srv_kinematics_plugin generates deadlock when calling the external IK service

[marco-faroni]

Description

I'm using the srv_kinematics_plugin to call an external service server with a custom IK solver. When I call the compute_ik service, the move_group node correctly enters the callback, but it never exits. Apparently there is a deadlock caused by the compute_ik callback and the client (calling the external IK) being in the same callback_group. I've tried to assign the client callback to a Reentrant callback group and it works properly (see a fork here: https://github.com/marco-faroni/moveit2/tree/humble).

ROS Distro

Humble

OS and version

Ubuntu 22.04

Source or binary build?

Binary

If binary, which release version?

2.5.9-1jammy.20251119.014812

If source, which branch?

No response

Which RMW are you using?

None

Steps to Reproduce

1. Create a MoveIt kinematics plugin based on SrvKinematicsPlugin. You can find an example here: https://github.com/marco-faroni/ik_solver_moveit_adapter
2. Launch move_group.
3. Request IK via MoveIt. E.g.,

ros2 service call /compute_ik moveit_msgs/srv/GetPositionIK "ik_request:
  group_name: 'manipulator'
  robot_state:
    joint_state:
      header:
        stamp:
          sec: 0
          nanosec: 0
        frame_id: ''
      name: []
      position: []
      velocity: []
      effort: []
    multi_dof_joint_state:
      header:
        stamp:
          sec: 0
          nanosec: 0
        frame_id: ''
      joint_names: []
      transforms: []
      twist: []
      wrench: []
    attached_collision_objects: []
    is_diff: false
  constraints:
    name: ''
    joint_constraints: []
    position_constraints: []
    orientation_constraints: []
    visibility_constraints: []
  avoid_collisions: false
  ik_link_name: ''
  pose_stamped:
    header:
      stamp:
        sec: 0
        nanosec: 0
      frame_id: ''
    pose:
      position:
        x: 1.0
        y: 1.0
        z: 0.5
      orientation:
        x: 0.0
        y: 0.0
        z: 0.0
        w: 1.0
  ik_link_names: []
  pose_stamped_vector: []
  timeout:
    sec: 0
    nanosec: 0"

Expected behavior

The service should return a response containing the IK solution (or NO_IK_SOLUTION error code).

Actual behavior

The external IK server correctly enters the callback, but it never returns a response. Apparently there is a deadlock caused by the compute_ik callback and the client (calling the external IK) being in the same callback_group. I've tried to assign the client callback to a Reentrant callback group and it works properly (see a fork here: https://github.com/marco-faroni/moveit2/tree/humble)

Backtrace or Console output

No response