Skip chunk coordinate enumeration in resize when array is only growing (#3650)

changes/3702.bugfix.md

@@ -0,0 +1 @@
+Skip chunk coordinate enumeration in resize when the array is only growing, avoiding unbounded memory usage for large arrays.

src/zarr/core/array.py

@@ -5990,7 +5990,10 @@ async def _resize(
     assert len(new_shape) == len(array.metadata.shape)
     new_metadata = array.metadata.update_shape(new_shape)
 
-    if delete_outside_chunks:
+    # ensure deletion is only run if array is shrinking as the delete_outside_chunks path is unbounded in memory
+    only_growing = all(new >= old for new, old in zip(new_shape, array.metadata.shape, strict=True))
+
+    if delete_outside_chunks and not only_growing:
         # Remove all chunks outside of the new shape
         old_chunk_coords = set(array.metadata.chunk_grid.all_chunk_coords(array.metadata.shape))
         new_chunk_coords = set(array.metadata.chunk_grid.all_chunk_coords(new_shape))

tests/test_array.py

@@ -781,6 +781,73 @@ def test_resize_2d(store: MemoryStore, zarr_format: ZarrFormat) -> None:
     assert new_shape == result.shape
 
 
+@pytest.mark.parametrize("store", ["memory"], indirect=True)
+def test_resize_growing_skips_chunk_enumeration(
+    store: MemoryStore, zarr_format: ZarrFormat
+) -> None:
+    """Growing an array should not enumerate chunk coords for deletion (#3650 mitigation)."""
+    from zarr.core.chunk_grids import RegularChunkGrid
+
+    z = zarr.create(
+        shape=(10, 10),
+        chunks=(5, 5),
+        dtype="i4",
+        fill_value=0,
+        store=store,
+        zarr_format=zarr_format,
+    )
+    z[:] = np.ones((10, 10), dtype="i4")
+
+    # growth only - ensure no chunk coords are enumerated
+    with mock.patch.object(
+        RegularChunkGrid,
+        "all_chunk_coords",
+        wraps=z.metadata.chunk_grid.all_chunk_coords,
+    ) as mock_coords:
+        z.resize((20, 20))
+        mock_coords.assert_not_called()
+
+    assert z.shape == (20, 20)
+    np.testing.assert_array_equal(np.ones((10, 10), dtype="i4"), z[:10, :10])
+    np.testing.assert_array_equal(np.zeros((10, 10), dtype="i4"), z[10:, 10:])
+
+    # shrink - ensure no regression of behaviour
+    with mock.patch.object(
+        RegularChunkGrid,
+        "all_chunk_coords",
+        wraps=z.metadata.chunk_grid.all_chunk_coords,
+    ) as mock_coords:
+        z.resize((5, 5))
+        assert mock_coords.call_count > 0
+
+    assert z.shape == (5, 5)
+    np.testing.assert_array_equal(np.ones((5, 5), dtype="i4"), z[:])
+
+    # mixed: grow dim 0, shrink dim 1 - ensure deletion path runs
+    z2 = zarr.create(
+        shape=(10, 10),
+        chunks=(5, 5),
+        dtype="i4",
+        fill_value=0,
+        store=store,
+        zarr_format=zarr_format,
+        overwrite=True,
+    )
+    z2[:] = np.ones((10, 10), dtype="i4")
+
+    with mock.patch.object(
+        RegularChunkGrid,
+        "all_chunk_coords",
+        wraps=z2.metadata.chunk_grid.all_chunk_coords,
+    ) as mock_coords:
+        z2.resize((20, 5))
+        assert mock_coords.call_count > 0
+
+    assert z2.shape == (20, 5)
+    np.testing.assert_array_equal(np.ones((10, 5), dtype="i4"), z2[:10, :])
+    np.testing.assert_array_equal(np.zeros((10, 5), dtype="i4"), z2[10:, :])
+
+
 @pytest.mark.parametrize("store", ["memory"], indirect=True)
 def test_append_1d(store: MemoryStore, zarr_format: ZarrFormat) -> None:
     a = np.arange(105)