copy backend vars from mixins (#3580)

* copy backend vars from mixins

* fix and improve backend var mixin test

reflex/state.py

@@ -200,6 +200,7 @@ def _no_chain_background_task(
 
 
 RESERVED_BACKEND_VAR_NAMES = {
+    "_abc_impl",
     "_backend_vars",
     "_was_touched",
 }
@@ -558,6 +559,14 @@ class BaseState(Base, ABC, extra=pydantic.Extra.allow):
                     cls.computed_vars[newcv._var_name] = newcv
                     cls.vars[newcv._var_name] = newcv
                     continue
+                if (
+                    types.is_backend_variable(name, cls)
+                    and name not in RESERVED_BACKEND_VAR_NAMES
+                    and name not in cls.inherited_backend_vars
+                    and not isinstance(value, FunctionType)
+                ):
+                    cls.backend_vars[name] = copy.deepcopy(value)
+                    continue
                 if events.get(name) is not None:
                     continue
                 if not cls._item_is_event_handler(name, value):

tests/test_state.py

@@ -2969,3 +2969,23 @@ config = rx.Config(
         state_manager = StateManager.create(state=State)
         assert state_manager.lock_expiration == expected_values[0]  # type: ignore
         assert state_manager.token_expiration == expected_values[1]  # type: ignore
+
+
+class MixinState(State, mixin=True):
+    """A mixin state for testing."""
+
+    num: int = 0
+    _backend: int = 0
+
+
+class UsesMixinState(MixinState, State):
+    """A state that uses the mixin state."""
+
+    pass
+
+
+def test_mixin_state() -> None:
+    """Test that a mixin state works correctly."""
+    assert "num" in UsesMixinState.base_vars
+    assert "num" in UsesMixinState.vars
+    assert UsesMixinState.backend_vars == {"_backend": 0}