fix: use flushable checkpoint handles on Windows

obliteratus/persistence_contracts.py

@@ -23,6 +23,16 @@ else:
     _DIRECTORY_SYNC_FLAGS = os.O_RDONLY | os.O_DIRECTORY
 
 
+def _file_sync_flags(platform_name: str, binary_flag: int) -> int:
+    """Return flags for a descriptor that the platform can durably flush."""
+    if platform_name == "nt":
+        return os.O_RDWR | binary_flag
+    return os.O_RDONLY
+
+
+_FILE_SYNC_FLAGS = _file_sync_flags(os.name, getattr(os, "O_BINARY", 0))
+
+
 class SizedTensor(Protocol):
     """Structural subset used to estimate a serialized state dictionary."""
 
@@ -160,7 +170,7 @@ def _sync_file(path: Path) -> None:
     mode = path.lstat().st_mode
     if not stat.S_ISREG(mode):
         raise OSError(f"Checkpoint artifact is not a regular file: {path}")
-    descriptor = os.open(path, os.O_RDONLY)
+    descriptor = os.open(path, _FILE_SYNC_FLAGS)
     try:
         os.fsync(descriptor)
     finally:

tests/test_persistence_contracts.py

@@ -498,6 +498,54 @@ def test_atomic_checkpoint_flushes_nested_directories(tmp_path, monkeypatch):
     assert "nested" in synced_directories
 
 
+def test_file_sync_flags_use_writable_binary_handle_on_windows():
+    binary_flag = 0x8000
+    expected = os.O_RDWR | binary_flag
+
+    assert persistence._file_sync_flags("nt", binary_flag) == expected
+    assert expected != os.O_RDONLY
+
+
+def test_file_sync_flags_use_read_handle_on_posix():
+    assert persistence._file_sync_flags("posix", 0x8000) == os.O_RDONLY
+
+
+def test_sync_file_uses_platform_flags_and_closes_descriptor(tmp_path, monkeypatch):
+    artifact = tmp_path / "artifact.bin"
+    artifact.write_bytes(b"checkpoint")
+    open_file = MagicMock(return_value=17)
+    sync_file = MagicMock()
+    close_file = MagicMock()
+    monkeypatch.setattr(persistence, "_FILE_SYNC_FLAGS", 123)
+    monkeypatch.setattr(persistence.os, "open", open_file)
+    monkeypatch.setattr(persistence.os, "fsync", sync_file)
+    monkeypatch.setattr(persistence.os, "close", close_file)
+
+    persistence._sync_file(artifact)
+
+    open_file.assert_called_once_with(artifact, 123)
+    sync_file.assert_called_once_with(17)
+    close_file.assert_called_once_with(17)
+
+
+def test_sync_file_closes_descriptor_after_fsync_failure(tmp_path, monkeypatch):
+    artifact = tmp_path / "artifact.bin"
+    artifact.write_bytes(b"checkpoint")
+    close_file = MagicMock()
+    monkeypatch.setattr(persistence.os, "open", MagicMock(return_value=17))
+    monkeypatch.setattr(
+        persistence.os,
+        "fsync",
+        MagicMock(side_effect=OSError("simulated fsync failure")),
+    )
+    monkeypatch.setattr(persistence.os, "close", close_file)
+
+    with pytest.raises(OSError, match="simulated fsync failure"):
+        persistence._sync_file(artifact)
+
+    close_file.assert_called_once_with(17)
+
+
 def test_sync_directory_uses_platform_flags_and_closes_descriptor(
     tmp_path,
     monkeypatch,