Fix #2467

httplib.h

@@ -1643,6 +1643,8 @@ public:
   using Expect100ContinueHandler =
       std::function<int(const Request &, Response &)>;
 
+  using StartHandler = std::function<void()>;
+
   using WebSocketHandler =
       std::function<void(const Request &, ws::WebSocket &)>;
   using SubProtocolSelector =
@@ -1694,6 +1696,9 @@ public:
   Server &set_pre_request_handler(HandlerWithResponse handler);
 
   Server &set_expect_100_continue_handler(Expect100ContinueHandler handler);
+
+  Server &set_start_handler(StartHandler handler);
+
   Server &set_logger(Logger logger);
   Server &set_pre_compression_logger(Logger logger);
   Server &set_error_logger(ErrorLogger error_logger);
@@ -1883,6 +1888,7 @@ private:
   Handler post_routing_handler_;
   HandlerWithResponse pre_request_handler_;
   Expect100ContinueHandler expect_100_continue_handler_;
+  StartHandler start_handler_;
 
   mutable std::mutex logger_mutex_;
   Logger logger_;
@@ -11100,6 +11106,11 @@ Server::set_expect_100_continue_handler(Expect100ContinueHandler handler) {
   return *this;
 }
 
+inline Server &Server::set_start_handler(StartHandler handler) {
+  start_handler_ = std::move(handler);
+  return *this;
+}
+
 inline Server &Server::set_address_family(int family) {
   address_family_ = family;
   return *this;
@@ -11795,6 +11806,8 @@ inline bool Server::listen_internal() {
   is_running_ = true;
   auto se = detail::scope_exit([&]() { is_running_ = false; });
 
+  if (start_handler_) { start_handler_(); }
+
   {
     std::unique_ptr<TaskQueue> task_queue(new_task_queue());
 

test/test.cc

@@ -810,6 +810,41 @@ TEST(ParseAcceptHeaderTest, ContentTypesPopulatedAndInvalidHeaderHandling) {
   }
 }
 
+TEST(ServerStartHandlerTest, CalledOnceWhenReady) {
+  Server svr;
+  svr.Get("/", [](const Request & /*req*/, Response &res) {
+    res.set_content("ok", "text/plain");
+  });
+
+  std::atomic<int> start_count{0};
+  std::atomic<bool> running_when_called{false};
+  svr.set_start_handler([&]() {
+    running_when_called = svr.is_running();
+    start_count++;
+  });
+
+  auto port = svr.bind_to_any_port(HOST);
+  std::thread t([&]() { svr.listen_after_bind(); });
+  auto se = detail::scope_exit([&] {
+    svr.stop();
+    t.join();
+    ASSERT_FALSE(svr.is_running());
+  });
+
+  svr.wait_until_ready();
+
+  // A successful request proves the accept loop is running, which the start
+  // handler precedes; so by now the handler must have run exactly once.
+  Client cli(HOST, port);
+  cli.set_connection_timeout(std::chrono::seconds(5));
+  auto res = cli.Get("/");
+  ASSERT_TRUE(res);
+  EXPECT_EQ(StatusCode::OK_200, res->status);
+
+  EXPECT_EQ(1, start_count.load());
+  EXPECT_TRUE(running_when_called.load());
+}
+
 TEST(DivideTest, DivideStringTests) {
   auto divide = [](const std::string &str, char d) {
     std::string lhs;