Atm, to get a TpContact we have those functions:

tp_connection_get_contacts_by_handle():
 - I think CONTACT TpHandle must totally disappear from our API, and this function should be killed. I think in general high-level APIs should expose TpContact objects. At least spec should always give handle+id and so the recommended way to get a TpContact is tp_client_factory_ensure_contact() + eventually upgrade. Note that 'next' already make immortal handles mandatory.

tp_connection_get_contacts_by_id():
 - It should be proper _async().
 - Should use GetContactAttributesByID (bug #30874) for massive simplification and single round-trip.

tp_connection_upgrade_contacts():
 - It should be proper _async().

tp_connection_dup_contact_if_possible():
 - Since we are making immortal handles mandatory, it is always possible to dup a contact with handle+id. In that case just use tp_client_factory_ensure_contact()
 - As said above, if you don't have id+handle, you should just never get a TpContact.
 - IIRC there are legitimate uses internally, when the spec gives only an handle but we are supposed to already have the TpContact (like removed members in group). So I suggest keeping some form of this but internal only.

One thing I would like is to ensure that at any moment, even internally, a TpContact has both id+handle. Making those 2 construct-only mandatory properties.