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segment.go
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segment.go
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package wal
import (
"encoding/binary"
"errors"
"fmt"
lru "github.com/hashicorp/golang-lru/v2"
"hash/crc32"
"io"
"os"
"sync"
)
type ChunkType = byte
type SegmentID = uint32
// 定义一个枚举值
const (
ChunkTypeFull ChunkType = iota
ChunkTypeFirst
ChunkTypeMiddle
ChunkTypeLast
)
var (
ErrClosed = errors.New("the segment file is closed")
ErrInvalidCRC = errors.New("invalid crc, the data may be corrupted")
)
const (
// 7 Bytes
// Checksum Length Type
// 4 2 1
chunkHeaderSize = 7
// 32 KB
blockSize = 32 * KB
fileModePerm = 0644
)
type segment struct {
id SegmentID
fd *os.File
currentBlockNumber uint32
currentBlockSize uint32
closed bool
cache *lru.Cache[uint64, []byte]
header []byte
blockPool sync.Pool
}
func (seg *segment) NewReader() *segmentReader {
return &segmentReader{
segment: seg,
blockNumber: 0,
chunkOffset: 0,
}
}
func (seg *segment) Sync() error {
if seg.closed {
return nil
}
return seg.fd.Sync()
}
func (seg *segment) Remove() error {
if !seg.closed {
seg.closed = true
_ = seg.fd.Close()
}
return os.Remove(seg.fd.Name())
}
func (seg *segment) Close() error {
if seg.closed {
return nil
}
seg.closed = true
return seg.fd.Close()
}
func (seg *segment) Size() int64 {
return int64(seg.currentBlockNumber*blockSize + seg.currentBlockSize)
}
func (seg *segment) Write(data []byte) (*ChunkPosition, error) {
if seg.closed {
return nil, ErrClosed
}
// The left block space is not enough for a chunk header
if seg.currentBlockSize+chunkHeaderSize >= blockSize {
// padding if necessary
if seg.currentBlockSize < blockSize {
padding := make([]byte, blockSize-seg.currentBlockSize)
if _, err := seg.fd.Write(padding); err != nil {
return nil, err
}
}
// A new block, clear the current block size.
seg.currentBlockNumber += 1
seg.currentBlockSize = 0
}
// the start position(for read operation)
position := &ChunkPosition{
SegmentId: seg.id,
BlockNumber: seg.currentBlockNumber,
ChunkOffset: int64(seg.currentBlockSize),
}
dataSize := uint32(len(data))
// The entire chunk can fit into the block.
if seg.currentBlockSize+dataSize+chunkHeaderSize <= blockSize {
err := seg.writeInternal(data, ChunkTypeFull)
if err != nil {
return nil, err
}
position.ChunkSize = dataSize + chunkHeaderSize
return position, nil
}
// If the size of the data exceeds the size of the block,
// the data should be written to the block in batches.
var leftSize = dataSize
var blockCount uint32 = 0
for leftSize > 0 {
chunkSize := blockSize - seg.currentBlockSize - chunkHeaderSize
if chunkSize > leftSize {
chunkSize = leftSize
}
chunk := make([]byte, chunkSize)
var end = dataSize - leftSize + chunkSize
if end > dataSize {
end = dataSize
}
copy(chunk[:], data[dataSize-leftSize:end])
// write the chunks
var err error
if leftSize == dataSize {
// First Chunk
err = seg.writeInternal(chunk, ChunkTypeFirst)
} else if leftSize == chunkSize {
// Last Chunk
err = seg.writeInternal(chunk, ChunkTypeLast)
} else {
// Middle Chunk
err = seg.writeInternal(chunk, ChunkTypeMiddle)
}
if err != nil {
return nil, err
}
leftSize -= chunkSize
blockCount += 1
}
position.ChunkSize = blockCount*chunkHeaderSize + dataSize
return position, nil
}
func (seg *segment) writeInternal(data []byte, chunkType ChunkType) error {
dataSize := uint32(len(data))
// Checksum Length Type
// 4 2 1
binary.LittleEndian.PutUint16(seg.header[4:6], uint16(dataSize))
seg.header[6] = chunkType
sum := crc32.ChecksumIEEE(seg.header[4:])
sum = crc32.Update(sum, crc32.IEEETable, data)
binary.LittleEndian.PutUint32(seg.header[:4], sum)
// 追加写入数据
if _, err := seg.fd.Write(seg.header); err != nil {
return err
}
if _, err := seg.fd.Write(data); err != nil {
return err
}
if seg.currentBlockSize > blockSize {
panic("wrong! can not exceed the block size")
}
// 写入溢出的判断,放在调用 writeInternal地方了
seg.currentBlockSize += dataSize + chunkHeaderSize
if seg.currentBlockSize == blockSize {
seg.currentBlockNumber += 1
seg.currentBlockSize = 0
}
return nil
}
type segmentReader struct {
segment *segment
blockNumber uint32
chunkOffset int64
}
type blockAndHeader struct {
block []byte
header []byte
}
type ChunkPosition struct {
SegmentId SegmentID
BlockNumber uint32
ChunkOffset int64
ChunkSize uint32
}
//func SegmentFileName(dirPath string, extName string, id SegmentID) string {
// return filepath.Join(dirPath, fmt.Sprintf("%09d"+extName, id))
//}
func newBlockAndHeader() interface{} {
return &blockAndHeader{
block: make([]byte, blockSize),
header: make([]byte, chunkHeaderSize),
}
}
func openSegmentFile(dirpath, extName string, id uint32, cache *lru.Cache[uint64, []byte]) (*segment, error) {
fd, err := os.OpenFile(
SegmentFileName(dirpath, extName, id),
os.O_CREATE|os.O_RDWR|os.O_APPEND,
fileModePerm,
)
if err != nil {
return nil, err
}
offset, err := fd.Seek(0, io.SeekEnd)
if err != nil {
panic(fmt.Errorf("seek to the end of segment file %d%s failed: %v", id, extName, err))
}
return &segment{
id: id,
fd: fd,
cache: cache,
header: make([]byte, chunkHeaderSize),
blockPool: sync.Pool{New: newBlockAndHeader},
currentBlockNumber: uint32(offset / blockSize),
currentBlockSize: uint32(offset % blockSize),
}, nil
}
func (seg *segment) Read(blockNumber uint32, chunkOffset int64) ([]byte, error) {
value, _, err := seg.readInternal(blockNumber, chunkOffset)
return value, err
}
func (seg *segment) readInternal(blockNumber uint32, chunkOffset int64) ([]byte, *ChunkPosition, error) {
if seg.closed {
return nil, nil, ErrClosed
}
var (
result []byte
bh = seg.blockPool.Get().(*blockAndHeader)
segSize = seg.Size()
nextChunk = &ChunkPosition{SegmentId: seg.id}
)
defer func() {
seg.blockPool.Put(bh)
}()
for {
size := int64(blockSize)
offset := int64(blockNumber * blockSize)
if size+offset > segSize {
size = segSize - offset
}
if chunkOffset >= size {
return nil, nil, io.EOF
}
var ok bool
var cachedBlock []byte
if seg.cache != nil {
cachedBlock, ok = seg.cache.Get(seg.getCacheKey(blockNumber))
}
if ok {
copy(bh.block, cachedBlock)
} else {
_, err := seg.fd.ReadAt(bh.block[0:size], offset)
if err != nil {
return nil, nil, err
}
if seg.cache != nil && size == blockSize && len(cachedBlock) == 0 {
cacheBlock := make([]byte, blockSize)
copy(cacheBlock, bh.block)
seg.cache.Add(seg.getCacheKey(blockNumber), cacheBlock)
}
}
copy(bh.header, bh.block[chunkOffset:chunkOffset+chunkHeaderSize])
length := binary.LittleEndian.Uint16(bh.header[4:6])
start := chunkOffset + chunkHeaderSize
result = append(result, bh.block[start:start+int64(length)]...)
// check sum
checksumEnd := chunkOffset + chunkHeaderSize + int64(length)
checksum := crc32.ChecksumIEEE(bh.block[chunkOffset+4 : checksumEnd])
savedSum := binary.LittleEndian.Uint32(bh.header[:4])
if savedSum != checksum {
return nil, nil, ErrInvalidCRC
}
chunkType := bh.header[6]
if chunkType == ChunkTypeFull || chunkType == ChunkTypeLast {
nextChunk.BlockNumber = blockNumber
nextChunk.ChunkOffset = checksumEnd
// If this is the last chunk in the block, and the left block
// space are paddings, the next chunk should be in the next block.
if checksumEnd+chunkHeaderSize >= blockSize {
nextChunk.BlockNumber += 1
nextChunk.ChunkOffset = 0
}
break
}
blockNumber += 1
chunkOffset = 0
}
return result, nextChunk, nil
}
func (seg *segment) getCacheKey(blockNumber uint32) uint64 {
return uint64(seg.id)<<32 | uint64(blockNumber)
}
func (segReader *segmentReader) Next() ([]byte, *ChunkPosition, error) {
// The segment file is closed
if segReader.segment.closed {
return nil, nil, ErrClosed
}
// this position describes the current chunk info
chunkPosition := &ChunkPosition{
SegmentId: segReader.segment.id,
BlockNumber: segReader.blockNumber,
ChunkOffset: segReader.chunkOffset,
}
value, nextChunk, err := segReader.segment.readInternal(
segReader.blockNumber,
segReader.chunkOffset,
)
if err != nil {
return nil, nil, err
}
// Calculate the chunk size.
// Remember that the chunk size is just an estimated value,
// not accurate, so don't use it for any important logic.
chunkPosition.ChunkSize =
nextChunk.BlockNumber*blockSize + uint32(nextChunk.ChunkOffset) -
(segReader.blockNumber*blockSize + uint32(segReader.chunkOffset))
// update the position
segReader.blockNumber = nextChunk.BlockNumber
segReader.chunkOffset = nextChunk.ChunkOffset
return value, chunkPosition, nil
}
// Encode encodes the chunk position to a byte slice.
// You can decode it by calling wal.DecodeChunkPosition().
func (cp *ChunkPosition) Encode() []byte {
maxLen := binary.MaxVarintLen32*3 + binary.MaxVarintLen64
buf := make([]byte, maxLen)
var index = 0
// SegmentId
index += binary.PutUvarint(buf[index:], uint64(cp.SegmentId))
// BlockNumber
index += binary.PutUvarint(buf[index:], uint64(cp.BlockNumber))
// ChunkOffset
index += binary.PutUvarint(buf[index:], uint64(cp.ChunkOffset))
// ChunkSize
index += binary.PutUvarint(buf[index:], uint64(cp.ChunkSize))
return buf[:index]
}
// DecodeChunkPosition decodes the chunk position from a byte slice.
// You can encode it by calling wal.ChunkPosition.Encode().
func DecodeChunkPosition(buf []byte) *ChunkPosition {
if len(buf) == 0 {
return nil
}
var index = 0
// SegmentId
segmentId, n := binary.Uvarint(buf[index:])
index += n
// BlockNumber
blockNumber, n := binary.Uvarint(buf[index:])
index += n
// ChunkOffset
chunkOffset, n := binary.Uvarint(buf[index:])
index += n
// ChunkSize
chunkSize, n := binary.Uvarint(buf[index:])
index += n
return &ChunkPosition{
SegmentId: uint32(segmentId),
BlockNumber: uint32(blockNumber),
ChunkOffset: int64(chunkOffset),
ChunkSize: uint32(chunkSize),
}
}