fix reading partial shards (#2397)

Author: normanrz

src/zarr/codecs/sharding.py

@@ -129,7 +129,7 @@ def get_chunk_slice(self, chunk_coords: ChunkCoords) -> tuple[int, int] | None:
         if (chunk_start, chunk_len) == (MAX_UINT_64, MAX_UINT_64):
             return None
         else:
-            return (int(chunk_start), int(chunk_start) + int(chunk_len))
+            return (int(chunk_start), int(chunk_len))
 
     def set_chunk_slice(self, chunk_coords: ChunkCoords, chunk_slice: slice | None) -> None:
         localized_chunk = self._localize_chunk(chunk_coords)
@@ -203,7 +203,7 @@ def create_empty(
     def __getitem__(self, chunk_coords: ChunkCoords) -> Buffer:
         chunk_byte_slice = self.index.get_chunk_slice(chunk_coords)
         if chunk_byte_slice:
-            return self.buf[chunk_byte_slice[0] : chunk_byte_slice[1]]
+            return self.buf[chunk_byte_slice[0] : (chunk_byte_slice[0] + chunk_byte_slice[1])]
         raise KeyError
 
     def __len__(self) -> int:

tests/test_codecs/test_sharding.py

@@ -118,6 +118,42 @@ def test_sharding_partial(
     assert np.array_equal(data, read_data)
 
 
+@pytest.mark.parametrize("index_location", ["start", "end"])
+@pytest.mark.parametrize("store", ["local", "memory", "zip"], indirect=["store"])
+@pytest.mark.parametrize(
+    "array_fixture",
+    [
+        ArrayRequest(shape=(128,) * 3, dtype="uint16", order="F"),
+    ],
+    indirect=["array_fixture"],
+)
+def test_sharding_partial_readwrite(
+    store: Store, array_fixture: npt.NDArray[Any], index_location: ShardingCodecIndexLocation
+) -> None:
+    data = array_fixture
+    spath = StorePath(store)
+    a = Array.create(
+        spath,
+        shape=data.shape,
+        chunk_shape=data.shape,
+        dtype=data.dtype,
+        fill_value=0,
+        codecs=[
+            ShardingCodec(
+                chunk_shape=(1, data.shape[1], data.shape[2]),
+                codecs=[BytesCodec()],
+                index_location=index_location,
+            )
+        ],
+    )
+
+    a[:] = data
+
+    for x in range(data.shape[0]):
+        read_data = a[x, :, :]
+        assert np.array_equal(data[x], read_data)
+
+
 @pytest.mark.parametrize(
     "array_fixture",
     [