You're right, it's unused.

My motivation for it was getting the whole certificate chain: the only place where the latter is available is as a callback parameter and I planned to get the certs from there.

Unfortunately this is not possible to pass a user pointer to this callback, effectively precluding the possibility to associate the cert chain with a data/conn structure.

There's a comment about it at

See also https://www.ibm.com/docs/api/v1/content/ssw_ibm_i_71/apis/gsk_attribute_set_callback.htm and search for pgsk_cert_validation_callback on the page.

I kept the function around in hope IBM would add such a parameter but they never did. 9 years later, I'm convinced they'll never do thus we'll never have the whole chain in CAINFO and I approve this removal.

Thanks for looking at it.

Thanks for clarifying!