memchunk.go

package chunkenc

import (
	"bufio"
	"bytes"
	"context"
	"encoding/binary"
	"fmt"
	"hash"
	"hash/crc32"
	"io"
	"sort"
	"time"

	"github.com/cespare/xxhash/v2"
	util_log "github.com/cortexproject/cortex/pkg/util/log"
	"github.com/go-kit/kit/log/level"
	"github.com/pkg/errors"
	"github.com/prometheus/prometheus/pkg/labels"

	"github.com/cortexproject/cortex/pkg/chunk/encoding"

	"github.com/grafana/loki/pkg/iter"
	"github.com/grafana/loki/pkg/logproto"
	"github.com/grafana/loki/pkg/logql/log"
	"github.com/grafana/loki/pkg/logqlmodel/stats"
)

const (
	_ byte = iota
	chunkFormatV1
	chunkFormatV2
	chunkFormatV3

	DefaultChunkFormat = chunkFormatV3 // the currently used chunk format

	blocksPerChunk = 10
	maxLineLength  = 1024 * 1024 * 1024

	// defaultBlockSize is used for target block size when cutting partially deleted chunks from a delete request.
	// This could wary from configured block size using `ingester.chunks-block-size` flag or equivalent yaml config resulting in
	// different block size in the new chunk which should be fine.
	defaultBlockSize = 256 * 1024
)

var magicNumber = uint32(0x12EE56A)

// The table gets initialized with sync.Once but may still cause a race
// with any other use of the crc32 package anywhere. Thus we initialize it
// before.
var castagnoliTable *crc32.Table

func init() {
	castagnoliTable = crc32.MakeTable(crc32.Castagnoli)
}

// newCRC32 initializes a CRC32 hash with a preconfigured polynomial, so the
// polynomial may be easily changed in one location at a later time, if necessary.
func newCRC32() hash.Hash32 {
	return crc32.New(castagnoliTable)
}

// MemChunk implements compressed log chunks.
type MemChunk struct {
	// The number of uncompressed bytes per block.
	blockSize int
	// Target size in compressed bytes
	targetSize int

	// The finished blocks.
	blocks []block
	// The compressed size of all the blocks
	cutBlockSize int

	// Current in-mem block being appended to.
	head *headBlock

	// the chunk format default to v2
	format   byte
	encoding Encoding
}

type block struct {
	// This is compressed bytes.
	b          []byte
	numEntries int

	mint, maxt int64

	offset           int // The offset of the block in the chunk.
	uncompressedSize int // Total uncompressed size in bytes when the chunk is cut.
}

// This block holds the un-compressed entries. Once it has enough data, this is
// emptied into a block with only compressed entries.
type headBlock struct {
	// This is the list of raw entries.
	entries []entry
	size    int // size of uncompressed bytes.

	mint, maxt int64
}

func (hb *headBlock) isEmpty() bool {
	return len(hb.entries) == 0
}

func (hb *headBlock) clear() {
	if hb.entries != nil {
		hb.entries = hb.entries[:0]
	}
	hb.size = 0
	hb.mint = 0
	hb.maxt = 0
}

func (hb *headBlock) append(ts int64, line string) error {
	if !hb.isEmpty() && hb.maxt > ts {
		return ErrOutOfOrder
	}

	hb.entries = append(hb.entries, entry{ts, line})
	if hb.mint == 0 || hb.mint > ts {
		hb.mint = ts
	}
	hb.maxt = ts
	hb.size += len(line)

	return nil
}

func (hb *headBlock) serialise(pool WriterPool) ([]byte, error) {
	inBuf := serializeBytesBufferPool.Get().(*bytes.Buffer)
	defer func() {
		inBuf.Reset()
		serializeBytesBufferPool.Put(inBuf)
	}()
	outBuf := &bytes.Buffer{}

	encBuf := make([]byte, binary.MaxVarintLen64)
	compressedWriter := pool.GetWriter(outBuf)
	defer pool.PutWriter(compressedWriter)
	for _, logEntry := range hb.entries {
		n := binary.PutVarint(encBuf, logEntry.t)
		inBuf.Write(encBuf[:n])

		n = binary.PutUvarint(encBuf, uint64(len(logEntry.s)))
		inBuf.Write(encBuf[:n])

		inBuf.WriteString(logEntry.s)
	}

	if _, err := compressedWriter.Write(inBuf.Bytes()); err != nil {
		return nil, errors.Wrap(err, "appending entry")
	}
	if err := compressedWriter.Close(); err != nil {
		return nil, errors.Wrap(err, "flushing pending compress buffer")
	}

	return outBuf.Bytes(), nil
}

// CheckpointBytes serializes a headblock to []byte. This is used by the WAL checkpointing,
// which does not want to mutate a chunk by cutting it (otherwise risking content address changes), but
// needs to serialize/deserialize the data to disk to ensure data durability.
func (hb *headBlock) CheckpointBytes(version byte, b []byte) ([]byte, error) {
	buf := bytes.NewBuffer(b[:0])
	err := hb.CheckpointTo(version, buf)
	return buf.Bytes(), err
}

// CheckpointSize returns the estimated size of the headblock checkpoint.
func (hb *headBlock) CheckpointSize(version byte) int {
	size := 1                                                                 // version
	size += binary.MaxVarintLen32 * 2                                         // total entries + total size
	size += binary.MaxVarintLen64 * 2                                         // mint,maxt
	size += (binary.MaxVarintLen64 + binary.MaxVarintLen32) * len(hb.entries) // ts + len of log line.

	for _, e := range hb.entries {
		size += len(e.s)
	}
	return size
}

// CheckpointTo serializes a headblock to a `io.Writer`. see `CheckpointBytes`.
func (hb *headBlock) CheckpointTo(version byte, w io.Writer) error {
	eb := EncodeBufferPool.Get().(*encbuf)
	defer EncodeBufferPool.Put(eb)

	eb.reset()

	eb.putByte(version)
	_, err := w.Write(eb.get())
	if err != nil {
		return errors.Wrap(err, "write headBlock version")
	}
	eb.reset()

	eb.putUvarint(len(hb.entries))
	eb.putUvarint(hb.size)
	eb.putVarint64(hb.mint)
	eb.putVarint64(hb.maxt)

	_, err = w.Write(eb.get())
	if err != nil {
		return errors.Wrap(err, "write headBlock metas")
	}
	eb.reset()

	for _, entry := range hb.entries {
		eb.putVarint64(entry.t)
		eb.putUvarint(len(entry.s))
		_, err = w.Write(eb.get())
		if err != nil {
			return errors.Wrap(err, "write headBlock entry ts")
		}
		eb.reset()

		_, err := io.WriteString(w, entry.s)
		if err != nil {
			return errors.Wrap(err, "write headblock entry line")
		}
	}
	return nil
}

func (hb *headBlock) FromCheckpoint(b []byte) error {
	if len(b) < 1 {
		return nil
	}

	db := decbuf{b: b}

	version := db.byte()
	if db.err() != nil {
		return errors.Wrap(db.err(), "verifying headblock header")
	}
	switch version {
	case chunkFormatV1, chunkFormatV2, chunkFormatV3:
	default:
		return errors.Errorf("incompatible headBlock version (%v), only V1,V2,V3 is currently supported", version)
	}

	ln := db.uvarint()
	hb.size = db.uvarint()
	hb.mint = db.varint64()
	hb.maxt = db.varint64()

	if err := db.err(); err != nil {
		return errors.Wrap(err, "verifying headblock metadata")
	}

	hb.entries = make([]entry, ln)
	for i := 0; i < ln && db.err() == nil; i++ {
		var entry entry
		entry.t = db.varint64()
		lineLn := db.uvarint()
		entry.s = string(db.bytes(lineLn))
		hb.entries[i] = entry
	}

	if err := db.err(); err != nil {
		return errors.Wrap(err, "decoding entries")
	}

	return nil
}

type entry struct {
	t int64
	s string
}

// NewMemChunk returns a new in-mem chunk.
func NewMemChunk(enc Encoding, blockSize, targetSize int) *MemChunk {
	c := &MemChunk{
		blockSize:  blockSize,  // The blockSize in bytes.
		targetSize: targetSize, // Desired chunk size in compressed bytes
		blocks:     []block{},

		head:   &headBlock{},
		format: DefaultChunkFormat,

		encoding: enc,
	}

	return c
}

// NewByteChunk returns a MemChunk on the passed bytes.
func NewByteChunk(b []byte, blockSize, targetSize int) (*MemChunk, error) {
	bc := &MemChunk{
		head:       &headBlock{}, // Dummy, empty headblock.
		blockSize:  blockSize,
		targetSize: targetSize,
	}
	db := decbuf{b: b}

	// Verify the header.
	m, version := db.be32(), db.byte()
	if db.err() != nil {
		return nil, errors.Wrap(db.err(), "verifying header")
	}
	if m != magicNumber {
		return nil, errors.Errorf("invalid magic number %x", m)
	}
	bc.format = version
	switch version {
	case chunkFormatV1:
		bc.encoding = EncGZIP
	case chunkFormatV2, chunkFormatV3:
		// format v2+ has a byte for block encoding.
		enc := Encoding(db.byte())
		if db.err() != nil {
			return nil, errors.Wrap(db.err(), "verifying encoding")
		}
		bc.encoding = enc
	default:
		return nil, errors.Errorf("invalid version %d", version)
	}

	metasOffset := binary.BigEndian.Uint64(b[len(b)-8:])
	mb := b[metasOffset : len(b)-(8+4)] // storing the metasOffset + checksum of meta
	db = decbuf{b: mb}

	expCRC := binary.BigEndian.Uint32(b[len(b)-(8+4):])
	if expCRC != db.crc32() {
		return nil, ErrInvalidChecksum
	}

	// Read the number of blocks.
	num := db.uvarint()
	bc.blocks = make([]block, 0, num)

	for i := 0; i < num; i++ {
		var blk block
		// Read #entries.
		blk.numEntries = db.uvarint()

		// Read mint, maxt.
		blk.mint = db.varint64()
		blk.maxt = db.varint64()

		// Read offset and length.
		blk.offset = db.uvarint()
		if version == chunkFormatV3 {
			blk.uncompressedSize = db.uvarint()
		}
		l := db.uvarint()
		blk.b = b[blk.offset : blk.offset+l]

		// Verify checksums.
		expCRC := binary.BigEndian.Uint32(b[blk.offset+l:])
		if expCRC != crc32.Checksum(blk.b, castagnoliTable) {
			_ = level.Error(util_log.Logger).Log("msg", "Checksum does not match for a block in chunk, this block will be skipped", "err", ErrInvalidChecksum)
			continue
		}

		bc.blocks = append(bc.blocks, blk)

		// Update the counter used to track the size of cut blocks.
		bc.cutBlockSize += len(blk.b)

		if db.err() != nil {
			return nil, errors.Wrap(db.err(), "decoding block meta")
		}
	}

	return bc, nil
}

// BytesWith uses a provided []byte for buffer instantiation
// NOTE: This does not cut the head block nor include any head block data.
func (c *MemChunk) BytesWith(b []byte) ([]byte, error) {
	buf := bytes.NewBuffer(b[:0])
	if _, err := c.WriteTo(buf); err != nil {
		return nil, err
	}
	return buf.Bytes(), nil
}

// Bytes implements Chunk.
// NOTE: Does not cut head block or include any head block data.
func (c *MemChunk) Bytes() ([]byte, error) {
	return c.BytesWith(nil)
}

// BytesSize returns the raw size of the chunk.
// NOTE: This does not account for the head block nor include any head block data.
func (c *MemChunk) BytesSize() int {
	size := 4 // magic number
	size++    // format
	if c.format > chunkFormatV1 {
		size++ // chunk format v2+ has a byte for encoding.
	}

	// blocks
	for _, b := range c.blocks {
		size += len(b.b) + crc32.Size // size + crc

		size += binary.MaxVarintLen32 // num entries
		size += binary.MaxVarintLen64 // mint
		size += binary.MaxVarintLen64 // maxt
		size += binary.MaxVarintLen32 // offset
		if c.format == chunkFormatV3 {
			size += binary.MaxVarintLen32 // uncompressed size
		}
		size += binary.MaxVarintLen32 // len(b)
	}

	// blockmeta
	size += binary.MaxVarintLen32 // len  blocks

	size += crc32.Size // metablock crc
	size += 8          // metaoffset
	return size
}

// WriteTo Implements io.WriterTo
// NOTE: Does not cut head block or include any head block data.
// For this to be the case you must call Close() first.
// This decision notably enables WAL checkpointing, which would otherwise
// result in different content addressable chunks in storage based on the timing of when
// they were checkpointed (which would cause new blocks to be cut early).
func (c *MemChunk) WriteTo(w io.Writer) (int64, error) {
	crc32Hash := crc32HashPool.Get().(hash.Hash32)
	defer crc32HashPool.Put(crc32Hash)
	crc32Hash.Reset()

	offset := int64(0)

	eb := EncodeBufferPool.Get().(*encbuf)
	defer EncodeBufferPool.Put(eb)

	eb.reset()

	// Write the header (magicNum + version).
	eb.putBE32(magicNumber)
	eb.putByte(c.format)
	if c.format > chunkFormatV1 {
		// chunk format v2+ has a byte for encoding.
		eb.putByte(byte(c.encoding))
	}

	n, err := w.Write(eb.get())
	if err != nil {
		return offset, errors.Wrap(err, "write blockMeta #entries")
	}
	offset += int64(n)

	// Write Blocks.
	for i, b := range c.blocks {
		c.blocks[i].offset = int(offset)

		crc32Hash.Reset()
		_, err := crc32Hash.Write(b.b)
		if err != nil {
			return offset, errors.Wrap(err, "write block")
		}

		n, err := w.Write(crc32Hash.Sum(b.b))
		if err != nil {
			return offset, errors.Wrap(err, "write block")
		}
		offset += int64(n)
	}

	metasOffset := offset
	// Write the number of blocks.
	eb.reset()
	eb.putUvarint(len(c.blocks))

	// Write BlockMetas.
	for _, b := range c.blocks {
		eb.putUvarint(b.numEntries)
		eb.putVarint64(b.mint)
		eb.putVarint64(b.maxt)
		eb.putUvarint(b.offset)
		if c.format == chunkFormatV3 {
			eb.putUvarint(b.uncompressedSize)
		}
		eb.putUvarint(len(b.b))
	}
	eb.putHash(crc32Hash)

	n, err = w.Write(eb.get())
	if err != nil {
		return offset, errors.Wrap(err, "write block metas")
	}
	offset += int64(n)

	// Write the metasOffset.
	eb.reset()
	eb.putBE64int(int(metasOffset))
	n, err = w.Write(eb.get())
	if err != nil {
		return offset, errors.Wrap(err, "write metasOffset")
	}
	offset += int64(n)

	return offset, nil
}

// SerializeForCheckpointTo serialize the chunk & head into different `io.Writer` for checkpointing use.
// This is to ensure eventually flushed chunks don't have different substructures depending on when they were checkpointed.
// In turn this allows us to maintain a more effective dedupe ratio in storage.
func (c *MemChunk) SerializeForCheckpointTo(chk, head io.Writer) error {
	_, err := c.WriteTo(chk)
	if err != nil {
		return err
	}

	if c.head.isEmpty() {
		return nil
	}

	err = c.head.CheckpointTo(c.format, head)
	if err != nil {
		return err
	}

	return nil
}

func (c *MemChunk) CheckpointSize() (chunk, head int) {
	return c.BytesSize(), c.head.CheckpointSize(c.format)
}

func MemchunkFromCheckpoint(chk, head []byte, blockSize int, targetSize int) (*MemChunk, error) {
	mc, err := NewByteChunk(chk, blockSize, targetSize)
	if err != nil {
		return nil, err
	}
	return mc, mc.head.FromCheckpoint(head)
}

// Encoding implements Chunk.
func (c *MemChunk) Encoding() Encoding {
	return c.encoding
}

// Size implements Chunk.
func (c *MemChunk) Size() int {
	ne := 0
	for _, blk := range c.blocks {
		ne += blk.numEntries
	}

	if !c.head.isEmpty() {
		ne += len(c.head.entries)
	}

	return ne
}

// BlockCount implements Chunk.
func (c *MemChunk) BlockCount() int {
	return len(c.blocks)
}

// SpaceFor implements Chunk.
func (c *MemChunk) SpaceFor(e *logproto.Entry) bool {
	if c.targetSize > 0 {
		// This is looking to see if the uncompressed lines will fit which is not
		// a great check, but it will guarantee we are always under the target size
		newHBSize := c.head.size + len(e.Line)
		return (c.cutBlockSize + newHBSize) < c.targetSize
	}
	// if targetSize is not defined, default to the original behavior of fixed blocks per chunk
	return len(c.blocks) < blocksPerChunk
}

// UncompressedSize implements Chunk.
func (c *MemChunk) UncompressedSize() int {
	size := 0

	if !c.head.isEmpty() {
		size += c.head.size
	}

	for _, b := range c.blocks {
		size += b.uncompressedSize
	}

	return size
}

// CompressedSize implements Chunk.
func (c *MemChunk) CompressedSize() int {
	size := 0
	// Better to account for any uncompressed data than ignore it even though this isn't accurate.
	if !c.head.isEmpty() {
		size += c.head.size
	}
	size += c.cutBlockSize
	return size
}

// Utilization implements Chunk.
func (c *MemChunk) Utilization() float64 {
	if c.targetSize != 0 {
		return float64(c.CompressedSize()) / float64(c.targetSize)
	}
	size := c.UncompressedSize()
	return float64(size) / float64(blocksPerChunk*c.blockSize)
}

// Append implements Chunk.
func (c *MemChunk) Append(entry *logproto.Entry) error {
	entryTimestamp := entry.Timestamp.UnixNano()

	// If the head block is empty but there are cut blocks, we have to make
	// sure the new entry is not out of order compared to the previous block
	if c.head.isEmpty() && len(c.blocks) > 0 && c.blocks[len(c.blocks)-1].maxt > entryTimestamp {
		return ErrOutOfOrder
	}

	if err := c.head.append(entryTimestamp, entry.Line); err != nil {
		return err
	}

	if c.head.size >= c.blockSize {
		return c.cut()
	}

	return nil
}

// Close implements Chunk.
// TODO: Fix this to check edge cases.
func (c *MemChunk) Close() error {
	return c.cut()
}

// cut a new block and add it to finished blocks.
func (c *MemChunk) cut() error {
	if c.head.isEmpty() {
		return nil
	}

	b, err := c.head.serialise(getWriterPool(c.encoding))
	if err != nil {
		return err
	}

	c.blocks = append(c.blocks, block{
		b:                b,
		numEntries:       len(c.head.entries),
		mint:             c.head.mint,
		maxt:             c.head.maxt,
		uncompressedSize: c.head.size,
	})

	c.cutBlockSize += len(b)

	c.head.clear()
	return nil
}

// Bounds implements Chunk.
func (c *MemChunk) Bounds() (fromT, toT time.Time) {
	var from, to int64
	if len(c.blocks) > 0 {
		from = c.blocks[0].mint
		to = c.blocks[len(c.blocks)-1].maxt
	}

	if !c.head.isEmpty() {
		if from == 0 || from > c.head.mint {
			from = c.head.mint
		}

		if to < c.head.maxt {
			to = c.head.maxt
		}
	}

	return time.Unix(0, from), time.Unix(0, to)
}

// Iterator implements Chunk.
func (c *MemChunk) Iterator(ctx context.Context, mintT, maxtT time.Time, direction logproto.Direction, pipeline log.StreamPipeline) (iter.EntryIterator, error) {
	mint, maxt := mintT.UnixNano(), maxtT.UnixNano()
	blockItrs := make([]iter.EntryIterator, 0, len(c.blocks)+1)
	var headIterator iter.EntryIterator

	for _, b := range c.blocks {
		if maxt < b.mint || b.maxt < mint {
			continue
		}
		blockItrs = append(blockItrs, encBlock{c.encoding, b}.Iterator(ctx, pipeline))
	}

	if !c.head.isEmpty() {
		headIterator = c.head.iterator(ctx, direction, mint, maxt, pipeline)
	}

	if direction == logproto.FORWARD {
		// add the headblock iterator at the end.
		if headIterator != nil {
			blockItrs = append(blockItrs, headIterator)
		}
		return iter.NewTimeRangedIterator(
			iter.NewNonOverlappingIterator(blockItrs, ""),
			time.Unix(0, mint),
			time.Unix(0, maxt),
		), nil
	}
	// reverse each block entries
	for i, it := range blockItrs {
		r, err := iter.NewEntryReversedIter(
			iter.NewTimeRangedIterator(it,
				time.Unix(0, mint),
				time.Unix(0, maxt),
			))
		if err != nil {
			return nil, err
		}
		blockItrs[i] = r
	}
	// except the head block which is already reversed via the heapIterator.
	if headIterator != nil {
		blockItrs = append(blockItrs, headIterator)
	}
	// then reverse all iterators.
	for i, j := 0, len(blockItrs)-1; i < j; i, j = i+1, j-1 {
		blockItrs[i], blockItrs[j] = blockItrs[j], blockItrs[i]
	}

	return iter.NewNonOverlappingIterator(blockItrs, ""), nil
}

// Iterator implements Chunk.
func (c *MemChunk) SampleIterator(ctx context.Context, from, through time.Time, extractor log.StreamSampleExtractor) iter.SampleIterator {
	mint, maxt := from.UnixNano(), through.UnixNano()
	its := make([]iter.SampleIterator, 0, len(c.blocks)+1)

	for _, b := range c.blocks {
		if maxt < b.mint || b.maxt < mint {
			continue
		}
		its = append(its, encBlock{c.encoding, b}.SampleIterator(ctx, extractor))
	}

	if !c.head.isEmpty() {
		its = append(its, c.head.sampleIterator(ctx, mint, maxt, extractor))
	}

	return iter.NewTimeRangedSampleIterator(
		iter.NewNonOverlappingSampleIterator(its, ""),
		mint,
		maxt,
	)
}

// Blocks implements Chunk
func (c *MemChunk) Blocks(mintT, maxtT time.Time) []Block {
	mint, maxt := mintT.UnixNano(), maxtT.UnixNano()
	blocks := make([]Block, 0, len(c.blocks))

	for _, b := range c.blocks {
		if maxt >= b.mint && b.maxt >= mint {
			blocks = append(blocks, encBlock{c.encoding, b})
		}
	}
	return blocks
}

// Rebound builds a smaller chunk with logs having timestamp from start and end(both inclusive)
func (c *MemChunk) Rebound(start, end time.Time) (Chunk, error) {
	// add a nanosecond to end time because the Chunk.Iterator considers end time to be non-inclusive.
	itr, err := c.Iterator(context.Background(), start, end.Add(time.Nanosecond), logproto.FORWARD, log.NewNoopPipeline().ForStream(labels.Labels{}))
	if err != nil {
		return nil, err
	}

	// Using defaultBlockSize for target block size.
	// The alternative here could be going over all the blocks and using the size of the largest block as target block size but I(Sandeep) feel that it is not worth the complexity.
	// For target chunk size I am using compressed size of original chunk since the newChunk should anyways be lower in size than that.
	newChunk := NewMemChunk(c.Encoding(), defaultBlockSize, c.CompressedSize())

	for itr.Next() {
		entry := itr.Entry()
		if err := newChunk.Append(&entry); err != nil {
			return nil, err
		}
	}

	if newChunk.Size() == 0 {
		return nil, encoding.ErrSliceNoDataInRange
	}

	if err := newChunk.Close(); err != nil {
		return nil, err
	}

	return newChunk, nil
}

// encBlock is an internal wrapper for a block, mainly to avoid binding an encoding in a block itself.
// This may seem roundabout, but the encoding is already a field on the parent MemChunk type. encBlock
// then allows us to bind a decoding context to a block when requested, but otherwise helps reduce the
// chances of chunk<>block encoding drift in the codebase as the latter is parameterized by the former.
type encBlock struct {
	enc Encoding
	block
}

func (b encBlock) Iterator(ctx context.Context, pipeline log.StreamPipeline) iter.EntryIterator {
	if len(b.b) == 0 {
		return iter.NoopIterator
	}
	return newEntryIterator(ctx, getReaderPool(b.enc), b.b, pipeline)
}

func (b encBlock) SampleIterator(ctx context.Context, extractor log.StreamSampleExtractor) iter.SampleIterator {
	if len(b.b) == 0 {
		return iter.NoopIterator
	}
	return newSampleIterator(ctx, getReaderPool(b.enc), b.b, extractor)
}

func (b block) Offset() int {
	return b.offset
}

func (b block) Entries() int {
	return b.numEntries
}

func (b block) MinTime() int64 {
	return b.mint
}

func (b block) MaxTime() int64 {
	return b.maxt
}

func (hb *headBlock) iterator(ctx context.Context, direction logproto.Direction, mint, maxt int64, pipeline log.StreamPipeline) iter.EntryIterator {
	if hb.isEmpty() || (maxt < hb.mint || hb.maxt < mint) {
		return iter.NoopIterator
	}

	chunkStats := stats.GetChunkData(ctx)

	// We are doing a copy everytime, this is because b.entries could change completely,
	// the alternate would be that we allocate a new b.entries everytime we cut a block,
	// but the tradeoff is that queries to near-realtime data would be much lower than
	// cutting of blocks.
	chunkStats.HeadChunkLines += int64(len(hb.entries))
	streams := map[uint64]*logproto.Stream{}

	process := func(e entry) {
		// apply time filtering
		if e.t < mint || e.t >= maxt {
			return
		}
		chunkStats.HeadChunkBytes += int64(len(e.s))
		newLine, parsedLbs, ok := pipeline.ProcessString(e.s)
		if !ok {
			return
		}
		var stream *logproto.Stream
		lhash := parsedLbs.Hash()
		if stream, ok = streams[lhash]; !ok {
			stream = &logproto.Stream{
				Labels: parsedLbs.String(),
			}
			streams[lhash] = stream
		}
		stream.Entries = append(stream.Entries, logproto.Entry{
			Timestamp: time.Unix(0, e.t),
			Line:      newLine,
		})
	}

	if direction == logproto.FORWARD {
		for _, e := range hb.entries {
			process(e)
		}
	} else {
		for i := len(hb.entries) - 1; i >= 0; i-- {
			process(hb.entries[i])
		}
	}

	if len(streams) == 0 {
		return iter.NoopIterator
	}
	streamsResult := make([]logproto.Stream, 0, len(streams))
	for _, stream := range streams {
		streamsResult = append(streamsResult, *stream)
	}
	return iter.NewStreamsIterator(ctx, streamsResult, direction)
}

func (hb *headBlock) sampleIterator(ctx context.Context, mint, maxt int64, extractor log.StreamSampleExtractor) iter.SampleIterator {
	if hb.isEmpty() || (maxt < hb.mint || hb.maxt < mint) {
		return iter.NoopIterator
	}
	chunkStats := stats.GetChunkData(ctx)
	chunkStats.HeadChunkLines += int64(len(hb.entries))
	series := map[uint64]*logproto.Series{}
	for _, e := range hb.entries {
		chunkStats.HeadChunkBytes += int64(len(e.s))
		value, parsedLabels, ok := extractor.ProcessString(e.s)
		if !ok {
			continue
		}
		var found bool
		var s *logproto.Series
		lhash := parsedLabels.Hash()
		if s, found = series[lhash]; !found {
			s = &logproto.Series{
				Labels: parsedLabels.String(),
			}
			series[lhash] = s
		}

		// []byte here doesn't create allocation because Sum64 has go:noescape directive
		// It specifies that the function does not allow any of the pointers passed as arguments to escape into the heap or into the values returned from the function.
		h := xxhash.Sum64([]byte(e.s))
		s.Samples = append(s.Samples, logproto.Sample{
			Timestamp: e.t,
			Value:     value,
			Hash:      h,
		})
	}

	if len(series) == 0 {
		return iter.NoopIterator
	}
	seriesRes := make([]logproto.Series, 0, len(series))
	for _, s := range series {
		// todo(ctovena) not sure we need this sort.
		sort.Sort(s)
		seriesRes = append(seriesRes, *s)
	}
	return iter.NewMultiSeriesIterator(ctx, seriesRes)
}

type bufferedIterator struct {
	origBytes []byte
	stats     *stats.ChunkData

	bufReader *bufio.Reader
	reader    io.Reader
	pool      ReaderPool

	err error

	decBuf   []byte // The buffer for decoding the lengths.
	buf      []byte // The buffer for a single entry.
	currLine []byte // the current line, this is the same as the buffer but sliced the the line size.
	currTs   int64

	closed bool
}

func newBufferedIterator(ctx context.Context, pool ReaderPool, b []byte) *bufferedIterator {
	chunkStats := stats.GetChunkData(ctx)
	chunkStats.CompressedBytes += int64(len(b))
	return &bufferedIterator{
		stats:     chunkStats,
		origBytes: b,
		reader:    nil, // will be initialized later
		bufReader: nil, // will be initialized later
		pool:      pool,
		decBuf:    make([]byte, binary.MaxVarintLen64),
	}
}

func (si *bufferedIterator) Next() bool {
	if !si.closed && si.reader == nil {
		// initialize reader now, hopefully reusing one of the previous readers
		si.reader = si.pool.GetReader(bytes.NewBuffer(si.origBytes))
		si.bufReader = BufReaderPool.Get(si.reader)
	}

	ts, line, ok := si.moveNext()
	if !ok {
		si.Close()
		return false
	}
	// we decode always the line length and ts as varint
	si.stats.DecompressedBytes += int64(len(line)) + 2*binary.MaxVarintLen64
	si.stats.DecompressedLines++

	si.currTs = ts
	si.currLine = line
	return true
}

// moveNext moves the buffer to the next entry
func (si *bufferedIterator) moveNext() (int64, []byte, bool) {
	ts, err := binary.ReadVarint(si.bufReader)
	if err != nil {
		if err != io.EOF {
			si.err = err
		}
		return 0, nil, false
	}

	l, err := binary.ReadUvarint(si.bufReader)
	if err != nil {
		if err != io.EOF {
			si.err = err
			return 0, nil, false
		}
	}
	lineSize := int(l)

	if lineSize >= maxLineLength {
		si.err = fmt.Errorf("line too long %d, maximum %d", lineSize, maxLineLength)
		return 0, nil, false
	}
	// If the buffer is not yet initialize or too small, we get a new one.
	if si.buf == nil || lineSize > cap(si.buf) {
		// in case of a replacement we replace back the buffer in the pool
		if si.buf != nil {
			BytesBufferPool.Put(si.buf)
		}
		si.buf = BytesBufferPool.Get(lineSize).([]byte)
		if lineSize > cap(si.buf) {
			si.err = fmt.Errorf("could not get a line buffer of size %d, actual %d", lineSize, cap(si.buf))
			return 0, nil, false
		}
	}
	// Then process reading the line.
	n, err := si.bufReader.Read(si.buf[:lineSize])
	if err != nil && err != io.EOF {
		si.err = err
		return 0, nil, false
	}
	for n < lineSize {
		r, err := si.bufReader.Read(si.buf[n:lineSize])
		if err != nil && err != io.EOF {
			si.err = err
			return 0, nil, false
		}
		n += r
	}
	return ts, si.buf[:lineSize], true
}

func (si *bufferedIterator) Error() error { return si.err }

func (si *bufferedIterator) Close() error {
	if !si.closed {
		si.closed = true
		si.close()
	}
	return si.err
}

func (si *bufferedIterator) close() {
	if si.reader != nil {
		si.pool.PutReader(si.reader)
		si.reader = nil
	}
	if si.bufReader != nil {
		BufReaderPool.Put(si.bufReader)
		si.bufReader = nil
	}

	if si.buf != nil {
		BytesBufferPool.Put(si.buf)
		si.buf = nil
	}
	si.origBytes = nil
	si.decBuf = nil
}

func newEntryIterator(ctx context.Context, pool ReaderPool, b []byte, pipeline log.StreamPipeline) iter.EntryIterator {
	return &entryBufferedIterator{
		bufferedIterator: newBufferedIterator(ctx, pool, b),
		pipeline:         pipeline,
	}
}

type entryBufferedIterator struct {
	*bufferedIterator
	pipeline log.StreamPipeline

	cur        logproto.Entry
	currLabels log.LabelsResult
}

func (e *entryBufferedIterator) Entry() logproto.Entry {
	return e.cur
}

func (e *entryBufferedIterator) Labels() string { return e.currLabels.String() }

func (e *entryBufferedIterator) Next() bool {
	for e.bufferedIterator.Next() {
		newLine, lbs, ok := e.pipeline.Process(e.currLine)
		if !ok {
			continue
		}
		e.cur.Timestamp = time.Unix(0, e.currTs)
		e.cur.Line = string(newLine)
		e.currLabels = lbs
		return true
	}
	return false
}

func newSampleIterator(ctx context.Context, pool ReaderPool, b []byte, extractor log.StreamSampleExtractor) iter.SampleIterator {
	it := &sampleBufferedIterator{
		bufferedIterator: newBufferedIterator(ctx, pool, b),
		extractor:        extractor,
	}
	return it
}

type sampleBufferedIterator struct {
	*bufferedIterator

	extractor log.StreamSampleExtractor

	cur        logproto.Sample
	currLabels log.LabelsResult
}

func (e *sampleBufferedIterator) Next() bool {
	for e.bufferedIterator.Next() {
		val, labels, ok := e.extractor.Process(e.currLine)
		if !ok {
			continue
		}
		e.currLabels = labels
		e.cur.Value = val
		e.cur.Hash = xxhash.Sum64(e.currLine)
		e.cur.Timestamp = e.currTs
		return true
	}
	return false
}
func (e *sampleBufferedIterator) Labels() string { return e.currLabels.String() }

func (e *sampleBufferedIterator) Sample() logproto.Sample {
	return e.cur
}