code: func getContext(marker interface{}) *context {
  	globals.lockContexts.Lock()
  	defer globals.lockContexts.Unlock()
  	if globals.contexts == nil {
  		return nil
  	}
  	c, ok := globals.contexts[marker]
  	if !ok {
  		return &consts.defaultContext
  	}
  	return c
  }
docs: // getContext returns a context if tracing is enabled.
  //
  // If the marker is unknown, the default {1, 1} context is returned.
  // It is implicitly locking.