Add ability to scan snaps (as a source) (#3929)

syft/internal/unionreader/union_reader.go

@@ -4,6 +4,9 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"sync"
+
+	"github.com/diskfs/go-diskfs/filesystem/squashfs"
 
 	macho "github.com/anchore/go-macholibre"
 	"github.com/anchore/syft/internal/log"
@@ -48,12 +51,15 @@ func GetUnionReader(readerCloser io.ReadCloser) (UnionReader, error) {
 	// file.LocationReadCloser embeds a ReadCloser, which is likely
 	// to implement UnionReader. Check whether the embedded read closer
 	// implements UnionReader, and just return that if so.
-	r, ok := readerCloser.(file.LocationReadCloser)
-	if ok {
-		ur, ok := r.ReadCloser.(UnionReader)
-		if ok {
-			return ur, nil
-		}
+
+	if r, ok := readerCloser.(file.LocationReadCloser); ok {
+		return GetUnionReader(r.ReadCloser)
+	}
+
+	if r, ok := readerCloser.(*squashfs.File); ok {
+		// seeking is implemented, but not io.ReaderAt. Lets wrap it to prevent from degrading performance
+		// by copying all data.
+		return newReaderAtAdapter(r), nil
 	}
 
 	b, err := io.ReadAll(readerCloser)
@@ -75,3 +81,58 @@ func GetUnionReader(readerCloser io.ReadCloser) (UnionReader, error) {
 
 	return reader, nil
 }
+
+type readerAtAdapter struct {
+	io.ReadSeekCloser
+	mu *sync.Mutex
+}
+
+func newReaderAtAdapter(rs io.ReadSeekCloser) UnionReader {
+	return &readerAtAdapter{
+		ReadSeekCloser: rs,
+		mu:             &sync.Mutex{},
+	}
+}
+
+func (r *readerAtAdapter) Read(p []byte) (n int, err error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	return r.ReadSeekCloser.Read(p)
+}
+
+func (r *readerAtAdapter) Seek(offset int64, whence int) (int64, error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	return r.ReadSeekCloser.Seek(offset, whence)
+}
+
+func (r *readerAtAdapter) ReadAt(p []byte, off int64) (n int, err error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	currentPos, err := r.ReadSeekCloser.Seek(0, io.SeekCurrent) // save current pos
+	if err != nil {
+		return 0, err
+	}
+
+	_, err = r.ReadSeekCloser.Seek(off, io.SeekStart) // seek to absolute position `off`
+	if err != nil {
+		return 0, err
+	}
+
+	n, err = r.ReadSeekCloser.Read(p) // read from that absolute position
+
+	// restore the position for the stateful read/seek operations
+	if restoreErr := r.restorePosition(currentPos); restoreErr != nil {
+		if err == nil {
+			err = restoreErr
+		}
+	}
+
+	return n, err
+}
+
+func (r *readerAtAdapter) restorePosition(pos int64) error {
+	_, err := r.ReadSeekCloser.Seek(pos, io.SeekStart)
+	return err
+}