[cross-repo from server#333] Conformance finding: child workflow result is null in parent (#131)

Author: durable-workflow-ops

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Fixed
+- Python parent workflows now decode successful child workflow completions from
+  the server's documented `ChildRunCompleted.output` history payload, while
+  still accepting the older `result` alias. This prevents completed child
+  workflow returns from replaying as `None`.
 - Condition-wait replay now binds signals that arrive during a leased
   workflow task to the next recorded wait when the server history records
   those signals before the task's `ConditionWaitOpened` row. This avoids

src/durable_workflow/workflow.py

@@ -1419,8 +1419,10 @@ def _decode_history_result(
     external_storage_cache: ExternalPayloadCache | None = None,
 ) -> Any:
     codec = payload.get("payload_codec") or fallback_codec
+    value = payload["result"] if "result" in payload else payload.get("output")
+
     return serializer.decode_envelope(
-        payload.get("result"),
+        value,
         codec=codec,
         external_storage=external_storage,
         external_storage_cache=external_storage_cache,

tests/test_replay.py

@@ -925,8 +925,16 @@ def test_first_replay_starts_child(self) -> None:
         assert cmd.workflow_type == "sub-workflow"
         assert cmd.arguments == ["alice"]
 
-    def test_child_completed(self) -> None:
-        history = [{"event_type": "ChildRunCompleted", "payload": {"result": '"sub-result"'}}]
+    @pytest.mark.parametrize(
+        "payload",
+        [
+            {"output": '"sub-result"'},
+            {"output": serializer.envelope("sub-result", codec="json")},
+            {"result": '"sub-result"'},
+        ],
+    )
+    def test_child_completed(self, payload: dict[str, object]) -> None:
+        history = [{"event_type": "ChildRunCompleted", "payload": payload}]
         outcome = replay(ChildWorkflow, history, ["alice"])
         assert len(outcome.commands) == 1
         cmd = outcome.commands[0]