storage: Weave end offset into unsealed reader code

lib/readerutil/readerutil.go

@@ -0,0 +1,44 @@
+package readerutil
+
+import (
+	"io"
+	"os"
+)
+
+// NewReadSeekerFromReaderAt returns a new io.ReadSeeker from a io.ReaderAt.
+// The returned io.ReadSeeker will read from the io.ReaderAt starting at the
+// given base offset.
+func NewReadSeekerFromReaderAt(readerAt io.ReaderAt, base int64) io.ReadSeeker {
+	return &readSeekerFromReaderAt{
+		readerAt: readerAt,
+		base:     base,
+		pos:      0,
+	}
+}
+
+type readSeekerFromReaderAt struct {
+	readerAt io.ReaderAt
+	base     int64
+	pos      int64
+}
+
+func (rs *readSeekerFromReaderAt) Read(p []byte) (n int, err error) {
+	n, err = rs.readerAt.ReadAt(p, rs.pos+rs.base)
+	rs.pos += int64(n)
+	return n, err
+}
+
+func (rs *readSeekerFromReaderAt) Seek(offset int64, whence int) (int64, error) {
+	switch whence {
+	case io.SeekStart:
+		rs.pos = offset
+	case io.SeekCurrent:
+		rs.pos += offset
+	case io.SeekEnd:
+		return 0, io.ErrUnexpectedEOF
+	default:
+		return 0, os.ErrInvalid
+	}
+
+	return rs.pos, nil
+}