GH-47417: [C++] Fix JSON parser losing nulls in a null-typed list

[advitrocks9]

Rationale for this change

{"a": [null, 1]} parses to [[1]]. Nothing raises, and validate(full=True) is the only thing
that notices:

Offset invariant failure: offset for slot 1 out of bounds: 2 > 1

Debug builds abort on a DCHECK instead, at parser.cc:540 if the second element is null and
parser.cc:1151 if it is typed.

What changes are included in this PR?

RawBuilderSet::AppendNull takes the child builder by value. Builders of Kind::kNull have no
arena, their null count is packed into BuilderPtr::index, so the increment updates the parent
but is lost to the caller. The next null then re-increments the same stale value, which leaves an
all-null list as NullArray(1) and a promoted list with its leading nulls missing.

It now takes a BuilderPtr*, matching MakeBuilder in the same class. The regression came in
with b7054c21aab and has shipped since 0.14.0.

Are these changes tested?

BlockParser.NullsInList covers both arms, and BlockParserWithSchema.NullsInList runs an
explicit list(null()) schema under Error and Ignore. arrow-json-test passes 262/262 in
Debug and Release. Reverting parser.cc with the tests in place aborts both in Debug and fails
both on null count in Release.

Are there any user-facing changes?

Those inputs parse correctly now. This is not inference-only: an explicit list(null()) schema
reaches the same arm under both Error and Ignore.

This PR contains a "Critical Fix". Well-formed JSON silently loses list elements in release
builds and aborts on a DCHECK in debug builds.

• GitHub Issue: [C++] Assertion fires when trying to parse json #47417

[github-actions]

⚠️ GitHub issue #47417 has been automatically assigned in GitHub to PR creator.

[Copilot]

Pull request overview

Fixes a long-standing JSON parsing bug in the C++ BlockParser where null elements could be silently dropped (or trigger DCHECKs in debug) when appending nulls to a null-typed list, by ensuring the inline null-count state in BuilderPtr is updated in-place.

Changes:

• Change RawBuilderSet::AppendNull to take a BuilderPtr* so increments to the inline null-count for Kind::kNull persist to the caller.
• Update all AppendNull call sites to pass builder pointers accordingly (including nested object-field null propagation).
• Add a regression test covering nulls inside lists (null-only and promoted-from-null cases).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
cpp/src/arrow/json/parser.cc	Fixes null appends by mutating the BuilderPtr in-place, preventing lost null counts in null-typed list builders.
cpp/src/arrow/json/parser_test.cc	Adds a regression test to ensure nulls inside lists are preserved and don’t break list offsets.

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[advitrocks9]

Added a test for the explicit schema under Error and Ignore.

[github-actions]

⚠️ GitHub issue #47417 has been automatically assigned in GitHub to PR creator.

[Copilot]

🟢 Approval recommended

The fix is narrowly scoped, updates all relevant call sites, and adds targeted regression tests exercising both the all-null and promotion scenarios described.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0 new
• Review effort level: Lite