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GH-47417: [C++] Fix JSON parser losing nulls in a null-typed list #51108

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GH-47417: [C++] Fix JSON parser losing nulls in a null-typed list #51108

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a5db8c8
GH-47417: [C++] Fix JSON parser dropping nulls at the start of a list
advitrocks9 Aug 25, 2026
609dc09
GH-47417: [C++] Cover the explicit list(null()) schema under Error an…
advitrocks9 Sep 3, 2026
1c376e9
Merge remote-tracking branch 'origin/main' into fix-gh-47417
advitrocks9 Oct 8, 2026
2bc1be2
GH-47417: [C++] Return the updated builder from AppendNull
advitrocks9 Oct 22, 2026
d29f49e
GH-47417: [C++] Inline the nullable check for absent fields
advitrocks9 Oct 22, 2026
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41 changes: 23 additions & 18 deletions cpp/src/arrow/json/parser.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -526,16 +526,15 @@ class RawBuilderSet {

/// Appending null is slightly tricky since null count is stored inline
/// for builders of Kind::kNull. Append nulls using this helper
Status AppendNull(BuilderPtr parent, int field_index, BuilderPtr builder) {
Result<BuilderPtr> AppendNull(BuilderPtr parent, int field_index) {
BuilderPtr builder = parent.kind == Kind::kArray
? Cast<Kind::kArray>(parent)->value_builder()
: Cast<Kind::kObject>(parent)->field_builder(field_index);
if (ARROW_PREDICT_FALSE(!builder.nullable)) {
return ParseError("a required field was null");
}
switch (builder.kind) {
case Kind::kNull: {
DCHECK_EQ(builder, parent.kind == Kind::kArray
? Cast<Kind::kArray>(parent)->value_builder()
: Cast<Kind::kObject>(parent)->field_builder(field_index));

// increment null count stored inline
builder.index += 1;

Expand All @@ -545,38 +544,43 @@ class RawBuilderSet {
} else {
Cast<Kind::kObject>(parent)->field_builder(field_index, builder);
}
return Status::OK();
break;
}
case Kind::kBoolean:
return Cast<Kind::kBoolean>(builder)->AppendNull();
RETURN_NOT_OK(Cast<Kind::kBoolean>(builder)->AppendNull());
break;

case Kind::kNumber:
return Cast<Kind::kNumber>(builder)->AppendNull();
RETURN_NOT_OK(Cast<Kind::kNumber>(builder)->AppendNull());
break;

case Kind::kString:
return Cast<Kind::kString>(builder)->AppendNull();
RETURN_NOT_OK(Cast<Kind::kString>(builder)->AppendNull());
break;

case Kind::kNumberOrString: {
return Cast<Kind::kNumberOrString>(builder)->AppendNull();
RETURN_NOT_OK(Cast<Kind::kNumberOrString>(builder)->AppendNull());
break;
}

case Kind::kArray:
return Cast<Kind::kArray>(builder)->AppendNull();
RETURN_NOT_OK(Cast<Kind::kArray>(builder)->AppendNull());
break;

case Kind::kObject: {
auto struct_builder = Cast<Kind::kObject>(builder);
RETURN_NOT_OK(struct_builder->AppendNull());

for (int i = 0; i < struct_builder->num_fields(); ++i) {
auto field_builder = struct_builder->field_builder(i);
RETURN_NOT_OK(AppendNull(builder, i, field_builder));
RETURN_NOT_OK(AppendNull(builder, i));
}
return Status::OK();
break;
}

default:
return Status::NotImplemented("invalid builder Kind");
}
return builder;
}

Status Finish(const std::shared_ptr<Array>& scalar_values, BuilderPtr builder,
Expand Down Expand Up @@ -669,7 +673,9 @@ class ParseImpl : public BlockParser {

Status Null() {
DCHECK(!builder_stack_.empty());
return builder_set_.AppendNull(builder_stack_.back(), field_index_, builder_);
ARROW_ASSIGN_OR_RAISE(builder_,
builder_set_.AppendNull(builder_stack_.back(), field_index_));
return Status::OK();
Comment on lines +676 to +678

@taepper taepper Oct 22, 2026

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I like this a lot! :)

}

Status HandleUnexpectedField(std::string_view key, sj::value& value) {
Expand Down Expand Up @@ -962,11 +968,10 @@ class ParseImpl : public BlockParser {
if (!absent_fields_stack_[i]) {
continue;
}
auto field_builder = Cast<Kind::kObject>(parent)->field_builder(i);
if (ARROW_PREDICT_FALSE(!field_builder.nullable)) {
if (ARROW_PREDICT_FALSE(!Cast<Kind::kObject>(parent)->field_builder(i).nullable)) {
return ParseError("a required field was absent");
}
RETURN_NOT_OK(builder_set_.AppendNull(parent, i, field_builder));
RETURN_NOT_OK(builder_set_.AppendNull(parent, i));
}
absent_fields_stack_.Pop();
EndNested();
Expand Down
27 changes: 27 additions & 0 deletions cpp/src/arrow/json/parser_test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -236,6 +236,22 @@ TEST(BlockParserWithSchema, FailOnIncompleteJson) {
ASSERT_RAISES(Invalid, ParseFromString(options, "{\"a\":0, \"b\"", &parsed));
}

TEST(BlockParserWithSchema, NullsInList) {
auto options = ParseOptions::Defaults();
options.explicit_schema = schema({field("a", list(null()))});
for (auto behavior :
{UnexpectedFieldBehavior::Error, UnexpectedFieldBehavior::Ignore}) {
options.unexpected_field_behavior = behavior;
AssertParseColumns(options, R"({"a": [null, null]})", {field("a", list(null()))},
{"[[null, null]]"});
}

auto type = list(struct_({field("b", null())}));
options.explicit_schema = schema({field("a", type)});
AssertParseColumns(options, R"({"a": [{"b": null}, null]})", {field("a", type)},
{R"([[{"b": null}, null]])"});
}

TEST(BlockParserWithSchema, ValidateIgnoredFields) {
auto options = ParseOptions::Defaults();
options.explicit_schema = schema({field("known", int64())});
Expand Down Expand Up @@ -286,6 +302,17 @@ TEST(BlockParser, Null) {
R"([{"plain": null}, {"plain": null}])"});
}

TEST(BlockParser, NullsInList) {
auto options = ParseOptions::Defaults();
options.unexpected_field_behavior = UnexpectedFieldBehavior::InferType;
AssertParseColumns(options, R"({"a": [null, null], "b": [null, "hi", null]})",
{field("a", list(null())), field("b", list(utf8()))},
{"[[null, null]]", R"([[null, "hi", null]])"});
AssertParseColumns(options, R"({"a": [{"b": null}, null]})",
{field("a", list(struct_({field("b", null())})))},
{R"([[{"b": null}, null]])"});
}

TEST(BlockParser, InferNewFields) {
std::string src = R"(
{}
Expand Down
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