fix: Recursively align when converting to ArrowArray (#19097)

crates/polars-arrow/src/array/ffi.rs

@@ -36,6 +36,7 @@ pub(crate) trait FromFfi<T: ffi::ArrowArrayRef>: Sized {
 macro_rules! ffi_dyn {
     ($array:expr, $ty:ty) => {{
         let array = $array.as_any().downcast_ref::<$ty>().unwrap();
+
         (
             array.offset().unwrap(),
             array.buffers(),
@@ -54,6 +55,7 @@ type BuffersChildren = (
 
 pub fn offset_buffers_children_dictionary(array: &dyn Array) -> BuffersChildren {
     use PhysicalType::*;
+
     match array.dtype().to_physical_type() {
         Null => ffi_dyn!(array, NullArray),
         Boolean => ffi_dyn!(array, BooleanArray),

crates/polars-arrow/src/ffi/array.rs

@@ -12,7 +12,7 @@ use crate::datatypes::{ArrowDataType, PhysicalType};
 use crate::ffi::schema::get_child;
 use crate::storage::SharedStorage;
 use crate::types::NativeType;
-use crate::{match_integer_type, with_match_primitive_type_full};
+use crate::{ffi, match_integer_type, with_match_primitive_type_full};
 
 /// Reads a valid `ffi` interface into a `Box<dyn Array>`
 /// # Errors
@@ -140,12 +140,19 @@ impl ArrowArray {
 
         let children_ptr = children
             .into_iter()
-            .map(|child| Box::into_raw(Box::new(ArrowArray::new(child))))
+            .map(|child| {
+                Box::into_raw(Box::new(ArrowArray::new(ffi::align_to_c_data_interface(
+                    child,
+                ))))
+            })
             .collect::<Box<_>>();
         let n_children = children_ptr.len() as i64;
 
-        let dictionary_ptr =
-            dictionary.map(|array| Box::into_raw(Box::new(ArrowArray::new(array))));
+        let dictionary_ptr = dictionary.map(|array| {
+            Box::into_raw(Box::new(ArrowArray::new(ffi::align_to_c_data_interface(
+                array,
+            ))))
+        });
 
         let length = array.len() as i64;
         let null_count = array.null_count() as i64;

crates/polars-arrow/src/ffi/mod.rs

@@ -8,6 +8,7 @@ mod schema;
 mod stream;
 
 pub(crate) use array::{try_from, ArrowArrayRef, InternalArrowArray};
+pub(crate) use bridge::align_to_c_data_interface;
 pub use generated::{ArrowArray, ArrowArrayStream, ArrowSchema};
 use polars_error::PolarsResult;
 pub use stream::{export_iterator, ArrowArrayStreamReader};

py-polars/tests/unit/interop/test_interop.py

@@ -766,3 +766,11 @@ def test_df_pycapsule_interface() -> None:
     df2 = pl.from_arrow(out)
     assert isinstance(df2, pl.DataFrame)
     assert df.equals(df2)
+
+
+def test_misaligned_nested_arrow_19097() -> None:
+    a = pl.Series("a", [1, 2, 3])
+    a = a.slice(1, 2)  # by slicing we offset=1 the values
+    a = a.replace(2, None)  # then we add a validity mask with offset=0
+    a = a.reshape((2, 1))  # then we make it nested
+    assert_series_equal(pl.Series("a", a.to_arrow()), a)