// concurrency guarantees.
	cancel := func(err error) {
		origCancel(err)
		t.reqMu.Lock()
		delete(t.reqCanceler, req)
		t.reqMu.Unlock()
	}
	t.reqMu.Lock()
	if t.reqCanceler == nil {
		t.reqCanceler = make(map[*Request]context.CancelCauseFunc)
	}
	t.reqCanceler[req] = cancel
	t.reqMu.Unlock()
	return cancel
}

// CancelRequest cancels an in-flight request by closing its connection.