Upgrade CancelCloser to also return seekers when possible

For #457

The download pipeline returns a CancelCloser, making this required.

util/readers/cancel_closer.go

@@ -2,24 +2,40 @@ package readers
 
 import "io"
 
+type CancellableCloser interface {
+	io.ReadCloser
+}
+
 type CancelCloser struct {
 	io.ReadCloser
-	r      io.ReadCloser
 	cancel func()
 }
 
-func NewCancelCloser(r io.ReadCloser, cancel func()) *CancelCloser {
-	return &CancelCloser{
-		r:      r,
-		cancel: cancel,
-	}
+type CancelSeekCloser struct {
+	io.ReadSeekCloser
+	cancel func()
 }
 
-func (c *CancelCloser) Read(p []byte) (int, error) {
-	return c.r.Read(p)
+func NewCancelCloser(r io.ReadCloser, cancel func()) CancellableCloser {
+	if rsc, ok := r.(io.ReadSeekCloser); ok {
+		return &CancelSeekCloser{
+			ReadSeekCloser: rsc,
+			cancel:         cancel,
+		}
+	} else {
+		return &CancelCloser{
+			ReadCloser: r,
+			cancel:     cancel,
+		}
+	}
 }
 
 func (c *CancelCloser) Close() error {
 	c.cancel()
-	return c.r.Close()
+	return c.ReadCloser.Close()
+}
+
+func (c *CancelSeekCloser) Close() error {
+	c.cancel()
+	return c.ReadSeekCloser.Close()
 }