lib/readers: add NoSeeker to adapt io.Reader to io.ReadSeeker

lib/readers/noseeker.go

@@ -0,0 +1,22 @@
+package readers
+
+import (
+	"errors"
+	"io"
+)
+
+var (
+	errCantSeek = errors.New("can't Seek")
+)
+
+// NoSeeker adapts an io.Reader into an io.ReadSeeker.
+//
+// However if Seek() is called it will return an error.
+type NoSeeker struct {
+	io.Reader
+}
+
+// Seek the stream - returns an error
+func (r NoSeeker) Seek(offset int64, whence int) (abs int64, err error) {
+	return 0, errCantSeek
+}

lib/readers/noseeker_test.go

@@ -0,0 +1,31 @@
+package readers
+
+import (
+	"bytes"
+	"io"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNoSeeker(t *testing.T) {
+	r := bytes.NewBufferString("hello")
+	rs := NoSeeker{Reader: r}
+
+	// Check read
+	b := make([]byte, 4)
+	n, err := rs.Read(b)
+	assert.NoError(t, err)
+	assert.Equal(t, 4, n)
+	assert.Equal(t, []byte("hell"), b)
+
+	// Check seek
+	_, err = rs.Seek(0, io.SeekCurrent)
+	assert.Equal(t, errCantSeek, err)
+}
+
+// check interfaces
+var (
+	_ io.Reader = NoSeeker{}
+	_ io.Seeker = NoSeeker{}
+)