storage rework

pkg/cowutils/writer.go

@@ -0,0 +1,39 @@
+package cowutils
+
+import (
+	"errors"
+	"io"
+)
+
+// sectionWriter is a helper used when we need to fallback into copying data manually
+type sectionWriter struct {
+	w    io.WriterAt // target file
+	base int64       // base position in file
+	off  int64       // current relative offset
+}
+
+// Write writes & updates offset
+func (s *sectionWriter) Write(p []byte) (int, error) {
+	n, err := s.w.WriteAt(p, s.base+s.off)
+	s.off += int64(n)
+	return n, err
+}
+
+func (s *sectionWriter) Seek(offset int64, whence int) (int64, error) {
+	switch whence {
+	case io.SeekStart:
+		// nothing needed
+	case io.SeekCurrent:
+		offset += s.off
+	case io.SeekEnd:
+		// we don't support io.SeekEnd
+		fallthrough
+	default:
+		return s.off, errors.New("Seek: invalid whence")
+	}
+	if offset < 0 {
+		return s.off, errors.New("Seek: invalid offset")
+	}
+	s.off = offset
+	return offset, nil
+}