[ckpt] fix: Reject Energon resume after DP changes

src/megatron/bridge/training/checkpointing.py

@@ -1857,6 +1857,20 @@ def maybe_load_dataloader_state(
         print_rank_0(f"no dataloader state for iteration {iteration}; dataloader starts from the beginning")
         return
 
+    state_file_pattern = join_paths(iter_dir, "train_dataloader_dprank*.pt")
+    saved_dp_size = (
+        len(msc.glob(state_file_pattern))
+        if msc is not None
+        else len(list(Path(iter_dir).glob("train_dataloader_dprank*.pt")))
+    )
+    current_dp_size = get_pg_size(pg_collection.dp)
+    if saved_dp_size != current_dp_size:
+        raise RuntimeError(
+            f"Dataloader state at {iter_dir} was saved for data-parallel size {saved_dp_size}, but the current "
+            f"data-parallel size is {current_dp_size}. Resuming would silently change the training data order; "
+            "refusing to continue."
+        )
+
     dp_rank = get_pg_rank(pg_collection.dp)
     data_state_load_path = join_paths(iter_dir, f"train_dataloader_dprank{dp_rank:03d}.pt")
     if not is_file(data_state_load_path):

tests/unit_tests/training/test_checkpointing.py

@@ -5011,7 +5011,7 @@ def _dist_initialized(self):
             yield
 
     @staticmethod
-    def _pg(cp: int | None = 0, pp: int = 0, tp: int = 0, dp: int = 0):
+    def _pg(cp: int | None = 0, pp: int = 0, tp: int = 0, dp: int = 0, dp_size: int = 1):
         """Mock a ProcessGroupCollection with the given per-dimension ranks. Only cp may be None —
         modeling a collection configured without context parallelism (tp/pp/dp are always
         populated); the real get_pg_rank then reads the absent cp group as rank 0."""
@@ -5023,6 +5023,7 @@ def _pg(cp: int | None = 0, pp: int = 0, tp: int = 0, dp: int = 0):
         pg.pp.rank.return_value = pp
         pg.tp.rank.return_value = tp
         pg.dp.rank.return_value = dp
+        pg.dp.size.return_value = dp_size
         return pg
 
     def test_noop_when_no_path(self):
@@ -5069,6 +5070,27 @@ def test_existing_dir_missing_file_raises(self, tmp_path):
         with pytest.raises(RuntimeError, match="data-parallel size"):
             maybe_load_dataloader_state(train_iterator, 10, str(tmp_path), pg_collection=self._pg())
 
+    @patch("megatron.bridge.training.checkpointing.energon_torch_load")
+    def test_extra_saved_dp_rank_file_raises(self, mock_load, tmp_path):
+        """A smaller current DP size must not silently discard saved rank state."""
+        train_iterator = Mock()
+        iter_dir = Path(get_checkpoint_name(str(tmp_path), 10))
+        iter_dir.mkdir(parents=True)
+        (iter_dir / "train_dataloader_dprank000.pt").touch()
+        (iter_dir / "train_dataloader_dprank001.pt").touch()
+        pg_collection = self._pg(dp=0, dp_size=1)
+
+        with pytest.raises(RuntimeError, match="data-parallel size"):
+            maybe_load_dataloader_state(
+                train_iterator,
+                10,
+                str(tmp_path),
+                pg_collection=pg_collection,
+            )
+
+        mock_load.assert_not_called()
+        train_iterator.iterable.restore_state.assert_not_called()
+
     @patch("megatron.bridge.training.checkpointing.energon_torch_load")
     def test_restores_on_every_cp_tp_pp_rank(self, mock_load, tmp_path):
         """Unlike save, restore must run on every cp/tp/pp because