What does "ACCEPTED" and "CANCELING" states do in the ros2 action state machine?

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With respect to the diagram:

• handle_goal is called when a goal is received (before ACCEPTED).

  If handle_goal returns "accept", then the goal state is set to ACCEPTED. The ACCEPTED state tells us (and the action client) that the goal is queued for execution. It's possible that the action server chooses to defer execution (e.g. queue the goal for later), hence we don't always jump straight to EXECUTING. The "ACCEPT_AND_EXECUTE" return code is a convenient shortcut if we want to skip straight to EXECUTING.

• handle_accepted is called after a goal enters the ACCEPTED state.

  This is a convenient way to let the code know that a particular goal handle could be executed. It is up to the action server to explicitly transition the goal to the EXECUTING state.

CANCELING and CANCELED work in a similar way:

• handle_cancel is called when a cancel request is received (before CANCELING).

  If the cancel request is accepted, then the goal enters in to the CANCELING state. Note, however, this does not mean the goal has been completely canceled yet. There may be a time delay between entering CANCELING and transitioning to CANCELED. It is up to the action server to explicitly signal that the goal is canceled.

• Calling canceled() on the goal handle is the signal to transition the state to CANCELED.

  I.e. canceled() indicates that the action server is finished canceling the goal.

Perhaps the demo code makes this more concrete:

if (goal_handle->is_canceling()) {
  result->sequence = sequence;
  goal_handle->canceled(result);
  RCLCPP_INFO(this->get_logger(), "Goal canceled");
  return;
}

Above, the goal execution thread checks if a cancel request has been accepted (goal_handle->is_canceling()), and if so it immediately transitions the goal to CANCELED (goal_handle->canceled()). In practice, it's possible the code might do something more complex before calling goal_handle->canceled() (e.g. wait for a robot should put down an object it is grasping).