmake flag I need to pass in to enable the EVENT
marco?
(2) What does the EVENT marco do and where does it output to?

Thanks!
Li