Function BuildCheckedDownloadTasks

engine/Poseidon/UI/OptionsUIApp.cpp:1381–1409

Download tasks for every ticked-but-not-downloaded row that carries a URL. Each task downloads the zstd-wrapped mod artifact next to its install dir then unpacks it into / (so GetModInstallStatus sees it installed). Returns true if any.

// libcurl download bridged to the agnostic DownloadFileFn. The C progress
// callback can't carry captures, so onBytes is reached through a void* context.
static DownloadFileFn MakeMasterServerTransport(std::string proxy)
{
    return [proxy](const DownloadTask& task, const std::function<void(int64_t, int64_t)>& onBytes,
                   const std::function<bool()>& cancelled, std::string& error) -> bool
    {
        struct Ctx
        {
            const std::function<void(int64_t, int64_t)>* cb;
            const std::function<bool()>* cancelled;
        } ctx{&onBytes, &cancelled};
        const bool ok = DownloadMasterServerServiceFile(
            task.url.c_str(), proxy.empty() ? nullptr : proxy.c_str(), task.destPath.c_str(), &ctx,
            [](void* instance, int64_t received, int64_t total)
            {
                auto* c = static_cast<Ctx*>(instance);
                if (c != nullptr && c->cb != nullptr && *c->cb)
                    (*c->cb)(received, total);
            },
            [](void* instance) -> bool
            {
                // Abort the libcurl transfer promptly when the worker is cancelled.
                auto* c = static_cast<Ctx*>(instance);
                return c != nullptr && c->cancelled != nullptr && *c->cancelled && (*c->cancelled)();
            });
        if (!ok)
            error = "download failed";
        return ok;
    };
}

// Bare mod id (no leading '@') — ModInstallDir/GetModInstallStatus prepend it.