code: func validateDatacentersCfg(c *validation.Context, cfg *configPB.Datacenters) {
  	// Datacenter filenames must be unique.
  	// Keep records of which ones we've already seen.
  	files := stringset.New(len(cfg.Datacenter))
  	for _, file := range cfg.Datacenter {
  		switch {
  		case file == "":
  			c.Errorf("datacenter filenames are required and must be non-empty")
  		case !files.Add(file):
  			c.Errorf("duplicate filename %q", file)
  		}
  	}
  }
docs: // validateDatacentersCfg validates datacenters.cfg.