apache / apache/arrow-java

[Java][IPC] AbstractCompressionCodec.compress() writes prefix=0 for empty buffers, incompatible with C++/Python readers

Aberta Para iniciantes
#1,196 1 comentário 0 reações 0 responsáveis Ver no GitHub
Type: bug
Linguagem predominante
Java
Estrelas
94
Forks
152
Merge médio
3d 16h
PRs com merge (30d)
11

Descrição

### Describe the bug

When a buffer has `writerIndex == 0` (e.g., a string column where all values
are empty string ""), `AbstractCompressionCodec.compress()` writes an 8-byte
buffer with `uncompressed_length = 0` as a "shortcut for empty buffer":

https://github.com/apache/arrow-java/blob/main/vector/src/main/java/org/apache/arrow/vector/compression/AbstractCompressionCodec.java#L32-L39

```java
if (uncompressedBuffer.writerIndex() == 0L) {
// shortcut for empty buffer
compressedBuffer.setLong(0, 0); // prefix = 0
...
}
```

This has been present since the initial implementation (ARROW-11899, 2021).

The Java `decompress()` handles this correctly (`if size==0 return empty`),
but **C++ and Python Arrow readers do not recognize `prefix=0`**. They attempt
to decompress 0 bytes of data, which fails:

- C++ (Arrow 1.0.0 ~ latest): `IOError: Lz4 compressed input contains less than one frame`
- PyArrow 21.0: same error

### Reproduction

Write an Arrow IPC stream with LZ4_FRAME (or ZSTD) compression where **one
string column has all values = ""** (empty string, not null). The string data
buffer has `writerIndex = 0`, triggering the empty buffer path.

```java
// Writer
ArrowStreamWriter writer = new ArrowStreamWriter(root, null, channel,
IpcOption.DEFAULT, CommonsCompressionFactory.INSTANCE, CodecType.LZ4_FRAME);

// All rows: stringVector.setSafe(i, "".getBytes());
```

Reading with C++ or Python fails at the first RecordBatch.

### Root cause

The Arrow IPC compression format defines:
- `prefix > 0`: compressed data follows, decompress to `prefix` bytes
- `prefix = -1`: buffer stored uncompressed (sentinel)
- `prefix = 0`: **undefined** — not in spec, not handled by C++/Python

Java writes `prefix=0` for empty buffers, but only Java itself knows how to
read it back. C++/Python treat it as "0 bytes to decompress" → fail.

### Suggested fix

Change the empty buffer path to use `-1` sentinel (which all readers support):

```java
if (uncompressedBuffer.writerIndex() == 0L) {
ArrowBuf compressedBuffer = allocator.buffer(SIZE_OF_UNCOMPRESSED_LENGTH);
compressedBuffer.setLong(0, -1L); // Use -1 instead of 0
compressedBuffer.writerIndex(SIZE_OF_UNCOMPRESSED_LENGTH);
uncompressedBuffer.close();
return compressedBuffer;
}
```

When a reader sees `prefix=-1`, it returns an empty/zero-length slice — which
is correct for an originally empty buffer.

### Environment

- Affected: All Arrow Java versions with IPC compression (1.0.0+)
- Readers that fail: Arrow C++ (all versions), PyArrow (all versions)
- Codec: Both LZ4_FRAME and ZSTD

### Related

- #1116 — similar symptom (prefix=0) but different root cause (race condition
in vector reuse, not the intentional empty buffer path)
- apache/arrow#15102 — C++ DecompressBuffer fix for prefix=-1 (does not
handle prefix=0)
```

Guia de contribuição

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Direção de pesquisa

Comece em vector/src/main/java/org/apache/arrow/vector/compression/AbstractCompressionCodec.java, no branch de buffer vazio, e compare o tratamento do prefixo com o sentinel -1 documentado. Reproduza o caso com uma coluna de strings contendo apenas strings vazias usando LZ4_FRAME ou ZSTD e, em seguida, verifique se os readers de C++ e Python conseguem consumir o stream IPC resultante sem erros de descompressão.

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Avaliação

Stack de tecnologia
java
Domínio
data-engineering
Tipo de issue
Bug
Dificuldade
2/5
Tempo estimado
1-3 horas
Status de atividade
Pouca atividade
Clareza
Claramente especificada
Facilidade para iniciantes
68/100

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