// WithDeadlineFraction returns a Context with a fraction of the
// original context's timeout. This is useful in sequential pipelines
// of work, where one might try options and fall back to others
// depending on the time available, or failure to respond. For example:
//
// // getPicture returns a picture from our encrypted database
// // we have a pipeline of multiple steps. we need to:
// // - get the data from a database
// // - decrypt it
// // - apply many transforms
// //
// // we **know** that each step takes increasingly more time.
// // The transforms are much more expensive than decryption, and
// // decryption is more expensive than the database lookup.
// // If our database takes too long (i.e. >0.2 of available time),
// // there's no use in continuing.
// func getPicture(ctx context.Context, key string) ([]byte, error) {
// // fractional timeout contexts to the rescue!
//
// // try the database with 0.2 of remaining time.
// ctx1, _ := ctxext.WithDeadlineFraction(ctx, 0.2)
// val, err := db.Get(ctx1, key)
// if err != nil {
// return nil, err
// }
//
// // try decryption with 0.3 of remaining time.
// ctx2, _ := ctxext.WithDeadlineFraction(ctx, 0.3)
// if val, err = decryptor.Decrypt(ctx2, val); err != nil {
// return nil, err
// }
//
// // try transforms with all remaining time. hopefully it's enough!
// return transformer.Transform(ctx, val)
// }
//
//
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)
}