Improve error message in _read_snapshot_metadata

Summary:
This is to mitigate confusion about what went wrong about the
snapshot.

Reviewed By: JKSenthil

Differential Revision: D54705863

Author: schwarzmx

tests/test_snapshot.py

@@ -10,6 +10,7 @@
 import copy
 from pathlib import Path
 from typing import Any, Dict, List
+from unittest.mock import MagicMock
 
 import pytest
 
@@ -226,3 +227,21 @@ def load_state_dict(self, state_dict: Dict[str, Any]) -> None:
     snapshot = Snapshot.take(app_state={"state": src}, path=str(tmp_path))
     snapshot.restore(app_state={"state": dst})
     assert check_state_dict_eq(src.state_dict(), dst.state_dict())
+
+
+@pytest.mark.usefixtures("toggle_batching")
+def test_snapshot_metadata_error(tmp_path: Path) -> None:
+    mock_storage_plugin = MagicMock()
+    mock_event_loop = MagicMock()
+    mock_storage_plugin.sync_read.side_effect = Exception(
+        "Mock error reading from storage"
+    )
+    with pytest.raises(
+        expected_exception=RuntimeError,
+        match=(
+            "Failed to read .snapshot_metadata. "
+            "Ensure path to snapshot is correct, "
+            "otherwise snapshot is likely incomplete or corrupted."
+        ),
+    ):
+        Snapshot._read_snapshot_metadata(mock_storage_plugin, mock_event_loop)

torchsnapshot/snapshot.py

@@ -840,7 +840,14 @@ def _read_snapshot_metadata(
         storage: StoragePlugin, event_loop: asyncio.AbstractEventLoop
     ) -> SnapshotMetadata:
         read_io = ReadIO(path=SNAPSHOT_METADATA_FNAME)
-        storage.sync_read(read_io=read_io, event_loop=event_loop)
+        try:
+            storage.sync_read(read_io=read_io, event_loop=event_loop)
+        except Exception as e:
+            raise RuntimeError(
+                f"Failed to read {SNAPSHOT_METADATA_FNAME}. "
+                "Ensure path to snapshot is correct, "
+                "otherwise snapshot is likely incomplete or corrupted."
+            ) from e
         yaml_str = read_io.buf.getvalue().decode("utf-8")
         return SnapshotMetadata.from_yaml(yaml_str)