Add error_handler to asio_server (#1602)

* Add error_handler to asio_server

* Update glaze_asio.hpp

include/glaze/ext/glaze_asio.hpp

@@ -386,6 +386,11 @@ namespace glz
    {
       uint16_t port{};
       uint32_t concurrency{1}; // How many threads to use (a call to .run() is inclusive on the main thread)
+
+      // Register a callback that takes a string error message on server/registry errors.
+      // Note that we use a std::string to support a wide source of errors and use e.what()
+      // IMPORTANT: The code within the callback must be thread safe, as multiple threads could call this simultaneously.
+      std::function<void(const std::string&)> error_handler{};
 
       ~asio_server() { stop(); }
 
@@ -485,7 +490,12 @@ namespace glz
             }
          }
          catch (const std::exception& e) {
-            std::fprintf(stderr, "%s\n", e.what());
+            if (error_handler) {
+               error_handler(e.what());
+            }
+            else {
+               std::fprintf(stderr, "glz::asio_server error: %s\n", e.what());
+            }
          }
       }
 

tests/asio_repe/asio_repe.cpp

@@ -332,6 +332,37 @@ void async_server_test()
    expect(result == 200);
 }
 
+struct error_api
+{
+   std::function<int()> func = []() {
+      throw std::runtime_error("func error");
+      return 0;
+   };
+};
+
+void server_error_test()
+{
+   static constexpr int16_t port = 8765;
+
+   glz::asio_server<> server{.port = port, .concurrency = 1};
+   server.error_handler = [](const std::string& error) {
+      expect(error == "func error");
+   };
+
+   error_api api{};
+   server.on(api);
+
+   server.run_async();
+
+   glz::asio_client<> client{"localhost", std::to_string(port)};
+   (void)client.init();
+
+   int result{};
+   glz::repe::message msg{};
+   client.call({"/func"}, msg, 100);
+   expect(bool(glz::repe::decode_message(result, msg)));
+}
+
 int main()
 {
    notify_test();
@@ -340,6 +371,7 @@ int main()
    async_calls();
    raw_json_tests();
    async_server_test();
+   server_error_test();
 
    return 0;
 }