nixos/etc: make re-mounting /etc overlay idempotent

nixos/modules/system/etc/etc.nix

@@ -238,7 +238,9 @@ in
       # this should not run because /etc is mounted via a systemd mount unit
       # instead. To a large extent this mimics what composefs does. Because
       # it's relatively simple, however, we avoid the composefs dependency.
-      if [[ ! $IN_NIXOS_SYSTEMD_STAGE1 ]]; then
+      # Since this script is not idempotent, it should not run when etc hasn't
+      # changed.
+      if [[ ! $IN_NIXOS_SYSTEMD_STAGE1 ]] && [[ "${config.system.build.etc}/etc" != "$(readlink -f /run/current-system/etc)" ]]; then
         echo "remounting /etc..."
 
         tmpMetadataMount=$(mktemp --directory)

nixos/tests/activation/etc-overlay-immutable.nix

@@ -20,7 +20,13 @@
   };
 
   testScript = ''
+    with subtest("/etc is mounted as an overlay"):
       machine.succeed("findmnt --kernel --type overlay /etc")
+
+    with subtest("switching to the same generation"):
+      machine.succeed("/run/current-system/bin/switch-to-configuration test")
+
+    with subtest("switching to a new generation"):
       machine.fail("stat /etc/newgen")
 
       machine.succeed("/run/current-system/specialisation/new-generation/bin/switch-to-configuration switch")

nixos/tests/activation/etc-overlay-mutable.nix

@@ -18,7 +18,13 @@
   };
 
   testScript = ''
+    with subtest("/etc is mounted as an overlay"):
       machine.succeed("findmnt --kernel --type overlay /etc")
+
+    with subtest("switching to the same generation"):
+      machine.succeed("/run/current-system/bin/switch-to-configuration test")
+
+    with subtest("switching to a new generation"):
       machine.fail("stat /etc/newgen")
       machine.succeed("echo -n 'mutable' > /etc/mutable")