+ var cancel context.CancelFunc
+ if c.Timeout > 0 {
+ ctx := req.Context()
+ ctx, cancel = context.WithDeadline(ctx, time.Now().Add(c.Timeout))
+ req = req.WithContext(ctx)
+ }
+ resp, err := c.httpClient().Do(req)
+ if err == nil && cancel != nil {
+ // We need to call cancel() eventually, but we can't
+ // use "defer cancel()" because the context has to
+ // stay alive until the caller has finished reading
+ // the response body.
+ resp.Body = cancelOnClose{ReadCloser: resp.Body, cancel: cancel}
+ } else if cancel != nil {
+ cancel()
+ }
+ return resp, err
+}
+
+// cancelOnClose calls a provided CancelFunc when its wrapped
+// ReadCloser's Close() method is called.
+type cancelOnClose struct {
+ io.ReadCloser
+ cancel context.CancelFunc
+}
+
+func (coc cancelOnClose) Close() error {
+ err := coc.ReadCloser.Close()
+ coc.cancel()
+ return err