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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
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37 changes: 19 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 @@ -529,50 +529,51 @@ 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) {
if (ARROW_PREDICT_FALSE(!builder.nullable)) {
Status AppendNull(BuilderPtr parent, int field_index, BuilderPtr* builder) {
if (ARROW_PREDICT_FALSE(!builder->nullable)) {
return ParseError("a required field was null");
}
switch (builder.kind) {
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));
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;
builder->index += 1;

// update the parent, since changing builder doesn't affect parent
if (parent.kind == Kind::kArray) {
Cast<Kind::kArray>(parent)->value_builder(builder);
Cast<Kind::kArray>(parent)->value_builder(*builder);
} else {
Cast<Kind::kObject>(parent)->field_builder(field_index, builder);
Cast<Kind::kObject>(parent)->field_builder(field_index, *builder);
}
return Status::OK();
}
case Kind::kBoolean:
return Cast<Kind::kBoolean>(builder)->AppendNull();
return Cast<Kind::kBoolean>(*builder)->AppendNull();

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

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

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

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

case Kind::kObject: {
auto struct_builder = Cast<Kind::kObject>(builder);
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, &field_builder));
}
return Status::OK();
}
Expand Down Expand Up @@ -672,7 +673,7 @@ class HandlerBase : public BlockParser,
///
/// @{
bool Null() {
status_ = builder_set_.AppendNull(builder_stack_.back(), field_index_, builder_);
status_ = builder_set_.AppendNull(builder_stack_.back(), field_index_, &builder_);
return status_.ok();
}

Expand Down Expand Up @@ -869,7 +870,7 @@ class HandlerBase : public BlockParser,
if (ARROW_PREDICT_FALSE(!field_builder.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, &field_builder));
}
absent_fields_stack_.Pop();
EndNested();
Expand Down
19 changes: 19 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 @@ -235,6 +235,17 @@ 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]]"});
}
}

TEST(BlockParser, Basics) {
auto options = ParseOptions::Defaults();
options.unexpected_field_behavior = UnexpectedFieldBehavior::InferType;
Expand Down Expand Up @@ -267,6 +278,14 @@ 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]])"});
}

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