GH#294: Fix a timing issue with the new garbage collector in Lua 5.5.

CHANGES.rst

@@ -1,6 +1,15 @@
 Lupa change log
 ===============
 
+2.9 (2026-??-??)
+----------------
+
+* GH#294: In Lua 5.5, passing Python objects temporarily into Lua could make the wrong
+  object show up inside of Lua due to a timing issue between marking the object as unused
+  and the following garbage collection run that frees it.  This issue surfaced due to
+  changes in the garbage collector of Lua 5.5.
+
+
 2.8 (2026-04-15)
 ----------------
 

lupa/_lupa.pyx

@@ -1697,9 +1697,11 @@ cdef bint py_to_lua_custom(LuaRuntime runtime, lua_State *L, object o, int type_
             pyref = <_PyReference>runtime._pyrefs_in_lua[refkey]
             lua.lua_rawgeti(L, -1, pyref._ref)              # tbl udata
             py_obj = <py_object*>lua.lua_touserdata(L, -1)
-            if py_obj:
+            if py_obj and py_obj.obj is <PyObject*> o:
                 lua.lua_remove(L, -2)                       # udata
                 return 1  # values pushed
+
+            # Object reference is no longer up to date. Create a new one.
             lua.lua_pop(L, 1)                               # tbl
 
         # create new wrapper for Python object
@@ -2078,6 +2080,10 @@ cdef int py_object_gc_with_gil(py_object *py_obj, lua_State* L) noexcept with gi
         try: runtime.store_raised_exception(L, b'error while cleaning up a Python object')
         finally: return -1
     else:
+        if py_obj.obj is not <PyObject*> pyref._obj:
+            # Object reference has been reused already. Nothing to clean up.
+            return 0
+
         lua.lua_getfield(L, lua.LUA_REGISTRYINDEX, PYREFST)  # tbl
         lua.luaL_unref(L, -1, pyref._ref)                    # tbl
         lua.lua_pop(L, 1)

lupa/tests/test.py

@@ -3111,6 +3111,55 @@ def test_functions(self):
         self.testmissingref({}, self.lupa.as_attrgetter) # attribute getter protocol
 
 
+class TestSafeObjectRoundtrip(SetupLuaRuntimeMixin, LupaTestCase):
+    lua_runtime_kwargs = {'max_memory': 0}  # Track memory allocations.
+
+    def setUp(self):
+        try:
+            super().setUp()
+        except self.lupa.LuaError:
+            # LuaJIT 2.0 doesn't support 'max_memory' option.
+            self.lua_runtime_kwargs = {}
+            super().setUp()
+
+    def test_python_object_identity_roundtrip_with_temporary_table_argument(self):
+        # Repeated object passing into Lua must not overwrite dead but uncollected objects.
+        # Make sure we don't leak memory when we handle that case.
+        # Issue reported in https://github.com/scoder/lupa/issues/294
+        lua = self.lua
+        if 'luajit' not in lua.lua_implementation.lower():
+            self.assertTrue(self.lua_runtime_kwargs, "Standard Lua should support 'max_memory' option.")
+
+        echo = lua.eval('function(entity, event) return entity end')
+        mktable = lua.table
+        objects = [object(), object(), object()]
+
+        def pass_objects_through_lua(chunk_size=100, obj_count=len(objects)):
+            for index in range(chunk_size):
+                expected = objects[index % obj_count]
+                actual = echo(expected, mktable(name="update"))
+                self.assertIs(actual, expected, (index, id(expected), id(actual)))
+
+        lua.gccollect()
+        peak_memory_initial = lua.get_memory_used()
+
+        pass_objects_through_lua()
+
+        lua.gccollect()
+        if self.lua_runtime_kwargs:
+            peak_memory_initial = max(peak_memory_initial, lua.get_memory_used())
+
+        peak_memory = peak_memory_initial
+        for _ in range(200):
+            pass_objects_through_lua()
+            lua.gccollect()
+            if self.lua_runtime_kwargs:
+                peak_memory = max(peak_memory, lua.get_memory_used())
+
+        if self.lua_runtime_kwargs:
+            self.assertLessEqual(peak_memory, int(peak_memory_initial * 1.05))  # at most 5% memory growth
+
+
 ################################################################################
 # test Lua object __str__ method