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package context
import "context"
// 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
preserveExpiredValues bool
valuesCtx context.Context
}
var _ context.Context = (*valueOnlyContext)(nil)
// 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:
}
}
return v.valuesCtx.Value(key)
}
// 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,
preserveExpiredValues: GetPreserveExpiredValues(ctx),
}
}
type preserveExpiredValuesKey struct{}
// 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)
}
// 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
}
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