fix(tests,engine): guard zarrista tests with importorskip; engine reads return writable arrays

Two fixes from roborev review 371:

- tests/zarrista lacked the importorskip guard its tests/zarrs siblings have,
  so any environment without the optional zarrista package failed at pytest
  collection (the default test env does not install zarrista). Guarded the
  module, and the obstore-dependent fixture/test likewise skip instead of
  erroring when obstore is absent. Verified by running the suite with the
  zarrista import blocked: clean skips, no collection errors.
- Engine reads copied for parity with the native path: read_region returns a
  read-only view over the backend's immutable bytes, which leaked through
  arr[...] as an immutable result. Pinned by a writability test.

Assisted-by: ClaudeCode:claude-opus-4.8

Author: d-v-b

src/zarr/core/array.py

@@ -5471,7 +5471,9 @@ async def _engine_get_selection(
     )
     if isinstance(indexer, BasicIndexer) and indexer.shape == ():
         return cast("NDArrayLikeOrScalar", result[()])
-    return result
+    # `read_region` returns a read-only view over the backend's immutable bytes;
+    # the native path returns a writable buffer, so copy for parity.
+    return result.copy()
 
 
 async def _get_selection(

tests/test_engine.py

@@ -111,6 +111,19 @@ def test_getitem_via_engine_matches_native(local: LocalStore, engine: str) -> No
     np.testing.assert_array_equal(arr[2:6, 1:5], data[2:6, 1:5])
 
 
+@pytest.mark.parametrize("engine", ENGINES)
+def test_getitem_result_is_writable_under_engine(local: LocalStore, engine: str) -> None:
+    """Engine reads return a writable array, matching the native path (the crud
+    facade's read-only view must not leak through `arr[...]`)."""
+    native = zarr.create_array(store=local, name="a", shape=(8, 8), chunks=(4, 4), dtype="uint16")
+    native[:] = _ramp()
+
+    arr = zarr.open_array(store=local, path="a", engine=engine)
+    result = np.asarray(arr[2:6, 1:5])
+    assert result.flags.writeable
+    result[0, 0] = 42  # must not raise
+
+
 @pytest.mark.parametrize("engine", ENGINES)
 def test_scalar_getitem_via_engine(local: LocalStore, engine: str) -> None:
     """A full-integer selection returns a scalar, matching native semantics."""

tests/zarrista/conftest.py

@@ -39,13 +39,13 @@ async def memory_store() -> AsyncIterator[Store]:
 async def object_store(tmp_path: Path) -> AsyncIterator[Store]:
     """A zarr ObjectStore wrapping an obstore LocalStore — ingested via
     zarrista's async API."""
-    import obstore.store
+    obstore_store = pytest.importorskip("obstore.store", reason="obstore is not installed")
 
     from zarr.storage import ObjectStore
 
     root = tmp_path / "obj"
     root.mkdir()
-    s: Store = await ObjectStore.open(obstore.store.LocalStore(str(root)))
+    s: Store = await ObjectStore.open(obstore_store.LocalStore(str(root)))
     try:
         yield s
     finally:

tests/zarrista/test_backend.py

@@ -13,6 +13,8 @@
 import numpy as np
 import pytest
 
+pytest.importorskip("zarrista", reason="zarrista is not installed", exc_type=ImportError)
+
 from zarr.crud import ReferenceBackend
 from zarr.zarrista import UnsupportedStoreError, ZarristaBackend
 
@@ -231,11 +233,11 @@ async def test_object_store_read_subset(object_store: Store) -> None:
 async def test_memory_backed_object_store_raises() -> None:
     """An ObjectStore wrapping an obstore MemoryStore is rejected at the gate:
     zarrista cannot ingest memory-backed obstore stores."""
-    import obstore.store
+    obstore_store = pytest.importorskip("obstore.store", reason="obstore is not installed")
 
     from zarr.storage import ObjectStore
 
-    store: Store = await ObjectStore.open(obstore.store.MemoryStore())
+    store: Store = await ObjectStore.open(obstore_store.MemoryStore())
     backend = ZarristaBackend()
     with pytest.raises(UnsupportedStoreError):
         await backend.create_array(store, "a", array_metadata(), overwrite=False)