Function compute_disk_cache_salt

server/src/server/http_server.cpp:825–860

─── Disk-cache identity salt ─────────────────────────────────────────── Compute a 16-byte salt from inputs that affect KV cache validity: model path + stat(size + mtime) [covers rope/yarn — GGUF-derived], max_ctx, and sha1(chat_template_src). Returns all-zeroes if model_path is empty (back-compat / disk disabled).

        for (size_t i = 0; i < system_parts.size(); i++) {
            if (i) merged_system += "\n\n";
            merged_system += system_parts[i];
        }
        chat_msgs.insert(chat_msgs.begin(), {"system", merged_system});
    }

    return chat_msgs;
}

// ─── Disk-cache identity salt ───────────────────────────────────────────
// Compute a 16-byte salt from inputs that affect KV cache validity:
//   model path + stat(size + mtime)  [covers rope/yarn — GGUF-derived],
//   max_ctx, and sha1(chat_template_src).
// Returns all-zeroes if model_path is empty (back-compat / disk disabled).
static std::array<uint8_t, 16> compute_disk_cache_salt(const ServerConfig & cfg) {
    std::array<uint8_t, 16> salt{};
    if (cfg.model_path.empty()) return salt;

    const std::string & path = cfg.model_path;
    struct stat st{};
    int64_t file_size  = 0;
    int64_t file_mtime = 0;
    if (::stat(path.c_str(), &st) == 0) {
        file_size  = (int64_t)st.st_size;
        file_mtime = (int64_t)st.st_mtime;
    } else {
        std::fprintf(stderr, "[disk-cache] salt: stat(%s) failed — path-only fingerprint\n",
                     path.c_str());
    }

    // Hash chat_template_src separately (can be large; fold as digest).
    uint8_t tmpl_digest[20] = {};
    sha1_hash(cfg.chat_template_src.data(), cfg.chat_template_src.size(), tmpl_digest);

    // Serialization: path_len(4) + path + file_size(8) + file_mtime(8) + max_ctx(4) + tmpl_digest(20).
    std::vector<uint8_t> buf;
    uint32_t plen = (uint32_t)path.size();
    buf.insert(buf.end(), (uint8_t *)&plen,        (uint8_t *)&plen        + 4);
    buf.insert(buf.end(), (uint8_t *)path.data(),  (uint8_t *)path.data()  + path.size());
    buf.insert(buf.end(), (uint8_t *)&file_size,   (uint8_t *)&file_size   + 8);