Sync the env file at startup, not just at install: updates append new settings

cmd/gpu-turnstile/main.go

@@ -172,6 +172,7 @@ func main() {
 	}
 	log, logOut, logCloser := newLogger(cfg)
 	defer logCloser.Close()
+	syncEnvFile(resolveConfigPath(configPath), cfg.LogFile, log)
 
 	if service.IsService() {
 		if err := service.Run(func(ctx context.Context) error { return run(ctx, cfg, log, logOut, true) }); err != nil {
@@ -188,6 +189,27 @@ func main() {
 	}
 }
 
+// syncEnvFile upgrades an installer-written config file after an update:
+// settings added since its CFG_VER are appended (commented out) and CFG_VER
+// is bumped. Files not written by the installer (no CFG_VER), up-to-date
+// files and dev builds are left untouched; a write failure is logged, not
+// fatal.
+func syncEnvFile(path, logFile string, log *slog.Logger) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return // no config file; nothing to upgrade
+	}
+	synced, changed := config.SyncSample(string(data), version, logFile)
+	if !changed {
+		return
+	}
+	if err := os.WriteFile(path, []byte(synced), 0o644); err != nil {
+		log.Warn("could not append new settings to the config file", "path", path, "err", err)
+		return
+	}
+	log.Warn("config file updated: new settings appended", "path", path, "version", version)
+}
+
 // defaultConfigPath returns gpu-turnstile.env next to the executable.
 func defaultConfigPath() string {
 	exe, err := os.Executable()