viirya commented on PR #41610:
URL: https://github.com/apache/arrow/pull/41610#issuecomment-2103964293
Hmm, I think I know why the tests are not failed in Java Arrow. This is how
Java Arrow imports an Utf8 array:
```
try (ArrowBuf offsets = importOffsets(type, VarCharVector.OFFSET_WIDTH)) {
final int start = offsets.getInt(0);
final int end = offsets.getInt(fieldNode.getLength() * (long)
VarCharVector.OFFSET_WIDTH);
final int len = end - start;
...
}
```
So even the offset buffer is not initialized, for empty array with one
element offset buffer, `end - start` is always 0 that is the length of data
buffer. That is why the added roundtrip tests are passed.
But in arrow-rs, it takes the last value of the offset buffer as the length
of data buffer, i.e., `end`. If the value is not initialized to zero, the
computed length of data buffer is incorrect.
That is what I found for the first offset value from the
[spec](https://arrow.apache.org/docs/format/Columnar.html#variable-size-binary-layout):
```
Generally the first slot in the offsets array is 0, and the last slot is the
length of the values array.
When serializing this layout, we recommend normalizing the offsets to start
at 0.
```
It looks like the first value doesn't have to be 0, although generally it
is. So seems Java Arrow's approach is correct without issue?
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