解决老中医提出的不能 syncAwait 然后捕获异常的问题。测试用例见 test7

简单的来说，就是

```c++
try
{
    start_coro(some_coro_that_throw());
}
catch(...)
{
    这里抓不到异常
}
```

原因是 start_coro 实际上同 asio 的 co_spawn 一样，分叉了。
这样就没办法抓异常了，除非修改some_coro_that_throw()本身的代码。

如果是
```c++
co_await start_coro(some_coro_that_throw());
```

固然这样写，就抓的到异常了。但是在非协程上下文里，就无法使用了。

因此需要一种在非协程上下文环境里，同步等待协程的方式，并且能抓到异常。

所以这个补丁就是新增了一个 sync_await() 函数。具体使用就是

```c++
void 常规函数()
{
    try
    {
        sync_await(some_coro_that_throw());
    }
    catch(...)
    {
        这里能抓到异常
    }
}
```

include/ucoro/awaitable.hpp

@@ -168,6 +168,22 @@ namespace ucoro
 		template<concepts::awaitable_type AwaitableType>
 		using awaitable_return_type = template_parameter_of<AwaitableType, awaitable>;
 
+		template<typename T>
+		struct exception_with_result
+		{
+			using type = std::variant<std::exception_ptr, T>;
+		};
+
+		template<>
+		struct exception_with_result<void>
+		{
+			using type = std::exception_ptr;
+		};
+
+		template<typename T>
+		using exception_with_result_t = typename exception_with_result<T>::type;
+
+
 	} // namespace traits
 
 	struct debug_coro_promise
@@ -239,7 +255,7 @@ namespace ucoro
 			exception_ = std::current_exception();
 		}
 
-		void get_value()
+		void get_value() const
 		{
 			if (exception_)
 			{
@@ -431,13 +447,7 @@ namespace ucoro
 			current_coro_handle_.promise().set_local(std::move(local));
 		}
 
-		auto detach()
-		{
-			auto launch_coro = [](awaitable<T> lazy) -> awaitable<T> { co_return co_await std::move(lazy); };
-			return launch_coro(std::move(*this));
-		}
-
-		auto detach(std::any local)
+		auto detach(std::any local = {})
 		{
 			auto launched_coro = [](awaitable<T> lazy) mutable -> awaitable<T>
 			{
@@ -452,6 +462,44 @@ namespace ucoro
 			return launched_coro;
 		}
 
+		template<typename Function> requires std::is_invocable_v<Function, ucoro::traits::exception_with_result_t<T>>
+		auto detach_with_callback(Function completion_handler)
+		{
+			return detach_with_callback<Function>(std::any{}, completion_handler);
+		}
+
+		template<typename Function> requires std::is_invocable_v<Function, ucoro::traits::exception_with_result_t<T>>
+		auto detach_with_callback(std::any local, Function completion_handler)
+		{
+			auto launched_coro = [](awaitable<T> lazy, auto completion_handler) mutable -> awaitable<void>
+			{
+				using result_wrapper = ucoro::traits::exception_with_result_t<T>;
+				try
+				{
+					if constexpr (std::is_void_v<T>)
+					{
+						co_await std::move(lazy);
+						completion_handler(result_wrapper{nullptr});
+					}
+					else
+					{
+						completion_handler(result_wrapper{co_await std::move(lazy)});
+					}
+				}
+				catch(...)
+				{
+					completion_handler(result_wrapper{std::current_exception()});
+				}
+			}(std::move(*this), std::move(completion_handler));
+
+			if (local.has_value())
+			{
+				launched_coro.set_local(local);
+			}
+
+			return launched_coro;
+		}
+
 		std::coroutine_handle<promise_type> current_coro_handle_;
 	};
 
@@ -463,6 +511,7 @@ namespace ucoro
 		auto result = awaitable<T>{std::coroutine_handle<awaitable_promise<T>>::from_promise(*this)};
 		return result;
 	}
+
 } // namespace ucoro
 
 //////////////////////////////////////////////////////////////////////////
@@ -598,9 +647,9 @@ namespace ucoro
 //////////////////////////////////////////////////////////////////////////
 
 template<typename T, typename callback>
-ucoro::CallbackAwaiter<T, callback> callback_awaitable(callback&& cb)
+auto callback_awaitable(callback&& cb) -> ucoro::awaitable<T>
 {
-	return ucoro::CallbackAwaiter<T, callback>{std::forward<callback>(cb)};
+	co_return co_await ucoro::CallbackAwaiter<T, callback>{std::forward<callback>(cb)};
 }
 
 template<typename Awaitable, typename Local>
@@ -614,3 +663,30 @@ auto coro_start(Awaitable&& coro)
 {
 	return coro.detach();
 }
+
+template<typename T>
+auto sync_await(ucoro::awaitable<T> lazy, std::any local_ = {}) -> T
+{
+	ucoro::traits::exception_with_result_t<T> result;
+
+	lazy.detach_with_callback(local_, [&](ucoro::traits::exception_with_result_t<T> result_)
+	{
+		result = result_;
+	});
+
+	if constexpr (std::is_void_v<T>)
+	{
+		if (result)
+		{
+			std::rethrow_exception(result);
+		}
+	}
+	else
+	{
+		if (std::holds_alternative<std::exception_ptr>(result))
+		{
+			std::rethrow_exception(std::get<std::exception_ptr>(result));
+		}
+		return std::get<T>(result);
+	}
+}

tests/CMakeLists.txt

@@ -3,6 +3,7 @@ set_target_properties(ucoro_tests PROPERTIES FOLDER "ucoro_tests")
 
 add_subdirectory(test1)
 add_subdirectory(test2)
+add_subdirectory(test7)
 
 find_package(Boost 1.60 COMPONENTS thread system atomic)
 if(Boost_FOUND)

tests/test7/CMakeLists.txt

@@ -0,0 +1,6 @@
+
+add_executable(test7 test.cpp)
+target_link_libraries(test7 ucoro)
+
+add_test(NAME test7 COMMAND test7)
+set_target_properties(test7 PROPERTIES FOLDER "ucoro_tests")

tests/test7/test.cpp

@@ -0,0 +1,45 @@
+
+#include "ucoro/awaitable.hpp"
+#include <iostream>
+
+ucoro::awaitable<int> test()
+{
+	throw std::runtime_error("test throw");
+    co_return 1;
+}
+
+
+ucoro::awaitable<void> test2()
+{
+	throw std::runtime_error("test throw");
+    co_return;
+}
+
+ucoro::awaitable<int> coro_compute()
+{
+	try
+	{
+		sync_await(test2());
+	}
+	catch(const std::exception& e)
+	{
+		std::cerr << "exception in test2: " << e.what() << '\n';
+	}
+
+	co_return co_await test();
+}
+
+int main(int argc, char** argv)
+{
+	try
+	{
+		std::string str = "hello";
+		sync_await(coro_compute(), str);
+	}
+	catch (std::exception& e)
+	{
+		std::cerr << "exception: " << e.what() << std::endl;
+	}
+
+	return 0;
+}