jorgecarleitao commented on a change in pull request #8960:
URL: https://github.com/apache/arrow/pull/8960#discussion_r548387772
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File path: rust/arrow/src/array/transform/variable_size.rs
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@@ -43,33 +47,33 @@ pub(super) fn build_extend<T: OffsetSizeTrait>(array:
&ArrayData) -> Extend {
// fast case where we can copy regions without null issues
Box::new(
move |mutable: &mut _MutableArrayData, _, start: usize, len:
usize| {
- let mutable_offsets = mutable.buffer::<T>(0);
- let last_offset = mutable_offsets[mutable_offsets.len() - 1];
- // offsets
- let buffer = &mut mutable.buffers[0];
+ let offset_buffer = &mut mutable.buffer1;
+ let values_buffer = &mut mutable.buffer2;
+
+ // this is safe due to how offset is built. See details on
`get_last_offset`
+ let last_offset = unsafe { get_last_offset(offset_buffer) };
Review comment:
I get your point.
`get_last_offset` must only be used in offset buffers, i.e. buffers whose
bytes represent a i32 or i64 and were specifically built from those types.
Doing so in other buffers is undefined behavior (even with the safeguards of
using `align_to`). We can remove the `unsafe` mark from `get_last_offset`,
though.
My proposal is that we refactor the `src/transform` code so that it has a
struct specific for each array type (that implements some trait for `dyn`
support). This will allow `last_offset` to be stored in the array-specific
struct, thereby avoiding this problem altogether (of having to read bytes
written to the `MutableBuffer`).
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