nixos/getty: add option to autologin once per boot

nixos/modules/services/ttys/getty.nix

@@ -7,14 +7,26 @@ let
 
   baseArgs = [
     "--login-program" "${cfg.loginProgram}"
-  ] ++ optionals (cfg.autologinUser != null) [
+  ] ++ optionals (cfg.autologinUser != null && !cfg.autologinOnce) [
     "--autologin" cfg.autologinUser
   ] ++ optionals (cfg.loginOptions != null) [
     "--login-options" cfg.loginOptions
   ] ++ cfg.extraArgs;
 
   gettyCmd = args:
-    "@${pkgs.util-linux}/sbin/agetty agetty ${escapeShellArgs baseArgs} ${args}";
+    "${pkgs.util-linux}/sbin/agetty ${escapeShellArgs baseArgs} ${args}";
+
+  autologinScript = ''
+    otherArgs="--noclear --keep-baud $TTY 115200,38400,9600 $TERM";
+    ${lib.optionalString cfg.autologinOnce ''
+      autologged="/run/agetty.autologged"
+      if test "$TTY" = tty1 && ! test -f "$autologged"; then
+        touch "$autologged"
+        exec ${gettyCmd "$otherArgs --autologin ${cfg.autologinUser}"}
+      fi
+    ''}
+    exec ${gettyCmd "$otherArgs"}
+  '';
 
 in
 
@@ -40,6 +52,16 @@ in
         '';
       };
 
+      autologinOnce = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          If enabled the automatic login will only happen in the first tty
+          once per boot. This can be useful to avoid retyping the account
+          password on systems with full disk encrypted.
+        '';
+      };
+
       loginProgram = mkOption {
         type = types.path;
         default = "${pkgs.shadow}/bin/login";
@@ -106,9 +128,11 @@ in
 
     systemd.services."getty@" =
       { serviceConfig.ExecStart = [
-          "" # override upstream default with an empty ExecStart
-          (gettyCmd "--noclear --keep-baud %I 115200,38400,9600 $TERM")
+          # override upstream default with an empty ExecStart
+          ""
+          (pkgs.writers.writeDash "getty" autologinScript)
         ];
+        environment.TTY = "%I";
         restartIfChanged = false;
       };