Make session index pruning explicit

api/models.py

@@ -244,7 +244,37 @@ def prune_session_from_index(session_id: str) -> None:
     sid = str(session_id or "")
     if not sid or not SESSION_INDEX_FILE.exists():
         return
-    _write_session_index(updates=[])
+    _tmp = SESSION_INDEX_FILE.with_suffix(f'.tmp.{os.getpid()}.{threading.current_thread().ident}')
+
+    _fallback = False
+    with _INDEX_WRITE_LOCK:
+        try:
+            with LOCK:
+                existing = json.loads(SESSION_INDEX_FILE.read_text(encoding='utf-8'))
+                if not isinstance(existing, list):
+                    raise ValueError("session index must be a list")
+                pruned = [e for e in existing if e.get('session_id') != sid]
+                if len(pruned) == len(existing):
+                    return
+                _payload = json.dumps(pruned, ensure_ascii=False, indent=2)
+
+            try:
+                with open(_tmp, 'w', encoding='utf-8') as f:
+                    f.write(_payload)
+                    f.flush()
+                    os.fsync(f.fileno())
+                os.replace(_tmp, SESSION_INDEX_FILE)
+            except Exception:
+                try:
+                    _tmp.unlink(missing_ok=True)
+                except Exception:
+                    pass
+                raise
+        except Exception:
+            _fallback = True
+
+    if _fallback:
+        _write_session_index(updates=None)
 
 
 def _active_stream_ids():

tests/test_regressions.py

@@ -325,10 +325,8 @@ def test_server_delete_prunes_session_index(cleanup_test_sessions):
         )
         if delete_idx >= 0:
             delete_block = text[delete_idx:delete_idx+1800]
-            assert "prune_session_from_index" in delete_block, \
+            assert "prune_session_from_index(sid)" in delete_block, \
                 f"{label} session/delete must prune SESSION_INDEX_FILE"
-            assert "SESSION_INDEX_FILE.unlink" not in delete_block, \
-                f"{label} session/delete must not discard the whole session index"
             return
     assert False, "session/delete handler not found in server.py or api/routes.py"
 

tests/test_session_index.py

@@ -83,20 +83,21 @@ def test_compact_exposes_last_message_at_from_message_timestamp():
     assert compact["last_message_at"] == 200.0
 
 
-def test_prune_session_from_index_removes_only_deleted_row():
+def test_prune_session_from_index_removes_requested_row_only():
     index_file = models.SESSION_INDEX_FILE
     s_a = _make_session("sess_a", "A", updated_at=100)
     s_b = _make_session("sess_b", "B", updated_at=200)
     s_a.save()
     s_b.save()
 
-    s_a.path.unlink()
     prune_session_from_index("sess_a")
 
     index = _read_index(index_file)
     ids = [entry["session_id"] for entry in index]
     assert ids == ["sess_b"]
     assert index_file.exists()
+    assert s_a.path.exists()
+    assert s_b.path.exists()
 
 
 def test_all_sessions_backfills_last_message_at_for_legacy_index_rows():