reqres.go
func (r *reqResReader) releasePreviousFragment() {
	fragment := r.previousFragment
	r.previousFragment = nil
	if fragment != nil {
		fragment.done()
	}
}
func (r *reqResReader) releasePreviousFragment() {
	fragment := r.previousFragment
	r.previousFragment = nil
	if fragment != nil {
		fragment.done()
	}
}
// releasePreviousFrament releases the last fragment returned by the reader if
// it's still around. This operation is idempotent.