/**
 * runs a callback in a vforked child process that shares its address space with
 * the current process. the child behaves much like a thread as a result and the
 * callback must not make changes to process memory that we cannot undo from the
 * parent, otherwise we may leak memory or fully trash the parent address space.
 *
 * NOTE: see `asVFork` for safety information regarding credentials and signals.
 *
 * throws an exception if the child exec's or otherwise doesn't return a result.
 */
static auto inVFork(int flags, auto fn)
{
    auto [pid, result] = asVFork(flags, fn);

    if (result) {
        return std::move(result->value());
    } else {
        throw Error("vfork child unexpectedly did not produce a value");
    }
}

this is the entrypoint of the magic