Guard host userdata identity against stale-reference reuse

src/luapyre/runtime.py

@@ -43,6 +43,13 @@ def _to_lua(self, value):
             for key, item in value.items():
                 t.rawset(self._to_lua(key), self._to_lua(item))
             return t
+
+        # Opaque host userdata is represented by the Python object itself.
+        # Do not replace this with a recyclable integer/ref-slot registry unless
+        # every reuse and finalization path validates the referent identity (or
+        # a generation token).  A stale reusable wrapper can otherwise become
+        # rebound to a different live host object; see Lupa GH-294 for the class
+        # of bug this direct-reference invariant deliberately avoids.
         return value
 
     def expose(self, name: str, fn):

tests/test_host_userdata_identity.py

@@ -0,0 +1,83 @@
+from __future__ import annotations
+
+from luapyre import LuaRuntime
+
+
+def test_host_userdata_identity_roundtrip_survives_temporary_tables_and_gc():
+    """Regression guard for the failure mode reported in Lupa GH-294.
+
+    Opaque host objects must round-trip through Lua by identity even while
+    temporary Lua objects are being created and collection cycles run.  A host
+    reference must never be silently rebound to another live Python object.
+    """
+
+    lua = LuaRuntime()
+    entities = [object(), object(), object()]
+
+    def entity_for(index):
+        return entities[index % len(entities)]
+
+    def verify_entity(index, actual):
+        expected = entities[index % len(entities)]
+        assert actual is expected, (index, id(expected), id(actual))
+        return True
+
+    lua.expose("entity_for", entity_for)
+    lua.expose("verify_entity", verify_entity)
+
+    result = lua.execute(
+        '''
+local function echo(entity, event)
+  return entity
+end
+
+for i = 0, 19999 do
+  local expected = entity_for(i)
+  local actual = echo(expected, {name = "update"})
+  if not verify_entity(i, actual) then
+    return false
+  end
+  if i % 97 == 0 then
+    collectgarbage("collect")
+  end
+end
+
+return true
+'''
+    )
+
+    assert result is True
+
+
+def test_distinct_host_userdata_remain_distinct_as_table_keys_across_gc():
+    """Identity-keyed Lua table entries must not alias after collection."""
+
+    lua = LuaRuntime()
+    entities = [object(), object(), object()]
+
+    def entity_for(index):
+        return entities[index % len(entities)]
+
+    lua.expose("entity_for", entity_for)
+
+    result = lua.execute(
+        '''
+local keys = {entity_for(0), entity_for(1), entity_for(2)}
+local seen = {}
+for i = 1, 3 do
+  seen[keys[i]] = i
+end
+
+for n = 1, 250 do
+  local temporary = {n = n}
+  if n % 17 == 0 then
+    collectgarbage("collect")
+  end
+end
+
+return seen[keys[1]], seen[keys[2]], seen[keys[3]],
+       keys[1] ~= keys[2] and keys[2] ~= keys[3] and keys[1] ~= keys[3]
+'''
+    )
+
+    assert result == (1, 2, 3, True)