bodduv opened a new issue, #1125:
URL: https://github.com/apache/arrow-java/issues/1125
### Describe the bug, including details regarding any error messages,
version, and platform.
## What happens
On 19.0.0 (and on master), calling `setPosition` on a `ListVector` that came
off the wire via IPC with `valueCount == 0` throws `IndexOutOfBoundsException`.
Some callers may hit this without touching `setPosition` themselves as
- `SingleStructReaderImpl.reader(String)` calls `setPosition(idx())` the
first time a child reader on a struct is looked up.
- `UnionMapReader` inherits from `UnionListReader` and breaks the same way.
- `UnionLargeListReader` has a related variant; capacity guard seems to be
missing and it fails whenever the offset buffer is shorter than `(idx + 2) * 8`
bytes.
The reader code hasn't changed. `ListVector.setReaderAndWriterIndex` changes
and now correctly emits the leading `[0]` offset when `valueCount == 0` (as per
the Arrow spec), so the receiver's offset buffer is no longer "zero-capacity"
and if block `capacity() == 0` never enters.
## Repro
The shape we actually hit — a struct with a list-typed child, struct itself
having `valueCount == 0`:
```java
StructVector parent = ...; // child "l" is LIST<INT>, valueCount
== 0
parent.getReader().reader("l"); // IOOBE
```
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