Initializing a shared variable safely

Imagine that we are designing a safety-critical class that is used from multiple threads, receives answers from a server, postprocesses them, and outputs the response:

struct postprocessor {
    typedef std::vector<std::string> answer_t;

    // Concurrent calls on the same variable are safe.
    answer_t act(const std::string& in) const {
        if (in.empty()) {
            // Extremely rare condition.
            return read_defaults();
        }

        // ...
    }
};

Note the return read_defaults(); line. There may be situations when server does not respond because of networking issues or some other problems. In those cases, we attempt to read defaults from file:

// Executes for a long time.
std::vector<std::string> read_defaults();

From the preceding code, we hit the problem: the server may be unreachable for some noticeable time...