code: func (c *certificateCache) set(addr string, certificate ssh.Signer, ttl time.Duration) error {
  	c.mu.Lock()
  	defer c.mu.Unlock()
  
  	err := c.cache.Set(addr, certificate, ttl)
  	if err != nil {
  		return trace.Wrap(err)
  	}
  
  	return nil
  }
docs: // set is goroutine safe and will set a ssh.Signer for a principal in
  // the cache.