Modify PUC Lua to wrap C++ exceptions (#12445)

src/script/cpp_api/s_base.cpp

@@ -109,12 +109,14 @@ ScriptApiBase::ScriptApiBase(ScriptingType type):
 	lua_rawseti(m_luastack, LUA_REGISTRYINDEX, CUSTOM_RIDX_BACKTRACE);
 	lua_pop(m_luastack, 1); // pop debug
 
-	// If we are using LuaJIT add a C++ wrapper function to catch
-	// exceptions thrown in Lua -> C++ calls
+	// Add a C++ wrapper function to catch exceptions thrown in Lua -> C++ calls
 #if USE_LUAJIT
 	lua_pushlightuserdata(m_luastack, (void*) script_exception_wrapper);
 	luaJIT_setmode(m_luastack, -1, LUAJIT_MODE_WRAPCFUNC | LUAJIT_MODE_ON);
 	lua_pop(m_luastack, 1);
+#else
+	// (This is a custom API from the bundled Lua.)
+	lua_atccall(m_luastack, script_exception_wrapper);
 #endif
 
 	// Add basic globals

src/unittest/test_lua.cpp

@@ -19,12 +19,27 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 */
 
 #include "test.h"
+#include "config.h"
+
+#include <stdexcept>
 
 extern "C" {
-#include <lua.h>
+#if USE_LUAJIT
+	#include <luajit.h>
+#else
+	#include <lua.h>
+#endif
 #include <lauxlib.h>
 }
 
+/*
+ * This class tests for two common issues that prevent correct error handling
+ * between Lua and C++.
+ * Further reading:
+ * - https://luajit.org/extensions.html#exceptions
+ * - http://lua-users.org/wiki/ErrorHandlingBetweenLuaAndCplusplus
+ */
+
 class TestLua : public TestBase
 {
 public:
@@ -34,6 +49,7 @@ public:
 	void runTests(IGameDef *gamedef);
 
 	void testLuaDestructors();
+	void testCxxExceptions();
 };
 
 static TestLua g_test_instance;
@@ -41,10 +57,16 @@ static TestLua g_test_instance;
 void TestLua::runTests(IGameDef *gamedef)
 {
 	TEST(testLuaDestructors);
+	TEST(testCxxExceptions);
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 
+/*
+	Check that Lua unwinds the stack correctly when it throws errors internally.
+	(This is not the case with PUC Lua unless it was compiled as C++.)
+*/
+
 namespace
 {
 
@@ -77,3 +99,57 @@ void TestLua::testLuaDestructors()
 
 	UASSERT(did_destruct);
 }
+
+namespace {
+
+	int wrapper(lua_State *L, lua_CFunction inner)
+	{
+		try {
+			return inner(L);
+		} catch (std::exception &e) {
+			lua_pushstring(L, e.what());
+			return lua_error(L);
+		}
+	}
+
+}
+
+/*
+	Check that C++ exceptions are caught and re-thrown as Lua errors.
+	This is handled by a wrapper we define ourselves.
+	(PUC Lua does not support use of such a wrapper, we have a patched version)
+*/
+
+void TestLua::testCxxExceptions()
+{
+	lua_State *L = luaL_newstate();
+
+#if USE_LUAJIT
+	lua_pushlightuserdata(L, reinterpret_cast<void*>(wrapper));
+	luaJIT_setmode(L, -1, LUAJIT_MODE_WRAPCFUNC | LUAJIT_MODE_ON);
+	lua_pop(L, 1);
+#else
+	lua_atccall(L, wrapper);
+#endif
+
+	lua_pushcfunction(L, [](lua_State *L) -> int {
+		throw std::runtime_error("example");
+	});
+
+	int caught = 0;
+	std::string errmsg;
+	try {
+		if (lua_pcall(L, 0, 0, 0) != 0) {
+			caught = 2;
+			errmsg = lua_isstring(L, -1) ? lua_tostring(L, -1) : "";
+		}
+	} catch (std::exception &e) {
+		caught = 1;
+	}
+
+	if (caught != 1)
+		lua_close(L);
+
+	UASSERTEQ(int, caught, 2);
+	UASSERT(errmsg.find("example") != std::string::npos);
+}