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[client-v2] readVariant discards the active type discriminant — colliding Variant alternatives are indistinguishable

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Evaluación

Dificultad
4/5
Tiempo estimado
3-5 días
Aptitud para principiantes
45/100
Tipo de issue
Error
Claridad
Bastante claro
Estado de actividad
Tranquilo
Stack tecnológico
java
Área
api, backend-api-design

Línea de trabajo

Empieza en client-v2/src/main/java/com/clickhouse/client/api/data_formats/internal/BinaryStreamReader.java, en readVariant alrededor de las líneas 892-898, y luego sigue el caso Variant en readValue alrededor de las líneas 248-250. Compara los enfoques propuestos de wrapper o subtipo resuelto y comprueba el procesador RowBinary legacy de clickhouse-data para ver el mismo patrón. Se considera terminado cuando los llamadores pueden distinguir alternativas en conflicto como DateTime y DateTime64, con pruebas que cubran el discriminante.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

area:data-type bug client-api-v2

Description

BinaryStreamReader.readVariant(ClickHouseColumn) reads the 1-byte Variant discriminant (ordNum) only to select which nested column's reader to run, then returns just the decoded value and throws the discriminant away:

// client-v2/src/main/java/com/clickhouse/client/api/data_formats/internal/BinaryStreamReader.java:892
public Object readVariant(ClickHouseColumn column) throws IOException {
    int ordNum = readByte() & 0xFF;
    if (ordNum == 0xFF) {
        return null;
    }
    return readValue(column.getNestedColumns().get(ordNum)); // ordNum (the active type) is discarded
}

For many Variants the active alternative can be inferred from the returned object's runtime class, but for alternatives whose readers produce the same Java runtime type the information is lost, and the consumer cannot recover which ClickHouse subtype was actually on the wire. Examples where the decoded objects are indistinguishable:

Variant both alternatives decode to why ambiguous
Variant(DateTime, DateTime64(3)) java.time.ZonedDateTime (lines 186-191 → convertDateTime, typeHint == null → returns ZonedDateTime) same class, same instant
Variant(String, FixedString(N)) java.lang.String same class
Variant(Decimal32(s), Decimal64(s)) java.math.BigDecimal same class/scale

Because readVariant returns a bare Object, a downstream consumer (text/JSON renderer, type-aware formatter, or any caller inspecting the value) has no way to choose the correct subtype for formatting in these cases.

This is the Java analogue of ClickHouse/clickhouse-js#910. Note the specific examples in that JS issue do not collide here — clickhouse-java maps Enum8/Enum16 to a dedicated EnumValue (lines 172-181) and Date to LocalDate (line 183), so Variant(UInt8, Enum8(...)) and Variant(Date, DateTime) are recoverable from the runtime type. The underlying limitation — the discriminant index is not surfaced — is the same, and other type pairs (above) hit it.

ClickHouse server version

26.6.1.1193 (collision behavior determined by code analysis of the type→class mapping; not exercised end-to-end against the server).

Reproduction

A Variant(DateTime, DateTime64(3)) column: whichever alternative the server picks, readVariant returns a ZonedDateTime, so the caller cannot tell DateTime from DateTime64.

// Conceptual unit-level repro against BinaryStreamReader
ClickHouseColumn col = ClickHouseColumn.of("v", "Variant(DateTime, DateTime64(3))");

// Wire bytes: discriminant 0x01 selects the DateTime64 alternative (alternatives are
// sorted by ClickHouse's global type-name ordering), followed by its encoded value.
byte[] payload = /* 0x01 ++ encoded DateTime64 value */;
BinaryStreamReader reader = new BinaryStreamReader(
        new ByteArrayInputStream(payload), null, LZ4_FACTORY, null);

Object value = reader.readVariant(col);

// EXPECTED: caller can determine the active alternative was DateTime64 (index 1)
// ACTUAL:   value is a java.time.ZonedDateTime, identical to what the DateTime
//           alternative (index 0) would have produced — the active type is unrecoverable.
assertTrue(value instanceof ZonedDateTime); // passes for BOTH alternatives

The same happens through the high-level reader path (readValue → case Variant at line 248-250): reading a Variant(String, FixedString(10)) column yields a String with no indication of which alternative it was.

Suggested fix

Surface the active alternative index alongside the value rather than discarding it — mirroring the upstream proposal. Options:

  • Return a small wrapper, e.g. VariantValue { int typeIndex; Object value; }, from readVariant (breaking change to the public Object readVariant(...) signature — gate behind a minor/major bump), or
  • Expose the resolved subtype (column.getNestedColumns().get(ordNum)) so a renderer can format with the exact ClickHouseColumn.

Buggy code: BinaryStreamReader.readVariant at client-v2/src/main/java/com/clickhouse/client/api/data_formats/internal/BinaryStreamReader.java:892-898 (dispatched from readValue case Variant at line 248-250). The legacy clickhouse-data RowBinary processor should be checked for the same pattern.

Link

Relayed from ClickHouse/clickhouse-js#910.

Lenguaje dominante
Java
Estrellas
1.6k
Forks
638
Merge medio
2 d 14 h
PR fusionados (30 d)
44

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