deadlock in chained request

#include <Wt/WServer>
#include <Wt/WResource>
#include <Wt/Http/Client>
#include <Wt/Http/Response>

#include <memory>
#include <atomic>

#include <Wt/Http/Client>
#include <Wt/Http/Response>
#include <atomic>

// $ while [ "$(curl -s 'http://localhost/TestCase')" == "done" ]; do   echo -n '.';   sleep 1; done
class TestCase : public Wt::WResource {
public:
   TestCase (Wt::WServer& server) :server(server) { }
   ~TestCase () { beingDeleted(); }

   void handleRequest (const Wt::Http::Request& request, Wt::Http::Response& response) override
   {
      const auto& parameterMap = request.getParameterMap();
      auto send = parameterMap.find("secondCall") == parameterMap.end();

      if (send) {
         SimulationOfSomeCalculationInAnExternalSystemThatConnectsBack();
      }

      response.out().write("done", 4);
   }

private:
   Wt::WServer& server;

   void SimulationOfSomeCalculationInAnExternalSystemThatConnectsBack ()
   {
      std::atomic<bool> done = false;
      auto client = std::make_unique<Wt::Http::Client>();

      client->done().connect([&] (boost::system::error_code err, const Http::Message& /*response*/, ...) {
         if (err) server.log("error") << err.message();
         done = true;
      });

      //if (client->get("http://localhost/TestCase?secondCall=true")) {
      if (client->get("http://localhost/TestCase2?secondCall=true")) {
         while (!done) {
            boost::this_thread::sleep_for(boost::chrono::milliseconds{20});
         }
      }
      else {
         server.log("error") << "something is wrong with the url";
      }
   }
};



int main (int argc, char** argv)
{
   Wt::WServer wserver{argc, argv};

   TestCase testcase(wserver);
   wserver.addResource(&testcase, "/TestCase");

   TestCase testcase2(wserver);
   wserver.addResource(&testcase2, "/TestCase2");

   wserver.run();

   return 0;
}