type deadliner interface {
		Deadline() (time.Time, bool)
	}
	if td, ok := t.(deadliner); ok {
		if deadline, ok := td.Deadline(); ok {
			timeout = time.Until(deadline)
			timeout = timeout * 19 / 20 // Give 5% headroom for cleanup and error-reporting.
		}
	}

	deadline := time.Now().Add(timeout)
	for {
		if fn() {
			return true
		}
		if time.Until(deadline) < pollDuration {
			return false
		}