1. func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
    2.     return WithDeadline(parent, time.Now().Add(timeout))
    3. }

    查看代码，WithTimeout其实就是调用了WithDeadline

    1. func WithDeadline(parent Context, d time.Time) (Context, CancelFunc) {
    2.     //1. 判断传入的Context是否为空
    3.     if parent == nil {
    4.         panic("cannot create context from nil parent")
    5.     }
    6.     //2.
    7.     if cur, ok := parent.Deadline(); ok && cur.Before(d) {
    8.         // The current deadline is already sooner than the new one.
    9.         return WithCancel(parent)
    10.     }
    11.     c := &timerCtx{
    12.         cancelCtx: newCancelCtx(parent),
    13.         deadline:  d,
    14.     }
    15.     propagateCancel(parent, c)
    16.     dur := time.Until(d)
    17.     if dur <= 0 {
    18.         c.cancel(true, DeadlineExceeded) // deadline has already passed
    19.         return c, func() { c.cancel(false, Canceled) }
    20.     }
    21.     c.mu.Lock()
    22.     defer c.mu.Unlock()
    23.     if c.err == nil {
    24.         c.timer = time.AfterFunc(dur, func() {
    25.             c.cancel(true, DeadlineExceeded)
    26.         })
    27.     }
    28.     return c, func() { c.cancel(true, Canceled) }
    29. }

    WithDeadline
    // 1.判断父context是否已经超时

    // 2.创建一个timerCtx

    // 3. 传播 timerCtx到所有子Context

    // 4.判断距离deadline还剩多少时间

    // 5.