我可以检查上下文是否已设置超时吗？

I have a custom http client, which has a default timeout value. The code is like this:

type Client struct {
  *http.Client
  timeout time.Duration
}

func (c *Client) Send(ctx context.Context, r *http.Request) (int, []byte, error) {
  // If ctx has timeout set, then don't change it.
  // Otherwise, create new context with ctx.WithTimeout(c.timeout)
}

How can I check if ctx has timeout set or not?

Check the bool value return from context.Deadline:

Deadline returns the time when work done on behalf of this context should be canceled. Deadline returns ok==false when no deadline is set. Successive calls to Deadline return the same results.

func (c *Client) Send(ctx context.Context, r *http.Request) (int, []byte, error) {
    if _, deadlineSet := ctx.Deadline(); !deadlineSet {
        ctx, _ = context.WithTimeout(ctx, c.timeout)
    }
}