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GH-51779: [C++][IPC] Read a DictionaryEncoding without indexType as int32 indices #51807

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GH-51779: [C++][IPC] Read a DictionaryEncoding without indexType as int32 indices #51807

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GH-51779: [C++][IPC] Read a DictionaryEncoding without indexType as i…
CaptainAni187 Oct 5, 2026
7453d8a
Use WriteFlatbufferBuilder in the test and check the memo registration
CaptainAni187 Oct 9, 2026
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25 changes: 25 additions & 0 deletions cpp/src/arrow/ipc/message_internal_test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -114,6 +114,31 @@ TEST(TestMessageInternal, TestEndiannessRoundtrip) {
}
}

// GH-51779: Schema.fbs allows DictionaryEncoding.indexType to be omitted, in
// which case the indices are signed int32.
TEST(TestMessageInternal, DictionaryEncodingWithoutIndexType) {
FBB fbb;
auto name = fbb.CreateString("f0");
auto value_type = flatbuf::CreateUtf8(fbb);
auto encoding = flatbuf::CreateDictionaryEncoding(fbb, /*id=*/0, /*indexType=*/0);
auto fb_field = flatbuf::CreateField(fbb, name, /*nullable=*/true, flatbuf::Type_Utf8,
value_type.Union(), encoding);
auto fb_schema = flatbuf::CreateSchema(fbb, flatbuf::Endianness_Little,
fbb.CreateVector({fb_field}));
fbb.Finish(flatbuf::CreateMessage(fbb, flatbuf::MetadataVersion_V5,
flatbuf::MessageHeader_Schema, fb_schema.Union()));
ASSERT_OK_AND_ASSIGN(auto metadata, WriteFlatbufferBuilder(fbb));

ASSERT_OK_AND_ASSIGN(auto message, Message::Open(metadata, /*body=*/nullptr));
DictionaryMemo memo;
ASSERT_OK_AND_ASSIGN(auto schema, ReadSchema(*message, &memo));

@ianmcook ianmcook Oct 9, 2026

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Could we also check that the field was registered in memo? The schema comparison passes even if the new code path never registers the field. A reader needs that registration to match the field to the DictionaryBatch and RecordBatch that follow, and that's where #51779 failed. Something like:

  ASSERT_OK_AND_EQ(0, memo.fields().GetFieldId({0}));
  ASSERT_OK_AND_ASSIGN(auto dict_value_type, memo.GetDictionaryType(0));
  AssertTypeEqual(utf8(), dict_value_type);

@CaptainAni187 CaptainAni187 Oct 9, 2026

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Added in 7453d8a: the test now checks the field ID and that the dictionary value type is utf8().

AssertSchemaEqual(*::arrow::schema({field("f0", dictionary(int32(), utf8()))}),
*schema);
ASSERT_OK_AND_EQ(0, memo.fields().GetFieldId({0}));
ASSERT_OK_AND_ASSIGN(auto dict_value_type, memo.GetDictionaryType(0));
AssertTypeEqual(utf8(), dict_value_type);
}

struct SampleMessageParams {
std::shared_ptr<const KeyValueMetadata> custom_metadata = {};
int64_t body_length = 0;
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8 changes: 6 additions & 2 deletions cpp/src/arrow/ipc/metadata_internal.cc
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Expand Up @@ -898,8 +898,12 @@ Status FieldFromFlatbuffer(const flatbuf::Field* field, FieldPosition field_pos,
// dictionary_memo
std::shared_ptr<DataType> index_type;
auto int_data = encoding->indexType();
CHECK_FLATBUFFERS_NOT_NULL(int_data, "DictionaryEncoding.indexType");
RETURN_NOT_OK(IntFromFlatbuffer(int_data, &index_type));
if (int_data == nullptr) {
// Schema.fbs: "If this field is null, the indices must be signed int32."
index_type = int32();
} else {
RETURN_NOT_OK(IntFromFlatbuffer(int_data, &index_type));
}
dict_value_type = type;
ARROW_ASSIGN_OR_RAISE(type,
DictionaryType::Make(index_type, type, encoding->isOrdered()));
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