feat: add callback for unhandled exception handler.

- add callback for unhandled exception handler to give programmer a chance to fetch log and coredump path, especially for GUI application because its stderr is invisible.
- fix fatal anto-recursive calling bug in unhandled exception handler.

src/ExceptionHelper.cpp

@@ -26,6 +26,7 @@ namespace YYCC::ExceptionHelper {
 		ExceptionRegister() :
 			m_CoreMutex(),
 			m_IsRegistered(false), m_IsProcessing(false), m_PrevProcHandler(nullptr),
+			m_UserCallback(nullptr),
 			m_SingletonMutex(NULL) {}
 		~ExceptionRegister() {
 			Unregister();
@@ -35,7 +36,7 @@ namespace YYCC::ExceptionHelper {
 		/**
 		 * @brief Try to register unhandled exception handler.
 		*/
-		void Register() {
+		void Register(ExceptionCallback callback) {
 			std::lock_guard<std::mutex> locker(m_CoreMutex);
 			// if we have registered, return
 			if (m_IsRegistered) return;
@@ -63,6 +64,8 @@ namespace YYCC::ExceptionHelper {
 			// okey, we can register it.
 			// backup old handler
 			m_PrevProcHandler = SetUnhandledExceptionFilter(UExceptionImpl);
+			// set user callback
+			m_UserCallback = callback;
 			// mark registered
 			m_IsRegistered = true;
 		}
@@ -115,6 +118,14 @@ namespace YYCC::ExceptionHelper {
 			std::lock_guard<std::mutex> locker(m_CoreMutex);
 			return m_PrevProcHandler;
 		}
+		/**
+		 * @brief Get user specified callback.
+		 * @return The function pointer to user callback. nullptr if no associated callback.
+		*/
+		ExceptionCallback GetUserCallback() const {
+			std::lock_guard<std::mutex> locker(m_CoreMutex);
+			return m_UserCallback;
+		}
 
 		/**
 		 * @brief Try to start process unhandled exception.
@@ -154,6 +165,12 @@ namespace YYCC::ExceptionHelper {
 		 * True if it is, otherwise false.
 		*/
 		bool m_IsProcessing;
+		/**
+		 * @brief User defined callback.
+		 * @details It will be called at the tail of unhandled exception handler, because it may raise exception.
+		 * We must make sure all log and coredump have been done before calling it.
+		*/
+		ExceptionCallback m_UserCallback;
 		/**
 		 * @brief The backup of old unhandled exception handler.
 		*/
@@ -166,6 +183,7 @@ namespace YYCC::ExceptionHelper {
 		HANDLE m_SingletonMutex;
 	};
 
+	/// @brief Core register singleton.
 	static ExceptionRegister g_ExceptionRegister;
 
 #pragma region Exception Handler Implementation
@@ -505,10 +523,14 @@ namespace YYCC::ExceptionHelper {
 			// write crash coredump
 			UExceptionCoreDump(coredump_path, info);
 
+			// call user callback
+			ExceptionCallback user_callback = g_ExceptionRegister.GetUserCallback();
+			if (user_callback != nullptr) 
+				user_callback(log_path, coredump_path);
 		}
 
 		// stop process
-		g_ExceptionRegister.StartProcessing();
+		g_ExceptionRegister.StopProcessing();
 
 	end_proc:
 		// if backup proc can be run, run it
@@ -523,8 +545,8 @@ namespace YYCC::ExceptionHelper {
 
 #pragma endregion
 
-	void Register() {
-		g_ExceptionRegister.Register();
+	void Register(ExceptionCallback callback) {
+		g_ExceptionRegister.Register(callback);
 	}
 
 	void Unregister() {

src/ExceptionHelper.hpp

@@ -11,6 +11,24 @@
 */
 namespace YYCC::ExceptionHelper {
 	
+	/**
+	 * @brief The callback function prototype which will be called when unhandled exception happened after registering.
+	 * @details
+	 * During registering unhandled exception handler,
+	 * caller can optionally provide a function pointer matching this prorotype to register.
+	 * Then it will be called if unhandled exception hanppened.
+	 * 
+	 * This callback will provide 2 readonly arguments.
+	 * First is the path to error log file.
+	 * Second is the path to core dump file.
+	 * These pathes may be empty if internal handler fail to create them.
+	 * 
+	 * This callback is convenient for programmer using an explicit way to tell user an exception happened.
+	 * Because in default, handler will only write error log to \c stderr and file.
+	 * It will be totally invisible on a GUI application.
+	*/
+	using ExceptionCallback = void(*)(const yycc_u8string& log_path, const yycc_u8string& coredump_path);
+
 	/**
 	 * @brief Register unhandled exception handler
 	 * @details
@@ -23,7 +41,7 @@ namespace YYCC::ExceptionHelper {
 	 * 
 	 * This function usually is called at the start of program.
 	*/
-	void Register();
+	void Register(ExceptionCallback callback = nullptr);
 	/**
 	 * @brief Unregister unhandled exception handler
 	 * @details 

testbench/main.cpp

@@ -338,7 +338,15 @@ namespace YYCCTestbench {
 	static void ExceptionTestbench() {
 #if YYCC_OS == YYCC_OS_WINDOWS
 
-		YYCC::ExceptionHelper::Register();
+		YYCC::ExceptionHelper::Register([](const YYCC::yycc_u8string& log_path, const YYCC::yycc_u8string& coredump_path) -> void {
+			MessageBoxW(
+				NULL,
+				YYCC::EncodingHelper::UTF8ToWchar(
+					YYCC::StringHelper::Printf(YYCC_U8("Log generated:\nLog path: %s\nCore dump path: %s"), log_path.c_str(), coredump_path.c_str())
+				).c_str(),
+				L"Fatal Error", MB_OK + MB_ICONERROR
+			);
+		});
 
 		// Perform a div zero exception.
 		int i = 1, j = 0;
@@ -389,7 +397,7 @@ namespace YYCCTestbench {
 
 		Assert(test_slashed_path == test_joined_path, YYCC_U8("YYCC::FsPathPatch"));
 
-	}
+}
 
 	enum class TestEnum : int8_t {
 		Test1, Test2, Test3