Welcome Moritz Buhl as #curl commit author 1291: https://github.com/curl/curl/pull/14394
Use NGHTTP3 prefix instead of NGTCP2 for errors in h3 callbacks. by moritzbuhl · Pull Request #14394 · curl/curl
To my understanding the functions are nghttp3 callbacks and should therefore return NGHTTP3_ERR_CALLBACK_FAILURE. However, this is not critical as the nghttp3 documentation states for all callbacks...GitHub
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