Adds guard for persisters returning bad state

burr/core/application.py

@@ -1404,6 +1404,14 @@ def _load_from_persister(self):
             self.state = self.state.update(**self.default_state)
             self.sequence_id = None  # has to start at None
         else:
+            if load_result["state"] is None:
+                raise ValueError(
+                    f"Error: {self.initializer.__class__.__name__} returned {load_result} for "
+                    f"partition_key:{self.partition_key}, app_id:{self.app_id}, "
+                    f"sequence_id:{self.sequence_id}, "
+                    f"but state was None! This is not allowed. Please return None in this case, or double "
+                    f"check that persisted state can never be a None value."
+                )
             # there was something
             last_position = load_result["position"]
             self.state = load_result["state"]

tests/core/test_application.py

@@ -1,7 +1,7 @@
 import asyncio
 import collections
 import logging
-from typing import Any, Awaitable, Callable, Dict, Generator, Tuple
+from typing import Any, Awaitable, Callable, Dict, Generator, Literal, Optional, Tuple
 
 import pytest
 
@@ -35,7 +35,7 @@
     _validate_start,
     _validate_transitions,
 )
-from burr.core.persistence import DevNullPersister
+from burr.core.persistence import BaseStatePersister, DevNullPersister, PersistedStateData
 from burr.lifecycle import (
     PostRunStepHook,
     PostRunStepHookAsync,
@@ -1670,6 +1670,57 @@ def test_application_builder_initialize_does_not_allow_state_setting():
         )
 
 
+class BrokenPersister(BaseStatePersister):
+    """Broken persistor."""
+
+    def load(
+        self, partition_key: str, app_id: Optional[str], sequence_id: Optional[int] = None, **kwargs
+    ) -> Optional[PersistedStateData]:
+        return dict(
+            partition_key="key",
+            app_id="id",
+            sequence_id=0,
+            position="foo",
+            state=None,
+            created_at="",
+            status="completed",
+        )
+
+    def list_app_ids(self, partition_key: str, **kwargs) -> list[str]:
+        return []
+
+    def save(
+        self,
+        partition_key: Optional[str],
+        app_id: str,
+        sequence_id: int,
+        position: str,
+        state: State,
+        status: Literal["completed", "failed"],
+        **kwargs,
+    ):
+        return
+
+
+def test_application_builder_initialize_raises_on_broken_persistor():
+    """Persisters should return None when there is no state to be loaded and the default used."""
+    with pytest.raises(ValueError, match="but state was None"):
+        counter_action = base_counter_action.with_name("counter")
+        result_action = Result("count").with_name("result")
+        (
+            ApplicationBuilder()
+            .with_actions(counter_action, result_action)
+            .with_transitions(("counter", "result", default))
+            .initialize_from(
+                BrokenPersister(),
+                resume_at_next_action=True,
+                default_state={},
+                default_entrypoint="foo",
+            )
+            .build()
+        )
+
+
 def test_application_builder_assigns_correct_actions_with_dual_api():
     counter_action = base_counter_action.with_name("counter")
     result_action = Result("count")