nixos/systemd: add units for capsule support

nixos/modules/system/boot/systemd.nix

···
   ++ [
     "systemd-exit.service"
     "systemd-update-done.service"
+
+    # Capsule support
+    "capsule@.service"
+    "capsule.slice"
   ]
   ++ cfg.additionalUpstreamSystemUnits;
 

nixos/modules/system/boot/systemd/user.nix

···
     "background.slice"
     "basic.target"
     "bluetooth.target"
+    "capsule@.target"
     "default.target"
     "exit.target"
     "graphical-session-pre.target"

nixos/tests/all-tests.nix

···
   systemd-binfmt = handleTestOn [ "x86_64-linux" ] ./systemd-binfmt.nix { };
   systemd-boot = handleTest ./systemd-boot.nix { };
   systemd-bpf = runTest ./systemd-bpf.nix;
+  systemd-capsules = runTest ./systemd-capsules.nix;
   systemd-confinement = handleTest ./systemd-confinement { };
   systemd-coredump = runTest ./systemd-coredump.nix;
   systemd-credentials-tpm2 = runTest ./systemd-credentials-tpm2.nix;

nixos/tests/systemd-capsules.nix

···
+{ lib, ... }:
+{
+  name = "systemd-capsules";
+
+  meta.maintainers = with lib.maintainers; [ fpletz ];
+
+  nodes.machine =
+    { pkgs, ... }:
+    {
+      environment.systemPackages = [ pkgs.hello ];
+      systemd.user.services.alice-sleep = {
+        wantedBy = [ "[e-mail]" ];
+        serviceConfig = {
+          ExecStart = "${pkgs.coreutils}/bin/sleep 999";
+        };
+      };
+    };
+
+  testScript = # python
+    ''
+      machine.wait_for_unit("multi-user.target")
+
+      with subtest("capsule setup"):
+        machine.succeed("systemctl start [e-mail]")
+
+      with subtest("imperative user service in capsule"):
+        machine.succeed("systemd-run --capsule=alice --unit=sleeptest.service /run/current-system/sw/bin/sleep 999")
+        machine.succeed("systemctl --capsule=alice status sleeptest.service")
+
+      with subtest("declarative user service in capsule"):
+        machine.succeed("systemctl --capsule=alice status alice-sleep.service")
+        machine.succeed("systemctl --capsule=alice stop alice-sleep.service")
+        machine.fail("systemctl --capsule=alice status alice-sleep.service")
+        machine.succeed("systemctl --capsule=alice start alice-sleep.service")
+        machine.succeed("systemctl --capsule=alice status alice-sleep.service")
+
+      with subtest("interactive shell with terminal in capsule"):
+        hello_output = machine.succeed("systemd-run -t --capsule=alice /run/current-system/sw/bin/bash -i -c 'hello | tee ~/hello'")
+        assert hello_output == "Hello, world!\r\n"
+        machine.copy_from_vm("/var/lib/capsules/alice/hello")
+
+      with subtest("capsule cleanup"):
+        machine.succeed("systemctl --capsule=alice stop sleeptest.service")
+        machine.succeed("systemctl stop [e-mail]")
+        machine.succeed("systemctl clean --all [e-mail]")
+    '';
+}