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reader.go
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reader.go
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package seekable
import (
"encoding/binary"
"errors"
"fmt"
"io"
"sync"
"github.com/cespare/xxhash"
"go.uber.org/zap"
"go.uber.org/zap/zapcore"
"github.com/google/btree"
)
var (
_ io.ReadSeeker = (*ReaderImpl)(nil)
_ io.ReaderAt = (*ReaderImpl)(nil)
)
type cachedFrame struct {
m sync.Mutex
offset uint64
data []byte
}
func (f *cachedFrame) replace(offset uint64, data []byte) {
f.m.Lock()
defer f.m.Unlock()
f.offset = offset
f.data = data
}
func (f *cachedFrame) get() (uint64, []byte) {
f.m.Lock()
defer f.m.Unlock()
return f.offset, f.data
}
// Environment can be used to inject a custom file reader that is different from normal ReadSeeker.
// This is useful when, for example there is a custom chunking code.
type REnvironment interface {
// GetFrameByIndex returns the compressed frame by its index.
GetFrameByIndex(index FrameOffsetEntry) ([]byte, error)
// ReadFooter returns buffer whose last 9 bytes are interpreted as a `Seek_Table_Footer`.
ReadFooter() ([]byte, error)
// ReadSkipFrame returns the full Seek Table Skippable frame
// including the `Skippable_Magic_Number` and `Frame_Size`.
ReadSkipFrame(skippableFrameOffset int64) ([]byte, error)
}
// readSeekerEnvImpl is the environment implementation for the io.ReadSeeker.
type readSeekerEnvImpl struct {
rs io.ReadSeeker
}
func (rs *readSeekerEnvImpl) GetFrameByIndex(index FrameOffsetEntry) (p []byte, err error) {
p = make([]byte, index.CompSize)
off := int64(index.CompOffset)
switch v := rs.rs.(type) {
case io.ReaderAt:
_, err = v.ReadAt(p, off)
if errors.Is(err, io.EOF) {
err = nil
}
default:
_, err = v.Seek(off, io.SeekStart)
if err != nil {
return nil, err
}
_, err = io.ReadFull(rs.rs, p)
}
return
}
func (rs *readSeekerEnvImpl) ReadFooter() ([]byte, error) {
n, err := rs.rs.Seek(-seekTableFooterOffset, io.SeekEnd)
if err != nil {
return nil, fmt.Errorf("failed to seek to: %d: %w", -seekTableFooterOffset, err)
}
buf := make([]byte, seekTableFooterOffset)
_, err = io.ReadFull(rs.rs, buf)
if err != nil {
return nil, fmt.Errorf("failed to read footer at: %d: %w", n, err)
}
return buf, nil
}
func (rs *readSeekerEnvImpl) ReadSkipFrame(skippableFrameOffset int64) ([]byte, error) {
n, err := rs.rs.Seek(-skippableFrameOffset, io.SeekEnd)
if err != nil {
return nil, fmt.Errorf("failed to seek to: %d: %w", -skippableFrameOffset, err)
}
buf := make([]byte, skippableFrameOffset)
_, err = io.ReadFull(rs.rs, buf)
if err != nil {
return nil, fmt.Errorf("failed to read skippable frame header at: %d: %w", n, err)
}
return buf, nil
}
type ReaderImpl struct {
dec ZSTDDecoder
index *btree.BTree
checksums bool
offset int64
numFrames int64
endOffset int64
o readerOptions
// TODO: Add simple LRU cache.
cachedFrame cachedFrame
}
type Reader interface {
io.ReadSeeker
io.ReaderAt
}
type ZSTDDecoder interface {
DecodeAll(input, dst []byte) ([]byte, error)
}
// NewReader returns ZSTD stream reader that can be randomly accessed using uncompressed data offset.
// Ideally, passed io.ReadSeeker should implement io.ReaderAt interface.
func NewReader(rs io.ReadSeeker, decoder ZSTDDecoder, opts ...ROption) (Reader, error) {
sr := ReaderImpl{
dec: decoder,
}
sr.o.setDefault()
for _, o := range opts {
err := o(&sr.o)
if err != nil {
return nil, err
}
}
if sr.o.env == nil {
sr.o.env = &readSeekerEnvImpl{
rs: rs,
}
}
tree, err := sr.indexFooter()
if err != nil {
return nil, err
}
sr.index = tree
var last *FrameOffsetEntry
sr.index.Descend(func(i btree.Item) bool {
last = i.(*FrameOffsetEntry)
return false
})
if last == nil {
return nil, fmt.Errorf("seek index is empty")
}
sr.endOffset = int64(last.DecompOffset) + int64(last.DecompSize)
sr.numFrames = last.ID + 1
return &sr, nil
}
// ReadAt implements io.ReaderAt interface to randomly access data.
// This method is goroutine-safe and can be called concurrently ONLY if
// the underlying reader supports io.ReaderAt interface.
func (s *ReaderImpl) ReadAt(p []byte, off int64) (n int, err error) {
for m := 0; n < len(p) && err == nil; n += m {
_, m, err = s.read(p[n:], off+int64(n))
}
return
}
// Read implements io.Reader interface to randomly access data.
// This method is NOT goroutine-safe and CAN NOT be called
// concurrently since it modifies the underlying offset.
func (s *ReaderImpl) Read(p []byte) (n int, err error) {
offset, n, err := s.read(p, s.offset)
if err != nil {
if errors.Is(err, io.EOF) {
s.offset = s.endOffset
}
return
}
s.offset = offset
return
}
func (s *ReaderImpl) read(dst []byte, off int64) (int64, int, error) {
if off >= s.endOffset {
return 0, 0, io.EOF
}
if off < 0 {
return 0, 0, fmt.Errorf("offset before the start of the file: %d", off)
}
index := s.GetIndexByDecompOffset(uint64(off))
if index == nil {
return 0, 0, fmt.Errorf("failed to get index by offest: %d", off)
}
if off < int64(index.DecompOffset) || off > int64(index.DecompOffset)+int64(index.DecompSize) {
return 0, 0, fmt.Errorf("offset outside of index bounds: %d: min: %d, max: %d",
off, int64(index.DecompOffset), int64(index.DecompOffset)+int64(index.DecompSize))
}
var decompressed []byte
cachedOffset, cachedData := s.cachedFrame.get()
if cachedOffset == index.DecompOffset && cachedData != nil {
// fastpath
decompressed = cachedData
} else {
// slowpath
src, err := s.o.env.GetFrameByIndex(*index)
if err != nil {
return 0, 0, fmt.Errorf("failed to read compressed data at: %d, %w", index.CompOffset, err)
}
if len(src) != int(index.CompSize) {
return 0, 0, fmt.Errorf("compressed size does not match index at: %d: expected: %d, index: %+v",
off, len(src), index)
}
decompressed, err = s.dec.DecodeAll(src, nil)
if err != nil {
return 0, 0, fmt.Errorf("failed to decompress data data at: %d, %w", index.CompOffset, err)
}
if s.checksums {
checksum := uint32((xxhash.Sum64(decompressed) << 32) >> 32)
if index.Checksum != checksum {
return 0, 0, fmt.Errorf("checksum verification failed at: %d: expected: %d, actual: %d",
index.CompOffset, index.Checksum, checksum)
}
}
s.cachedFrame.replace(index.DecompOffset, decompressed)
}
if len(decompressed) != int(index.DecompSize) {
panic(fmt.Sprintf("index corruption: len: %d, expected: %d",
len(decompressed), int(index.DecompSize)))
}
offsetWithinFrame := uint64(off) - index.DecompOffset
size := uint64(len(decompressed)) - offsetWithinFrame
if size > uint64(len(dst)) {
size = uint64(len(dst))
}
s.o.logger.Debug("decompressed", zap.Uint64("offsetWithinFrame", offsetWithinFrame), zap.Uint64("end", offsetWithinFrame+size),
zap.Uint64("size", size), zap.Int("lenDecompressed", len(decompressed)), zap.Int("lenDst", len(dst)), zap.Object("index", index))
copy(dst, decompressed[offsetWithinFrame:offsetWithinFrame+size])
return off + int64(size), int(size), nil
}
// Seek implements io.Seeker interface to randomly access data.
// This method is NOT goroutine-safe and CAN NOT be called
// concurrently since it modifies the underlying offset.
func (s *ReaderImpl) Seek(offset int64, whence int) (int64, error) {
newOffset := s.offset
switch whence {
case io.SeekCurrent:
newOffset += offset
case io.SeekStart:
newOffset = offset
case io.SeekEnd:
newOffset = s.endOffset + offset
}
if newOffset < 0 {
return 0, fmt.Errorf("offset before the start of the file: %d (%d + %d)",
newOffset, s.offset, offset)
}
s.offset = newOffset
return s.offset, nil
}
// FrameOffsetEntry is the post-proccessed view of the Seek_Table_Entries suitable for indexing.
type FrameOffsetEntry struct {
// ID is the is the sequence number of the frame in the index.
ID int64
// CompOffset is the offset within compressed stream.
CompOffset uint64
// DecompOffset is the offset within decompressed stream.
DecompOffset uint64
// CompSize is the size of the compressed frame.
CompSize uint32
// DecompSize is the size of the original data.
DecompSize uint32
// Checksum is the lower 32 bits of the XXH64 hash of the uncompressed data.
Checksum uint32
}
func (o *FrameOffsetEntry) MarshalLogObject(enc zapcore.ObjectEncoder) error {
enc.AddInt64("ID", o.ID)
enc.AddUint64("CompOffset", o.CompOffset)
enc.AddUint64("DecompOffset", o.DecompOffset)
enc.AddUint32("CompSize", o.CompSize)
enc.AddUint32("DecompSize", o.DecompSize)
enc.AddUint32("Checksum", o.Checksum)
return nil
}
func (o *FrameOffsetEntry) Less(than btree.Item) bool {
return o.DecompOffset < than.(*FrameOffsetEntry).DecompOffset
}
func (s *ReaderImpl) indexFooter() (*btree.BTree, error) {
// read SeekTableFooter
buf, err := s.o.env.ReadFooter()
if err != nil {
return nil, fmt.Errorf("failed to read footer: %w", err)
}
if len(buf) < seekTableFooterOffset {
return nil, fmt.Errorf("footer is too small: %d", len(buf))
}
// parse SeekTableFooter
footer := SeekTableFooter{}
err = footer.UnmarshalBinary(buf[len(buf)-seekTableFooterOffset:])
if err != nil {
return nil, fmt.Errorf("failed to parse footer %+v: %w", buf, err)
}
s.o.logger.Debug("loaded", zap.Object("footer", &footer))
s.checksums = footer.SeekTableDescriptor.ChecksumFlag
// read SeekTableEntries
seekTableEntrySize := int64(8)
if footer.SeekTableDescriptor.ChecksumFlag {
seekTableEntrySize += 4
}
skippableFrameOffset := seekTableFooterOffset + seekTableEntrySize*int64(footer.NumberOfFrames)
skippableFrameOffset += frameSizeFieldSize
skippableFrameOffset += skippableMagicNumberFieldSize
buf, err = s.o.env.ReadSkipFrame(skippableFrameOffset)
if err != nil {
return nil, fmt.Errorf("failed to read footer: %w", err)
}
if len(buf) <= frameSizeFieldSize+skippableMagicNumberFieldSize+seekTableFooterOffset {
return nil, fmt.Errorf("skip frame is too small: %d", len(buf))
}
// parse SeekTableEntries
magic := binary.LittleEndian.Uint32(buf[0:4])
if magic != SkippableFrameMagic+seekableTag {
return nil, fmt.Errorf("skippable frame magic mismatch %d vs %d",
magic, SkippableFrameMagic+seekableTag)
}
expectedFrameSize := int64(len(buf)) - frameSizeFieldSize - skippableMagicNumberFieldSize
frameSize := int64(binary.LittleEndian.Uint32(buf[4:8]))
if frameSize != expectedFrameSize {
return nil, fmt.Errorf("skippable frame size mismatch: expected: %d, actual: %d",
expectedFrameSize, frameSize)
}
t, err := s.indexSeekTableEntries(buf[8:len(buf)-seekTableFooterOffset], uint64(seekTableEntrySize))
return t, err
}
func (s *ReaderImpl) indexSeekTableEntries(p []byte, entrySize uint64) (*btree.BTree, error) {
// TODO: Rewrite btree using generics.
t := btree.New(16)
entry := SeekTableEntry{}
var compOffset, decompOffset uint64
var i int64
for indexOffset := uint64(0); indexOffset < uint64(len(p)); indexOffset += entrySize {
err := entry.UnmarshalBinary(p[indexOffset : indexOffset+entrySize])
if err != nil {
return nil, fmt.Errorf("failed to parse entry %+v at: %d: %w",
p[indexOffset:indexOffset+entrySize], indexOffset, err)
}
t.ReplaceOrInsert(&FrameOffsetEntry{
ID: i,
CompOffset: compOffset,
DecompOffset: decompOffset,
CompSize: entry.CompressedSize,
DecompSize: entry.DecompressedSize,
Checksum: entry.Checksum,
})
compOffset += uint64(entry.CompressedSize)
decompOffset += uint64(entry.DecompressedSize)
i++
}
return t, nil
}