Add state load/dump logic to the main server binary

The server will attempt to load state on startup, and it will dump
state on (graceful) shutdown.

cmd/keystored/main.go

@@ -28,6 +28,7 @@ var (
 // server config in a single object for YAML deserialization.
 type Config struct {
 	server.Config `yaml:",inline"`
+	StateFile     string                   `yaml:"state_file"`
 	ServerConfig  *serverutil.ServerConfig `yaml:"http_server"`
 }
 
@@ -45,6 +46,39 @@ func loadConfig() (*Config, error) {
 	return &config, nil
 }
 
+func loadState(ks *server.KeyStore, path string) error {
+	f, err := os.Open(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil
+		}
+		return err
+	}
+	defer f.Close()
+
+	log.Printf("loading state from %s", path)
+	if err := ks.Load(f); err != nil {
+		return err
+	}
+
+	if err := os.Remove(path); err != nil {
+		// Non-fatal error.
+		log.Printf("error removing state file: %v", err)
+	}
+	return nil
+}
+
+func dumpState(ks *server.KeyStore, path string) error {
+	f, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0600)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	log.Printf("dumping state to %s", path)
+	return ks.Dump(f)
+}
+
 func main() {
 	log.SetFlags(0)
 	flag.Parse()
@@ -60,9 +94,21 @@ func main() {
 	}
 	prometheus.MustRegister(server.NewKeystoreCollector(ks))
 
+	if config.StateFile != "" {
+		if err := loadState(ks, config.StateFile); err != nil {
+			log.Printf("error loading state: %v", err)
+		}
+	}
+
 	srv := server.NewServer(ks)
 
 	if err := serverutil.Serve(srv, config.ServerConfig, *addr); err != nil {
 		log.Fatal(err)
 	}
+
+	if config.StateFile != "" {
+		if err := dumpState(ks, config.StateFile); err != nil {
+			log.Fatalf("error dumping state: %v", err)
+		}
+	}
 }