fix(models): make session-index rebuild thread ownership explicit (#3894)

api/models.py

@@ -200,6 +200,7 @@ def _session_dir_has_persisted_session_files() -> bool:
 
 def _rebuild_session_index_background(expected_session_dir: Path, expected_index_file: Path) -> None:
     global _SESSION_INDEX_REBUILD_THREAD, _SESSION_INDEX_REBUILD_THREAD_TARGET
+    current_thread = threading.current_thread()
     try:
         with _SESSION_INDEX_REBUILD_LOCK:
             if SESSION_DIR != expected_session_dir or SESSION_INDEX_FILE != expected_index_file:
@@ -213,7 +214,7 @@ def _rebuild_session_index_background(expected_session_dir: Path, expected_index
         logger.debug("Background session-index rebuild failed", exc_info=True)
     finally:
         with _SESSION_INDEX_REBUILD_LOCK:
-            if _SESSION_INDEX_REBUILD_THREAD_TARGET == (
+            if _SESSION_INDEX_REBUILD_THREAD is current_thread and _SESSION_INDEX_REBUILD_THREAD_TARGET == (
                 expected_session_dir,
                 expected_index_file,
             ):

tests/test_session_index.py

@@ -1666,3 +1666,30 @@ def test_background_index_rebuild_skips_after_session_dir_switch(tmp_path, monke
     assert not new_index_file.exists()
     rows = _read_index(original_index_file)
     assert [row["session_id"] for row in rows] == ["late_switch_sid"]
+
+
+def test_background_rebuild_old_thread_finally_preserves_new_same_target_owner(tmp_path, monkeypatch):
+    session_dir = tmp_path / "sessions"
+    session_dir.mkdir(exist_ok=True)
+    index_file = session_dir / "_index.json"
+    target = (session_dir, index_file)
+
+    monkeypatch.setattr(models, "SESSION_DIR", session_dir)
+    monkeypatch.setattr(models, "SESSION_INDEX_FILE", index_file)
+
+    old_thread = object()
+    new_thread = object()
+    monkeypatch.setattr(models, "_SESSION_INDEX_REBUILD_THREAD", old_thread)
+    monkeypatch.setattr(models, "_SESSION_INDEX_REBUILD_THREAD_TARGET", target)
+    monkeypatch.setattr(models.threading, "current_thread", lambda: old_thread)
+
+    def _handoff_then_write(*args, **kwargs):
+        monkeypatch.setattr(models, "_SESSION_INDEX_REBUILD_THREAD", new_thread)
+        monkeypatch.setattr(models, "_SESSION_INDEX_REBUILD_THREAD_TARGET", target)
+
+    monkeypatch.setattr(models, "_write_session_index", _handoff_then_write)
+
+    models._rebuild_session_index_background(*target)
+
+    assert models._SESSION_INDEX_REBUILD_THREAD is new_thread
+    assert models._SESSION_INDEX_REBUILD_THREAD_TARGET == target