move HTTP server to separate thread

src/EventUtils.hh

@@ -4,5 +4,31 @@
 
 #include <functional>
 #include <memory>
+#include <mutex>
+#include <optional>
+#include <stdexcept>
 
 void forward_to_event_thread(std::shared_ptr<struct event_base> base, std::function<void()>&& fn);
+
+template <typename T>
+T call_on_event_thread(std::shared_ptr<struct event_base> base, std::function<T()>&& compute) {
+  std::optional<T> ret;
+  std::string exc_what;
+  std::mutex ret_lock;
+  std::condition_variable ret_cv;
+  std::unique_lock<std::mutex> g(ret_lock);
+  forward_to_event_thread(base, [&]() -> void {
+    std::lock_guard<std::mutex> g(ret_lock);
+    try {
+      ret = compute();
+    } catch (const std::exception& e) {
+      exc_what = e.what();
+    }
+    ret_cv.notify_one();
+  });
+  ret_cv.wait(g);
+  if (!ret.has_value()) {
+    throw std::runtime_error(exc_what);
+  }
+  return ret.value();
+}