Cleanup all the deprecations warning from the recent ros2_control variant changes

[saikishor]

Recently after the new variants feature in Handles (#1688) We have a bunch of deprecation warnings that needs to be cleanup. This could be a nice first issue.

[MartinPeris]

I'd like to take this one. Quick couple of questions. For example in this situation:

/root/ws_ros2_control/src/ros2_control/hardware_interface/include/hardware_interface/system_interface.hpp: In member function ‘double hardware_interface::SystemInterface::get_state(const std::string&) const’:
/root/ws_ros2_control/src/ros2_control/hardware_interface/include/hardware_interface/system_interface.hpp:410:56: warning: ‘double hardware_interface::Handle::get_value() const’ is deprecated: Use bool get_value(double & value) instead to retrieve the value. [-Wdeprecated-declarations]
  410 |     return system_states_.at(interface_name)->get_value();
      |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~

The original code is:

double get_state(const std::string & interface_name) const
  {
    return system_states_.at(interface_name)->get_value();
  }

Since the new API signature is bool get_value(double &value), how do you want me to handle the bool returned by get_value?

Ignoring it would not be an option for me, so this leaves 3 possibilities:

1. Throw an Exception
2. Return a Sentinel Value (e.g., NaN)
3. Log an Error and Return Default Value

Either 1 or 2 would be acceptable for me (leaning towards 2, but that is just personal preference). In any case, new unit tests will have to be added... I charge extra karma for that :P

Also, I see a lot of -Wunused-result:

In file included from /root/ws_ros2_control/src/ros2_control/hardware_interface/include/mock_components/generic_system.hpp:25,
                 from /root/ws_ros2_control/src/ros2_control/hardware_interface/src/mock_components/generic_system.cpp:17:
/root/ws_ros2_control/src/ros2_control/hardware_interface/include/hardware_interface/system_interface.hpp: In member function ‘void hardware_interface::SystemInterface::set_state(const std::string&, const double&)’:
/root/ws_ros2_control/src/ros2_control/hardware_interface/include/hardware_interface/system_interface.hpp:405:49: warning: ignoring return value of ‘bool hardware_interface::Handle::set_value(double)’, declared with attribute ‘nodiscard’ [-Wunused-result]
  405 |     system_states_.at(interface_name)->set_value(value);
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~

Should I deal with those too?

Is it OK to remove the deprecated API as I refactor? or should I leave them for backwards compatibility?

[MartinPeris]

mmmm looking at the documentation source code looks like a quiet NaN is the way to go:

[[deprecated("Use bool get_value(double & value) instead to retrieve the value.")]]
  double get_value() const
  {
    std::shared_lock<std::shared_mutex> lock(handle_mutex_, std::try_to_lock);
    if (!lock.owns_lock())
    {
      return std::numeric_limits<double>::quiet_NaN();
    }
    // BEGIN (Handle export change): for backward compatibility
    // TODO(Manuel) return value_ if old functionality is removed
    THROW_ON_NULLPTR(value_ptr_);
    return *value_ptr_;
    // END
  }