cmd/natc: allow specifying the tsnet state dir

Which can make operating the service more convenient.
It makes sense to put the cluster state with this if specified, so
rearrange the logic to handle that.

Updates #14667

Signed-off-by: Fran Bull <fran@tailscale.com>

cmd/natc/natc.go

@@ -18,6 +18,7 @@ import (
 	"net/http"
 	"net/netip"
 	"os"
+	"path/filepath"
 	"strings"
 	"time"
 
@@ -59,7 +60,7 @@ func main() {
 		wgPort          = fs.Uint("wg-port", 0, "udp port for wireguard and peer to peer traffic")
 		clusterTag      = fs.String("cluster-tag", "", "optionally run in a consensus cluster with other nodes with this tag")
 		server          = fs.String("login-server", ipn.DefaultControlURL, "the base URL of control server")
-		clusterStateDir = fs.String("cluster-state-dir", "", "path to directory in which to store raft state")
+		stateDir        = fs.String("state-dir", "", "path to directory in which to store app state")
 	)
 	ff.Parse(fs, os.Args[1:], ff.WithEnvVarPrefix("TS_NATC"))
 
@@ -96,6 +97,7 @@ func main() {
 	}
 	ts := &tsnet.Server{
 		Hostname: *hostname,
+		Dir:      *stateDir,
 	}
 	ts.ControlURL = *server
 	if *wgPort != 0 {
@@ -156,7 +158,11 @@ func main() {
 	var ipp ippool.IPPool
 	if *clusterTag != "" {
 		cipp := ippool.NewConsensusIPPool(addrPool)
-		err = cipp.StartConsensus(ctx, ts, *clusterTag, *clusterStateDir)
+		clusterStateDir, err := getClusterStatePath(*stateDir)
+		if err != nil {
+			log.Fatalf("Creating cluster state dir failed: %v", err)
+		}
+		err = cipp.StartConsensus(ctx, ts, *clusterTag, clusterStateDir)
 		if err != nil {
 			log.Fatalf("StartConsensus: %v", err)
 		}
@@ -570,3 +576,28 @@ func proxyTCPConn(c net.Conn, dest string, ctor *connector) {
 
 	p.Start()
 }
+
+func getClusterStatePath(stateDirFlag string) (string, error) {
+	var dirPath string
+	if stateDirFlag != "" {
+		dirPath = stateDirFlag
+	} else {
+		confDir, err := os.UserConfigDir()
+		if err != nil {
+			return "", err
+		}
+		dirPath = filepath.Join(confDir, "nat-connector-state")
+	}
+	dirPath = filepath.Join(dirPath, "cluster")
+
+	if err := os.MkdirAll(dirPath, 0700); err != nil {
+		return "", err
+	}
+	if fi, err := os.Stat(dirPath); err != nil {
+		return "", err
+	} else if !fi.IsDir() {
+		return "", fmt.Errorf("%v is not a directory", dirPath)
+	}
+
+	return dirPath, nil
+}