Bug Fix: Resuming Twice Resets the Dataloader

src/datasets/iterable_dataset.py

@@ -2712,6 +2712,8 @@ def _iter_pytorch(self):
             }
             if self._starting_state_dict and self.epoch == self._starting_state_dict["epoch"]:
                 ex_iterable.load_state_dict(self._starting_state_dict["examples_iterable"])
+                # re-point at the live ex_iterable state so progress tracking
+                self._state_dict["examples_iterable"] = ex_iterable._state_dict
 
             if self._formatting and (ex_iterable.iter_arrow or self._formatting.is_table):
                 formatter = get_formatter(self._formatting.format_type, features=self.features)
@@ -2796,6 +2798,8 @@ def _prepare_ex_iterable_for_iteration(
         }
         if self._starting_state_dict and self.epoch == self._starting_state_dict["epoch"]:
             ex_iterable.load_state_dict(self._starting_state_dict["examples_iterable"])
+            # re-point at the live ex_iterable state so progress tracking
+            self._state_dict["examples_iterable"] = ex_iterable._state_dict
         return ex_iterable
 
     def __iter__(self):

tests/test_iterable_dataset.py

@@ -2833,6 +2833,39 @@ def test_resume_dataloader(dataset: IterableDataset):
     assert remaining == list(dl)
 
 
+@require_torchdata_stateful_dataloader
+@pytest.mark.parametrize("num_workers", [0, 1, 2])
+def test_resume_dataloader_twice(num_workers):
+    from torchdata.stateful_dataloader import StatefulDataLoader
+
+    ex_iterable = ExamplesIterable(generate_examples_fn, {"filepaths": [f"file{i}.txt" for i in range(4)]})
+    dataset = IterableDataset(ex_iterable)
+
+    def make_dataloader():
+        return StatefulDataLoader(dataset, batch_size=None, num_workers=num_workers)
+
+    all_examples = list(make_dataloader())
+
+    # consume 2 examples, then checkpoint #1
+    dl = make_dataloader()
+    it = iter(dl)
+    consumed = [next(it) for _ in range(2)]
+    state_1 = dl.state_dict()
+
+    # resume from #1, consume 2 more, then checkpoint #2 (taken from a resumed loader)
+    dl = make_dataloader()
+    dl.load_state_dict(state_1)
+    it = iter(dl)
+    consumed += [next(it) for _ in range(2)]
+    state_2 = dl.state_dict()
+
+    # resuming from #2 must continue from where it left off, not restart from the beginning
+    dl = make_dataloader()
+    dl.load_state_dict(state_2)
+    remainder = list(dl)
+    assert consumed + remainder == all_examples
+
+
 @pytest.mark.parametrize("num_shards", [1, 2, 3, 7])
 def test_iterable_dataset_batch(num_shards: int):
     # Create a simple IterableDataset