[humble binary] BT: „missing port [value]” for blackboard “{goal}”

[lmendyk]

I’m writing my first BT in which I want to reuse the default one for navigate to pose in a way to make a robot go first to the goal and after that goes back so I can use the GUI for navigation the turtlebot3.

To do that I’m using the subtree which definition is from the default BT “navigate_to_pose_w_replanning_and_recovery.xml” but used as a subtree. In the main tree in the sequence I intend to call the subtree which uses {goal} (and {start} variables and next I want to call the same tree but with {goal} and {start} swapped to make the robot come back.

First I wanted to use the re-mapping mechanism, but from my understanding there are bugs which fixes may not have been released yet ( #3640 and BehaviorTree/BehaviorTree.CPP#563) so I decided to use the “workaround” and use the mechanism with __shared_blackboard="true" but for this to work I need to save the {goal} (and {start} variables.
I try doing it with that:
<SetBlackboard value="{goal}" output_key="orignalGoal" />

But I'm getting an error:

Behavior tree threw exception: missing port [value]. Exiting with failure

My understating of this error is that {goal} must be returning Null or similar causing that error. But As I understand the Nav stack wrapping BT should have defined the {goal} key value in the blackboard as
<ComputePathToPose goal="{goal}" path="{path}" planner_id="GridBased"/>
Does not cause the problem.

Could you advice what is wrong with the idea?
The snippet of my BT is as follows (there are commented lines – the effect of my debugging :

<root main_tree_to_execute="MainTree">
  <BehaviorTree ID="MainTree">
    <Sequence>
      <!--<Script code=" orignalStart:=start; orignalGoal:=goal; "/>-->
      <!--<SetBlackboard value="{start}" output_key="orignalStart" />-->
      <!--<SetBlackboard value="{goal}" output_key="orignalGoal" />-->
      <!--<SubTree ID="NavigateToPoseTree" goal="{orignalGoal}" node="{node}" bt_loop_duration="{bt_loop_duration}" server_timeout="{server_timeout}"/> -->
      <!-- <SubTree ID="NavigateToPoseTree" goal="{orignalStart}" node="{node}" bt_loop_duration="{bt_loop_duration}" server_timeout="{server_timeout}"/> -->
      <SubTree ID="NavigateToPoseTree" __shared_blackboard="true"/>
      <SetBlackboard value="{goal}" output_key="orignalGoal" />   <!-- when commented out - there is no error -->
      <!--<SetBlackboard value="{orignalGoal}" output_key="start" /> -->
      <!--<SetBlackboard value="{orignalStart}" output_key="goal" /> -->
      <SubTree ID="NavigateToPoseTree" __shared_blackboard="true" /> 

    </Sequence>
  </BehaviorTree>

[SteveMacenski]

Asking RSE, this is not the right place to obtain general behavior tree Q&A.

[lmendyk]

I would consider it it is a bug and probably tricky one BT vs Nav2 wrapper, not just generic question on how BT behaves as I don't see the reason it should not work correctly.

The bug is
<SetBlackboard value="{goal}" output_key="orignalGoal" />
yields

Behavior tree threw exception: missing port [value]. Exiting with failure

the resto of the description provided above is just the "context"

[SteveMacenski]

That is not a question for Nav2, but BT.CPP. This is not the right place for generic help - please read BT.CPP documentation and if you still have any questions, ask on RSE.

[lmendyk]

I think it is a bug between Nav2 and BT.CPP (BT.CPP may not know anything how {goal} is defined by Nav2.) My suspicion is that {goal} may not serialize to String).

Not quite sure what you mean asking on RSE - as I'm reporting a bug not just asking a question.

[facontidavide]

FYI, I just wrote a unit test and both version 3.8 and 4.4 pass without any issue