StrictMetricsEvaluator treats unknown record_count as rows must match

[kevinjqliu]

StrictMetricsEvaluator::Evaluate currently returns ROWS_MUST_MATCH when data_file.record_count <= 0:

iceberg-cpp/src/iceberg/expression/strict_metrics_evaluator.cc

Lines 534 to 537 in c0c6b01

	Result<bool> StrictMetricsEvaluator::Evaluate(const DataFile& data_file) const {
	if (data_file.record_count <= 0) {
	return kRowsMustMatch;
	}

This is correct for record_count == 0, but not for record_count == -1. iceberg-cpp uses -1 as an unknown row-count sentinel when writer metrics do not include row_count:

• data writer:

  iceberg-cpp/src/iceberg/data/data_writer.cc

  Lines 80 to 86 in c0c6b01

  	auto data_file = std::make_shared<DataFile>(DataFile{
  	.content = DataFile::Content::kData,
  	.file_path = options_.path,
  	.file_format = options_.format,
  	.partition = options_.partition,
  	.record_count = metrics.row_count.value_or(-1),
  	.file_size_in_bytes = length,

• equality delete writer:

  iceberg-cpp/src/iceberg/data/equality_delete_writer.cc

  Lines 80 to 86 in c0c6b01

  	auto data_file = std::make_shared<DataFile>(DataFile{
  	.content = DataFile::Content::kEqualityDeletes,
  	.file_path = options_.path,
  	.file_format = options_.format,
  	.partition = options_.partition,
  	.record_count = metrics.row_count.value_or(-1),
  	.file_size_in_bytes = length,

• position delete writer:

  iceberg-cpp/src/iceberg/data/position_delete_writer.cc

  Lines 140 to 146 in c0c6b01

  	auto data_file = std::make_shared<DataFile>(DataFile{
  	.content = DataFile::Content::kPositionDeletes,
  	.file_path = options_.path,
  	.file_format = options_.format,
  	.partition = options_.partition,
  	.record_count = metrics.row_count.value_or(-1),
  	.file_size_in_bytes = length,

Other code already treats negative record count as unknown/missing:

• inclusive metrics:

  iceberg-cpp/src/iceberg/expression/inclusive_metrics_evaluator.cc

  Lines 507 to 516 in c0c6b01

  	Result<bool> InclusiveMetricsEvaluator::Evaluate(const DataFile& data_file) const {
  	if (data_file.record_count == 0) {
  	return kRowCannotMatch;
  	}
  	if (data_file.record_count < 0) {
  	// we haven't implemented parsing record count from avro file and thus set record
  	// count -1 when importing avro tables to iceberg tables. This should be updated once
  	// we implemented and set correct record count.
  	return kRowsMightMatch;
  	}

• count aggregate:

  iceberg-cpp/src/iceberg/expression/aggregate.cc

  Lines 379 to 388 in c0c6b01

  	Result<int64_t> CountStarAggregate::CountFor(const DataFile& file) const {
  	if (!HasValue(file)) {
  	return NotFound("Record count is missing");
  	}
  	return file.record_count;
  	}
  	bool CountStarAggregate::HasValue(const DataFile& file) const {
  	return file.record_count >= 0;
  	}

Impact: a file with unknown row count can be treated as if every row must match, even for predicates like AlwaysFalse.

Suggested fix: only special-case record_count == 0; let negative/unknown counts fall through to normal strict metrics evaluation.

A focused regression test with record_count = -1 and AlwaysFalse fails before changing <= 0 to == 0, and passes after.