code: func WithGoRand(c context.Context, fn func(r *rand.Rand) error) error {
  	if r := getRand(c); r != nil {
  		return r.WithGoRand(fn)
  	}
  
  	// Return our globalRandBase. We MUST hold globalRand's lock in order for this
  	// to be safe.
  	l, base := getGlobalRand()
  	l.Lock()
  	defer l.Unlock()
  	return fn(base)
  }
docs: // WithGoRand invokes the supplied "fn" while holding an exclusive lock
  // for it. This can be used by callers to pull and use a *rand.Rand instance
  // out of the Context safely.
  //
  // The callback's r must not be retained or used outside of the scope of the
  // callback.