DDS15 — Add DataTagQosPolicy

The DDS Security specification (see 7.2.5) added a DataTagQosPolicy defined as:

@extensibility(APPENDABLE)
struct Tag {
    string name;
    string value;
};

typedef sequence<Tag> TagSeq;

@extensibility(APPENDABLE)
struct DataTags {
    TagSeq tags;
};

typedef DataTags DataTagQosPolicy

This is actually a generic policy that does not require DDS Security. It makes sense even without security as a way to associate tags with DataWriters and DataReaders.

Therefore it would be better to move that definition to the DDS specification. This is a user-visible API that is expected to be explicitly set. If it is not part of the DDS API than applications would not be directly portable between DDS and DDS security.

The spec should also define values for DATATAG_QOS_POLICY_NAME and DATATAG_QOS_POLICY_ID. For example:

 const string DATATAG_QOS_POLICY_NAME = "DataTag";
 const QosPolicyId_t  DATATAG_QOS_POLICY_ID = 25;

Note that currently DDS-XTYPES defines IDs 23 and 24. Probably XTYPES should have used numbers in a different range so the IDs in DDS could remain contiguous.

See also DDS15-209.