/
reader.go
169 lines (147 loc) · 3.02 KB
/
reader.go
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package log
import (
"bufio"
"errors"
"hash/crc32"
"io"
)
var (
BadCRC = errors.New("bad CRC in message")
UnexpectedEOF = errors.New("unexpected EOF")
)
type Reader struct {
ReaderBackend
position int64
bufferSize int
buf *bufio.Reader
}
type ReaderBackend interface {
io.Reader
io.Seeker
io.Closer
}
func NewReader(backend ReaderBackend, position int64, bufferSize int) *Reader {
if bufferSize == 0 {
bufferSize = 4096
}
r := &Reader{backend, position, bufferSize, nil}
r.resetBufio()
return r
}
func (lr *Reader) Position() int64 {
return lr.position
}
func (lr *Reader) SeekToEnd() (uint64, error) {
var lastValidOffset uint64 = 0
for {
offset, err := lr.FastRead()
switch err {
case nil:
lastValidOffset = offset
continue
case io.EOF:
return lastValidOffset, nil
default:
return 0, err
}
}
}
func (lr *Reader) SeekToOffset(offset uint64) error {
// back to the start
if _, err := lr.Seek(0, 0); err != nil {
return err
}
for {
positionBeforeRead := lr.position
offset, err := lr.FastRead()
if err != nil {
return err
}
if offset >= offset {
lr.position = positionBeforeRead
return lr.rewind()
}
}
}
// Read and check the next message but don't parse it.
func (lr *Reader) FastRead() (uint64, error) {
offset, size, r := lr.readPreMessage()
if r.err != nil {
return 0, r.err
}
failure := func(err error) (uint64, error) {
lr.rewind()
if err == io.EOF {
return 0, UnexpectedEOF
}
return 0, err
}
crc := r.ReadUint32()
if err := r.err; err != nil {
return failure(err)
}
h := crc32.NewIEEE()
// we hash only "size-4" because we just read the CRC (4 bytes)
if _, err := io.CopyN(h, lr, int64(size)-4); err != nil {
return failure(err)
}
if crc != h.Sum32() {
return failure(BadCRC)
}
lr.updatePosition(size)
return offset, nil
}
func (lr *Reader) readPreMessage() (uint64, uint32, *BinaryReader) {
r := &BinaryReader{lr, nil}
offset := r.ReadUint64()
size := r.ReadUint32()
if r.err == nil && size == 0 {
r.err = BadCRC
}
if r.err != nil {
lr.rewind()
}
return offset, size, r
}
// Read and check the next message.
func (lr *Reader) Next() (uint64, *Message, error) {
var offset uint64
offset, size, r := lr.readPreMessage()
if r.err != nil {
return 0, nil, r.err
}
msg, err := lr.readMessage(r)
if err != nil {
lr.rewind()
}
if err == io.EOF {
err = UnexpectedEOF
}
lr.updatePosition(size)
return offset, msg, err
}
func (lr *Reader) readMessage(r *BinaryReader) (*Message, error) {
var size uint32
l := &Message{}
if err := l.ReadFrom(lr); err != nil {
return nil, err
}
if l.ComputeCRC() != l.CRC {
return nil, BadCRC
}
lr.updatePosition(size)
return l, nil
}
func (lr *Reader) updatePosition(messageSize uint32) {
// 8 bytes for the offset
// 4 bytes for the size
lr.position += 8 + 4 + int64(messageSize)
}
func (lr *Reader) rewind() error {
_, err := lr.Seek(lr.position, 0)
lr.resetBufio()
return err
}
func (lr *Reader) resetBufio() {
lr.buf = bufio.NewReaderSize(lr, lr.bufferSize)
}