deleting parameters doesn't actually remove them

[dirk-thomas]

When deleting a parameter from a node in C++ it is not being removed from the storage. Instead a parameter with the value type "not set" is being kept.

For future list requests that is confusing since the parameter was explicitly deleted. It should be removed from the entirely.

The line in question currently unconditionally sets the parameter in the map:

rclcpp/rclcpp/src/rclcpp/node_interfaces/node_parameters.cpp

Line 107 in d298fa4

tmp_map[p.get_name()] = p;

In the case where the type is "not set" it should instead remove the entry.

[dirk-thomas]

The existing comment mentions that the parameter is intentionally kept in the map with a "not set" type:

rclcpp/rclcpp/src/rclcpp/node_interfaces/node_parameters.cpp

Lines 102 to 104 in d298fa4

	// it is not necessary to erase the parameter from parameters_
	// because the new value for this key (p.get_name()) will be a
	// Parameter with type "NOT_SET"

Imo that is not a good idea:

• First that makes it impossible to remove a once added parameter completely and the system can never get back into the previous state (before the parameter was added).
• Second keeping a not set parameter around is not useful since the semantic is intended to be the same as if the parameter doesn't exist at all.
• Third with the not set parameter being kept the caller has to actively filter not set parameter from a list response (which can't be done if only the name is transferred and the data has to be communicated unnecessarily).