Possible fix for #22

Add sparse support for fragments (undocumented)

internal/data/reader.go

@@ -16,15 +16,17 @@ type Reader struct {
 	blockSizes []uint32
 	blockSize  uint32
 	resetable  bool
+	fileSize   uint64
 }
 
-func NewReader(r io.Reader, d decompress.Decompressor, blockSizes []uint32, blockSize uint32) *Reader {
+func NewReader(r io.Reader, d decompress.Decompressor, blockSizes []uint32, blockSize uint32, fileSize uint64) *Reader {
 	return &Reader{
 		d:          d,
 		master:     r,
 		blockSizes: blockSizes,
 		blockSize:  blockSize,
 		resetable:  true,
+		fileSize:   fileSize,
 	}
 }
 
@@ -49,7 +51,11 @@ func (r *Reader) advance() (err error) {
 	} else {
 		size := realSize(r.blockSizes[0])
 		if size == 0 {
-			r.cur = bytes.NewReader(make([]byte, r.blockSize))
+			outSize := r.blockSize
+			if r.fileSize < uint64(r.blockSize) {
+				outSize = uint32(r.fileSize)
+			}
+			r.cur = bytes.NewReader(make([]byte, outSize))
 		} else {
 			r.cur = io.LimitReader(r.master, int64(size))
 			if size == r.blockSizes[0] {

reader_frag.go

@@ -1,6 +1,7 @@
 package squashfs
 
 import (
+	"bytes"
 	"io"
 
 	"github.com/CalebQ42/squashfs/internal/toreader"
@@ -12,8 +13,11 @@ type fragEntry struct {
 	_     uint32
 }
 
-func (r Reader) fragReader(index uint32) (io.Reader, error) {
+func (r Reader) fragReader(index uint32, fragSize uint32) (io.Reader, error) {
 	realSize := r.fragEntries[index].Size &^ (1 << 24)
+	if realSize == 0 {
+		return bytes.NewReader(make([]byte, fragSize)), nil
+	}
 	rdr := io.LimitReader(toreader.NewReader(r.r, int64(r.fragEntries[index].Start)), int64(realSize))
 	if realSize != r.fragEntries[index].Size {
 		return rdr, nil

reader_inode.go

@@ -54,12 +54,12 @@ func (r Reader) getReaders(i inode.Inode) (full *data.FullReader, rdr *data.Read
 	} else {
 		return nil, nil, errors.New("getReaders called on non-file type")
 	}
-	rdr = data.NewReader(toreader.NewReader(r.r, int64(blockOffset)), r.d, blockSizes, r.s.BlockSize)
+	rdr = data.NewReader(toreader.NewReader(r.r, int64(blockOffset)), r.d, blockSizes, r.s.BlockSize, fileSize)
 	full = data.NewFullReader(r.r, uint64(blockOffset), r.d, blockSizes, r.s.BlockSize, fileSize)
 	if fragInd != 0xFFFFFFFF {
 		full.AddFragment(func() (io.Reader, error) {
 			var fragRdr io.Reader
-			fragRdr, err = r.fragReader(fragInd)
+			fragRdr, err = r.fragReader(fragInd, fragSize)
 			if err != nil {
 				return nil, err
 			}
@@ -75,7 +75,7 @@ func (r Reader) getReaders(i inode.Inode) (full *data.FullReader, rdr *data.Read
 			return fragRdr, nil
 		})
 		var fragRdr io.Reader
-		fragRdr, err = r.fragReader(fragInd)
+		fragRdr, err = r.fragReader(fragInd, fragSize)
 		if err != nil {
 			return nil, nil, err
 		}