lf.eval.v2: Handle potential race condition when saving 'index.html'.

There might be race conditions when 'index.html` is written by the background thread and from the thread that completes an example.

PiperOrigin-RevId: 711879944

langfun/core/eval/v2/reporting.py

@@ -51,6 +51,7 @@ def _on_bound(self):
     self._update_thread = None
     self._stop_update = False
     self._stop_update_experiment_ids = set()
+    self._experiment_index_lock = None
 
   def on_run_start(
       self,
@@ -64,6 +65,7 @@ def on_run_start(
     self._update_thread = threading.Thread(
         target=self._update_thread_func, args=(runner,)
     )
+    self._experiment_index_lock = {}
     self._update_thread.start()
 
   def on_run_complete(
@@ -100,6 +102,7 @@ def on_experiment_start(
       experiment: Experiment
   ) -> None:
     if experiment.is_leaf:
+      self._experiment_index_lock[experiment.id] = threading.Lock()
       self._maybe_update_experiment_html(runner, experiment)
 
   def on_experiment_complete(
@@ -170,7 +173,8 @@ def _save():
                   card_view=False,
               ),
           )
-          html.save(index_html_path)
+          with self._experiment_index_lock[experiment.id]:
+            html.save(index_html_path)
           experiment.info(
               f'Updated {index_html_path!r} in {t.elapse:.2f} seconds.',
           )
@@ -185,11 +189,11 @@ def _save():
         time.time() - self._last_experiment_report_time[experiment.id]
         > self.experiment_report_interval
     ):
+      self._last_experiment_report_time[experiment.id] = time.time()
       if background:
         runner.background_run(_save)
       else:
         _save()
-      self._last_experiment_report_time[experiment.id] = time.time()
 
   def _save_example_html(
       self, runner: Runner, experiment: Experiment, example: Example