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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,81 @@ | ||
| package context | ||
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| import "context" | ||
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| // valueOnlyContext provides a utility to preserve only the values of a | ||
| // Context. Suppressing any cancellation or deadline on that context being | ||
| // propagated downstream of this value. | ||
| // | ||
| // If preserveExpiredValues is false (default), and the valueCtx is canceled, | ||
| // calls to lookup values with the Values method, will always return nil. Setting | ||
| // preserveExpiredValues to true, will allow the valueOnlyContext to lookup | ||
| // values in valueCtx even if valueCtx is canceled. | ||
| // | ||
| // Based on the Go standard libraries net/lookup.go onlyValuesCtx utility. | ||
| // https://github.com/golang/go/blob/da2773fe3e2f6106634673a38dc3a6eb875fe7d8/src/net/lookup.go | ||
| type valueOnlyContext struct { | ||
| context.Context | ||
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| preserveExpiredValues bool | ||
| valuesCtx context.Context | ||
| } | ||
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| var _ context.Context = (*valueOnlyContext)(nil) | ||
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| // Value looks up the key, returning its value. If configured to not preserve | ||
| // values of expired context, and the wrapping context is canceled, nil will be | ||
| // returned. | ||
| func (v *valueOnlyContext) Value(key interface{}) interface{} { | ||
| if !v.preserveExpiredValues { | ||
| select { | ||
| case <-v.valuesCtx.Done(): | ||
| return nil | ||
| default: | ||
| } | ||
| } | ||
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| return v.valuesCtx.Value(key) | ||
| } | ||
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| // WithSuppressCancel wraps the Context value, suppressing its deadline and | ||
| // cancellation events being propagated downstream to consumer of the returned | ||
| // context. | ||
| // | ||
| // By default the wrapped Context's Values are available downstream until the | ||
| // wrapped Context is canceled. Once the wrapped Context is canceled, Values | ||
| // method called on the context return will no longer lookup any key. As they | ||
| // are now considered expired. | ||
| // | ||
| // To override this behavior, use WithPreserveExpiredValues on the Context | ||
| // before it is wrapped by WithSuppressCancel. This will make the Context | ||
| // returned by WithSuppressCancel allow lookup of expired values. | ||
| func WithSuppressCancel(ctx context.Context) context.Context { | ||
| return &valueOnlyContext{ | ||
| Context: context.Background(), | ||
| valuesCtx: ctx, | ||
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| preserveExpiredValues: GetPreserveExpiredValues(ctx), | ||
| } | ||
| } | ||
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| type preserveExpiredValuesKey struct{} | ||
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| // WithPreserveExpiredValues adds a Value to the Context if expired values | ||
| // should be preserved, and looked up by a Context wrapped by | ||
| // WithSuppressCancel. | ||
| // | ||
| // WithPreserveExpiredValues must be added as a value to a Context, before that | ||
| // Context is wrapped by WithSuppressCancel | ||
| func WithPreserveExpiredValues(ctx context.Context, enable bool) context.Context { | ||
| return context.WithValue(ctx, preserveExpiredValuesKey{}, enable) | ||
| } | ||
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| // GetPreserveExpiredValues looks up, and returns the PreserveExpressValues | ||
| // value in the context. Returning true if enabled, false otherwise. | ||
| func GetPreserveExpiredValues(ctx context.Context) bool { | ||
| v := ctx.Value(preserveExpiredValuesKey{}) | ||
| if v != nil { | ||
| return v.(bool) | ||
| } | ||
| return false | ||
| } |