func WithDeadlineFraction(ctx context.Context, fraction float64) (context.Context, context.CancelFunc) {
d, found := ctx.Deadline()
if !found { // no deadline
return context.WithCancel(ctx)
}
left := d.Sub(time.Now())
if left < 0 { // already passed...
return context.WithCancel(ctx)
}
left = time.Duration(float64(left) * fraction)
return context.WithTimeout(ctx, left)
}