Right now, there is no way to represent a non-nullable, "degenerate" FixedSizeListArray with arbitrary length. By degenerate, I mean that the size of each list scalar is equal to 0, meaning the FixedSizeList does not actually store any data.

The try_new constructor will use the length of the null buffer to determine the length of the array in the degenerate case, but if the null buffer is None (indicating a non-nullable array), then the code incorrectly defaults to 0.

Possible Solutions

You cannot correctly handle this case by relying on the length of the null buffer, so I believe there are only 2 sane solutions. The first is to fix the try_new constructor and require it to take in a len: usize.

Since a breaking change like this might not be acceptable, then a try_new_with_length constructor can be made that does take in a len parameter, and the existing try_new can call into try_new_with_length while documenting that if users want this specific degenerate and non-nullable case, they must use try_new_with_length.

Edit: I've made a first step in #8624

Edit: I also just realized that if a caller passes in a values array that is not an exact multiple of size to try_new, we immediately have an incorrect FixedSizeList. So if I pass in an array of length 6, and I specify an list size of 4 and a single null in my null buffer, that creates an invalid(?) array (though technically you can never access those last two values?)

Related Issues

• FixedSizeListArray::try_new Errors on Entirely Null Array With Size 0 #5614
• Empty lists in FixedSizeListArray::try_new is not handled #4623