mirror of
https://github.com/Luzifer/ansible-role-version.git
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63 lines
1.9 KiB
Go
63 lines
1.9 KiB
Go
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// Package ctxext provides multiple useful context constructors.
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package ctxext
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import (
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"time"
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context "golang.org/x/net/context"
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)
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// WithDeadlineFraction returns a Context with a fraction of the
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// original context's timeout. This is useful in sequential pipelines
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// of work, where one might try options and fall back to others
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// depending on the time available, or failure to respond. For example:
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//
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// // getPicture returns a picture from our encrypted database
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// // we have a pipeline of multiple steps. we need to:
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// // - get the data from a database
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// // - decrypt it
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// // - apply many transforms
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// //
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// // we **know** that each step takes increasingly more time.
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// // The transforms are much more expensive than decryption, and
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// // decryption is more expensive than the database lookup.
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// // If our database takes too long (i.e. >0.2 of available time),
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// // there's no use in continuing.
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// func getPicture(ctx context.Context, key string) ([]byte, error) {
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// // fractional timeout contexts to the rescue!
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//
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// // try the database with 0.2 of remaining time.
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// ctx1, _ := ctxext.WithDeadlineFraction(ctx, 0.2)
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// val, err := db.Get(ctx1, key)
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// if err != nil {
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// return nil, err
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// }
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//
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// // try decryption with 0.3 of remaining time.
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// ctx2, _ := ctxext.WithDeadlineFraction(ctx, 0.3)
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// if val, err = decryptor.Decrypt(ctx2, val); err != nil {
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// return nil, err
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// }
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//
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// // try transforms with all remaining time. hopefully it's enough!
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// return transformer.Transform(ctx, val)
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// }
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//
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//
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func WithDeadlineFraction(ctx context.Context, fraction float64) (
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context.Context, context.CancelFunc) {
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d, found := ctx.Deadline()
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if !found { // no deadline
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return context.WithCancel(ctx)
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}
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left := d.Sub(time.Now())
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if left < 0 { // already passed...
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return context.WithCancel(ctx)
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}
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left = time.Duration(float64(left) * fraction)
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return context.WithTimeout(ctx, left)
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}
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