all: add flag for specifying snapshot file

We still support directly setting LastSnapshot to support embedding the
snapshot in the binary (like we do internally).  But this allows an
alternate way have restoring backups which doesn't require recompiling
binaries.

cmd/golink/link-snapshot.json

@@ -1 +0,0 @@
-

cmd/golink/main.go

@@ -8,11 +8,7 @@ import (
 	"github.com/tailscale/golink"
 )
 
-//go:embed link-snapshot.json
-var lastSnapshot []byte
-
 func main() {
-	golink.LastSnapshot = lastSnapshot
 	if err := golink.Run(); err != nil {
 		log.Fatal(err)
 	}

golink.go

@@ -16,6 +16,7 @@ import (
 	"log"
 	"net/http"
 	"net/url"
+	"os"
 	"path/filepath"
 	"regexp"
 	"sort"
@@ -33,6 +34,7 @@ var (
 	verbose    = flag.Bool("verbose", false, "be verbose")
 	sqlitefile = flag.String("sqlitedb", "", "path of SQLite database to store links")
 	dev        = flag.String("dev-listen", "", "if non-empty, listen on this addr and run in dev mode; auto-set sqlitedb if empty and don't use tsnet")
+	snapshot   = flag.String("snapshot", "", "file path of snapshot file")
 )
 
 var stats struct {
@@ -76,6 +78,17 @@ func Run() error {
 		return fmt.Errorf("NewSQLiteDB(%q): %w", *sqlitefile, err)
 	}
 
+	if *snapshot != "" {
+		if LastSnapshot != nil {
+			log.Printf("LastSnapshot already set; ignoring --snapshot")
+		} else {
+			var err error
+			LastSnapshot, err = os.ReadFile(*snapshot)
+			if err != nil {
+				log.Fatalf("error reading snapshot file %q: %v", *snapshot, err)
+			}
+		}
+	}
 	if err := restoreLastSnapshot(); err != nil {
 		log.Printf("restoring snapshot: %v", err)
 	}