test

        if (acquired) g_activeWsTunnels.fetch_sub(1, std::memory_order_relaxed);

    }

    ScopedTunnelSlot(const ScopedTunnelSlot &) = delete;

    ScopedTunnelSlot &operator=(const ScopedTunnelSlot &) = delete;

    // Move-only: a user-declared destructor + deleted copy ctor suppress the

    // implicit move ctor, which std::thread's constructor needs to relocate

    // this into its internal storage (see relayLoop's thread launch below).

    ScopedTunnelSlot(ScopedTunnelSlot &&other) noexcept : acquired(other.acquired) {

        other.acquired = false;

    }

};

// Opens a raw (not HttpClient) connection to the upstream, blocking until