When I was reading the gpgme documentation, I found that the type of the “next“ member in the detailed introduction to tofu_info_t was wrong.
The type of "next" must be gpgme_tofu_info_t not the gpgme_key_sig_t.
I downloaded the latest document and the problem still exists.
location: page 50 of gpgme.pdf(Edition 1.16.0)