refactor to support init options and context

[wjwwood]

Connects to ros2/rmw#154

@serge-nikulin FYI this is an example of what the disruption to an existing rmw implementation will look like for the init options and context changes.

[serge-nikulin]

Ack.

I will also join the chorus "do we really need rmw_init_options_t ?"

Please give me an example when a context is not enough.

[wjwwood]

rmw_init_options_t is given as input to init, and context is an output, so if you want to influence the initialization then you need the init options.

[serge-nikulin]

OK, I like it.

[jacobperron]

LGTM, just some questions about pushing common implementation to shared functions.

[sloretz]

It looks like rmw_init() sets context->impl to nullptr, why is this expecting it to be non-null?

[sloretz]

same comment here, context->impl is set to nullptr in rmw_init()

[serge-nikulin]

Connects to ros2/rmw#154

FYI this is an example of what the disruption to an existing rmw implementation will look like for the init options and context changes.

@wjwwood, do you have an example of a user-side customization?

[wjwwood]

@serge-nikulin no, but I think it would look something like this:

#include <rclcpp/rclcpp.hpp>
// note this next include breaks portability of ROS code
#include <rmw_connext_cpp/add_options_to_context.hpp>

auto context = std::make_shared<rclcpp::Context>()
context->init(...);

rmw_connext_cpp::add_option_to_context(context, "do_a_thing", true);

auto node = std::make_shared<rclcpp::Node>("my_node", "", context);

And the implementation of the add_option_to_context() function could looks something like this:

template<typename ContextT>
void
rmw_connext_cpp::add_option_to_context(
  std::shared_ptr<ContextT> context,
  const std::string & option_name,
  bool option_value)
{
  // see: http://docs.ros2.org/crystal/api/rclcpp/classrclcpp_1_1Context.html#a28596bd16aad20e057160f8a66d4697f
  auto rcl_context_shared_ptr = context->get_rcl_context();
  // some how assert that rcl_context_shared_ptr is not nullptr

  rcl_context_t * rcl_context = rcl_context_shared_ptr.get();
  // this function doesn't exist yet :'(
  rmw_context_t * rmw_context = rcl_context_get_rmw_context(rcl_context);
  // some how assert that rmw_context is not NULL

  // now you can cast the `void *` impl pointer to the connext cpp specific version
  // and then mutate it how ever you like
  rmw_context_impl_t * rmw_context_impl = (rmw_context_impl_t *)rmw_context->impl;
  // some how insert the option into the connext impl for later use
}

And so currently you cannot 🙃. I'll add the rcl_context_get_rmw_context function to the work related to passing the context to the wait set. Sorry about that.

[serge-nikulin]

@wjwwood,

And so currently you cannot

Do you have an issue to address it?

[wjwwood]

@serge-nikulin see: ros2/rcl#372