Downloader: Resolve race condition (#19348)

src/lightning/data/streaming/downloader.py

@@ -20,6 +20,7 @@ from urllib import parse
 from filelock import FileLock, Timeout
 
 from lightning.data.streaming.client import S3Client
+from lightning.data.streaming.constants import _INDEX_FILENAME
 
 
 class Downloader(ABC):
@@ -56,7 +57,7 @@ class S3Downloader(Downloader):
             return
 
         try:
-            with FileLock(local_filepath + ".lock", timeout=0):
+            with FileLock(local_filepath + ".lock", timeout=1 if obj.path.endswith(_INDEX_FILENAME) else 0):
                 if self._s5cmd_available:
                     proc = subprocess.Popen(
                         f"s5cmd cp {remote_filepath} {local_filepath}",