jorgecarleitao commented on a change in pull request #9588:
URL: https://github.com/apache/arrow/pull/9588#discussion_r584293489
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File path: rust/arrow/src/array/array_binary.rs
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@@ -258,6 +258,8 @@ where
}
}
+ // calculate actual data_len, which may be different from the
iterator's upper bound
+ let data_len = offsets.len() - 1;
Review comment:
I think that @alamb is right (my comment is actually wrong).
@yordan-pavlov , the Iterator spec recommends, does not require, that the
iterator reports a correct length. Consumer that lead to undefined behavior
from an incorrect `size_hint` are the causers of said undefined behavior.
The only case where consumers can trust the iterators' length is when the
interator implement `unsafe trait TrustedLen`. Unfortunately, `TrustedLen` is
still in unstable. For that reason, we have been exposing `unsafe
Buffer::from_trusted_len_iter` and the like for those cases.
The issue here is that to correctly declare a FromIterator in this case, we
must use `BooleanBufferBuilder` for the null bitmap, which is significantly
slower than a `MutableBuffer`. This is why I added that `panic!` there, even
though it is unsound. This PR is just surfacing this problem where the current
implementation is `unsound` (because the iterator can be larger or smaller than
`upper_bound`).
Since we have been seeing people prefer the `FromIter` over the `Builder`
APIs, my suggestion is that we use `BooleanBufferBuilder`, like we are doing
for `PrimitiveArray`, and offer (in a future PR), a `from_trusted_len_iter` for
string arrays.
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