bugfix: skip write index.json if no data is wrote. (#19439)

(cherry picked from commit 47c8f4c)

src/lightning/data/streaming/writer.py

@@ -322,6 +322,8 @@ def write_chunk_to_file(
 
     def write_chunks_index(self) -> str:
         """Write the chunks index to a JSON file."""
+        if len(self._chunks_info) == 0:
+            return ""
         filepath = os.path.join(self._cache_dir, f"{self.rank}.{_INDEX_FILENAME}")
         config = self.get_config()
         with open(filepath, "w") as out:
@@ -393,7 +395,6 @@ def _merge_no_wait(self, node_rank: Optional[int] = None) -> None:
                     config = data["config"]
 
                 elif config != data["config"]:
-                    breakpoint()
                     raise Exception("The config isn't consistent between chunks. This shouldn't have happened.")
 
                 chunks_info.extend(data["chunks"])

tests/tests_data/processing/test_data_processor.py

@@ -564,6 +564,9 @@ def test_data_processsor_nlp(tmpdir, monkeypatch):
     data_processor = DataProcessor(input_dir=str(tmpdir), num_workers=1, num_downloaders=1)
     data_processor.run(TextTokenizeRecipe(chunk_size=1024 * 11))
 
+    data_processor_more_wokers = DataProcessor(input_dir=str(tmpdir), num_workers=2, num_downloaders=1)
+    data_processor_more_wokers.run(TextTokenizeRecipe(chunk_size=1024 * 11))
+
 
 class ImageResizeRecipe(DataTransformRecipe):
     def prepare_structure(self, input_dir: str):