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AVRO-4228: [c++] Fix BinaryDecoder::arrayNext() to handle negative block counts #3646

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AVRO-4228: [c++] Fix BinaryDecoder::arrayNext() to handle negative block counts #3646

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8be6e9c
AVRO-4228: Fix BinaryDecoder::arrayNext() to handle negative block co…
Feb 8, 2026
dfe6815
AVRO-4228: Add test for arrayNext() with negative block counts
Feb 8, 2026
cd52d25
AVRO-4228: Move negative block count to second block in test
Feb 9, 2026
8e7dd3a
AVRO-4228: Avoid undefined behavior when negating INT64_MIN in doDeco…
Feb 9, 2026
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4 changes: 2 additions & 2 deletions lang/c++/impl/BinaryDecoder.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -164,13 +164,13 @@ size_t BinaryDecoder::doDecodeItemCount() {
auto result = doDecodeLong();
if (result < 0) {
doDecodeLong();
return static_cast<size_t>(-result);
return static_cast<size_t>(-(result + 1)) + 1;
}
return static_cast<size_t>(result);
}

size_t BinaryDecoder::arrayNext() {
return static_cast<size_t>(doDecodeLong());
return doDecodeItemCount();
}

size_t BinaryDecoder::skipArray() {
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35 changes: 35 additions & 0 deletions lang/c++/test/CodecTests.cc
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Expand Up @@ -2100,6 +2100,40 @@ static void testJsonCodecReinit() {
}
}

static void testArrayNegativeBlockCount() {
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@martin-g martin-g Feb 9, 2026

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This test pass even without the fix above.
The reason is that the negative count is in the first block and this uses arrayStart() which already delegates to doDecodeItemCount().

Please update the test to use a negative count [also] in the second block.

// Array of ints [10, 20, 30, 40, 50] encoded with a negative block count
// in the second block, which exercises arrayNext().
// Per the Avro spec, a negative count means: abs(count) items follow,
// preceded by a long byte-size of the block.
//
// Block 1: count=2, items: 10, 20 (read by arrayStart)
// Block 2: count=-3, bytesize=3, items: 30, 40, 50 (read by arrayNext)
// Terminal: count=0
const uint8_t data[] = {
0x04, // zigzag(2) = 4: block count = 2
0x14, 0x28, // zigzag ints: 10, 20
0x05, // zigzag(-3) = 5: block count = -3
0x06, // zigzag(3) = 6: byte-size of block
0x3c, 0x50, 0x64, // zigzag ints: 30, 40, 50
0x00 // terminal
};

InputStreamPtr is = memoryInputStream(data, sizeof(data));
DecoderPtr d = binaryDecoder();
d->init(*is);

std::vector<int32_t> result;
for (size_t n = d->arrayStart(); n != 0; n = d->arrayNext()) {
for (size_t i = 0; i < n; ++i) {
result.push_back(d->decodeInt());
}
}

const std::vector<int32_t> expected = {10, 20, 30, 40, 50};
BOOST_CHECK_EQUAL_COLLECTIONS(result.begin(), result.end(),
expected.begin(), expected.end());
}

static void testByteCount() {
OutputStreamPtr os1 = memoryOutputStream();
EncoderPtr e1 = binaryEncoder();
Expand All @@ -2125,6 +2159,7 @@ init_unit_test_suite(int, char *[]) {
ts->add(BOOST_PARAM_TEST_CASE(&avro::testJson, avro::jsonData,
ENDOF(avro::jsonData)));
ts->add(BOOST_TEST_CASE(avro::testJsonCodecReinit));
ts->add(BOOST_TEST_CASE(avro::testArrayNegativeBlockCount));
ts->add(BOOST_TEST_CASE(avro::testByteCount));

return ts;
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