Validate state in template helper preview (#99455)

* Validate state in template helper preview

* Deduplicate state validation

homeassistant/components/template/template_entity.py

@@ -25,6 +25,7 @@ from homeassistant.core import (
     HomeAssistant,
     State,
     callback,
+    validate_state,
 )
 from homeassistant.exceptions import TemplateError
 import homeassistant.helpers.config_validation as cv
@@ -413,8 +414,8 @@ class TemplateEntity(Entity):
             return
 
         for update in updates:
-            for attr in self._template_attrs[update.template]:
-                attr.handle_result(
+            for template_attr in self._template_attrs[update.template]:
+                template_attr.handle_result(
                     event, update.template, update.last_result, update.result
                 )
 
@@ -422,7 +423,13 @@ class TemplateEntity(Entity):
             self.async_write_ha_state()
             return
 
-        self._preview_callback(*self._async_generate_attributes(), None)
+        try:
+            state, attrs = self._async_generate_attributes()
+            validate_state(state)
+        except Exception as err:  # pylint: disable=broad-exception-caught
+            self._preview_callback(None, None, str(err))
+        else:
+            self._preview_callback(state, attrs, None)
 
     @callback
     def _async_template_startup(self, *_: Any) -> None:

homeassistant/core.py

@@ -174,6 +174,16 @@ def valid_entity_id(entity_id: str) -> bool:
     return VALID_ENTITY_ID.match(entity_id) is not None
 
 
+def validate_state(state: str) -> str:
+    """Validate a state, raise if it not valid."""
+    if len(state) > MAX_LENGTH_STATE_STATE:
+        raise InvalidStateError(
+            f"Invalid state with length {len(state)}. "
+            "State max length is 255 characters."
+        )
+    return state
+
+
 def callback(func: _CallableT) -> _CallableT:
     """Annotation to mark method as safe to call from within the event loop."""
     setattr(func, "_hass_callback", True)
@@ -1255,11 +1265,7 @@ class State:
                 "Format should be <domain>.<object_id>"
             )
 
-        if len(state) > MAX_LENGTH_STATE_STATE:
-            raise InvalidStateError(
-                f"Invalid state encountered for entity ID: {entity_id}. "
-                "State max length is 255 characters."
-            )
+        validate_state(state)
 
         self.entity_id = entity_id
         self.state = state

tests/test_core.py

@@ -2465,3 +2465,10 @@ async def test_cancellable_hassjob(hass: HomeAssistant) -> None:
 
     # Cleanup
     timer2.cancel()
+
+
+async def test_validate_state(hass: HomeAssistant) -> None:
+    """Test validate_state."""
+    assert ha.validate_state("test") == "test"
+    with pytest.raises(InvalidStateError):
+        ha.validate_state("t" * 256)