chore(dataobj): add logs tracking and encoding

This commit is the equivalent of grafana#15713 for the logs section: log
records are accumulated into column builders and then flushed into a
section at encoding time.

Some minor API changes have been made to sections/streams to ensure
consistency with sections/logs.

pkg/dataobj/internal/sections/logs/iter.go

@@ -0,0 +1,136 @@
+package logs
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/grafana/loki/pkg/push"
+
+	"github.com/grafana/loki/v3/pkg/dataobj/internal/dataset"
+	"github.com/grafana/loki/v3/pkg/dataobj/internal/encoding"
+	"github.com/grafana/loki/v3/pkg/dataobj/internal/metadata/datasetmd"
+	"github.com/grafana/loki/v3/pkg/dataobj/internal/metadata/filemd"
+	"github.com/grafana/loki/v3/pkg/dataobj/internal/metadata/logsmd"
+	"github.com/grafana/loki/v3/pkg/dataobj/internal/result"
+)
+
+// Iter iterates over records in the provided decoder. All logs sections are
+// iterated over in order.
+func Iter(ctx context.Context, dec encoding.Decoder) result.Seq[Record] {
+	return result.Iter(func(yield func(Record) bool) error {
+		sections, err := dec.Sections(ctx)
+		if err != nil {
+			return err
+		}
+
+		logsDec := dec.LogsDecoder()
+
+		for _, section := range sections {
+			if section.Type != filemd.SECTION_TYPE_LOGS {
+				continue
+			}
+
+			for result := range iterSection(ctx, logsDec, section) {
+				if result.Err() != nil || !yield(result.MustValue()) {
+					return result.Err()
+				}
+			}
+		}
+
+		return nil
+	})
+}
+
+func iterSection(ctx context.Context, dec encoding.LogsDecoder, section *filemd.SectionInfo) result.Seq[Record] {
+	return result.Iter(func(yield func(Record) bool) error {
+		// We need to pull the columns twice: once from the dataset implementation
+		// and once for the metadata to retrieve column type.
+		//
+		// TODO(rfratto): find a way to expose this information from
+		// encoding.StreamsDataset to avoid the double call.
+		streamsColumns, err := dec.Columns(ctx, section)
+		if err != nil {
+			return err
+		}
+
+		dset := encoding.LogsDataset(dec, section)
+
+		columns, err := result.Collect(dset.ListColumns(ctx))
+		if err != nil {
+			return err
+		}
+
+		for result := range dataset.Iter(ctx, columns) {
+			row, err := result.Value()
+			if err != nil {
+				return err
+			}
+
+			record, err := decodeRecord(streamsColumns, row)
+			if err != nil {
+				return err
+			} else if !yield(record) {
+				return nil
+			}
+		}
+
+		return nil
+	})
+}
+
+func decodeRecord(columns []*logsmd.ColumnDesc, row dataset.Row) (Record, error) {
+	record := Record{
+		// Preallocate metadata to exact number of metadata columns to avoid
+		// oversizing.
+		Metadata: make(push.LabelsAdapter, 0, metadataColumns(columns)),
+	}
+
+	for columnIndex, columnValue := range row.Values {
+		if columnValue.IsNil() || columnValue.IsZero() {
+			continue
+		}
+
+		column := columns[columnIndex]
+		switch column.Type {
+		case logsmd.COLUMN_TYPE_STREAM_ID:
+			if ty := columnValue.Type(); ty != datasetmd.VALUE_TYPE_INT64 {
+				return Record{}, fmt.Errorf("invalid type %s for %s", ty, column.Type)
+			}
+			record.StreamID = columnValue.Int64()
+
+		case logsmd.COLUMN_TYPE_TIMESTAMP:
+			if ty := columnValue.Type(); ty != datasetmd.VALUE_TYPE_INT64 {
+				return Record{}, fmt.Errorf("invalid type %s for %s", ty, column.Type)
+			}
+			record.Timestamp = time.Unix(0, columnValue.Int64()).UTC()
+
+		case logsmd.COLUMN_TYPE_METADATA:
+			if ty := columnValue.Type(); ty != datasetmd.VALUE_TYPE_STRING {
+				return Record{}, fmt.Errorf("invalid type %s for %s", ty, column.Type)
+			}
+			record.Metadata = append(record.Metadata, push.LabelAdapter{
+				Name:  column.Info.Name,
+				Value: columnValue.String(),
+			})
+
+		case logsmd.COLUMN_TYPE_MESSAGE:
+			if ty := columnValue.Type(); ty != datasetmd.VALUE_TYPE_STRING {
+				return Record{}, fmt.Errorf("invalid type %s for %s", ty, column.Type)
+			}
+			record.Line = columnValue.String()
+		}
+	}
+
+	return record, nil
+}
+
+func metadataColumns(columns []*logsmd.ColumnDesc) int {
+	var count int
+	for _, column := range columns {
+		if column.Type == logsmd.COLUMN_TYPE_METADATA {
+			count++
+		}
+	}
+	return count
+}