[HuaHuaY]

Rationale for this change

The Parquet writer checks the leaf array's physical null count without accounting for the parent struct's validity bitmap. As a result, it rejects nulls in required child fields even when they are masked by null parent rows.

What changes are included in this PR?

Corrected the logic used by the column writer to validate the null values for required leaf fields.

Are these changes tested?

Yes.

Are there any user-facing changes?

No.

Was AI used for this PR?

In accordance to the AI generation guidelines, please disclose below whether and how AI was used in this PR.

PR code and description written by:

• Human
• AI

Reviewed before submission by:

• Human
• AI
• Not reviewed

• GitHub Issue: [C++][Parquet] Null value in nested non-nullable columns fails to write when the parent is null #55124

[Copilot]

🟢 Approval recommended

The focused validation change follows definition-level semantics and includes regression coverage for both accepted and rejected cases.

No open issues 🎉

What changed in this PR

Fixes Parquet writing of required child fields whose null values are masked by null ancestors, addressing GH-55124 while preserving rejection of unmasked nulls.

Changes:

• Validate required leaf nulls against ancestor definition levels.
• Preserve definition levels for required leaves so invalid nulls remain detectable.
• Add regression coverage for masked nulls, null and empty lists, and invalid unmasked nulls.

File	Description
cpp/​src/​parquet/​column_writer.cc	Distinguishes masked from unmasked required-leaf nulls.
cpp/​src/​parquet/​arrow/​path_internal.cc	Prevents required-leaf nulls from lowering definition levels.
cpp/​src/​parquet/​arrow/​arrow_reader_writer_test.cc	Tests valid masked nulls and rejection of unmasked nulls.

_{Review effort: Balanced}