nixos/tests: add nspawn-daemon-reexec-dbus repro

Loops `systemctl daemon-reexec` inside a systemd-nspawn test container and
asserts the in-container D-Bus stays usable afterwards. On affected systemd
under nspawn, one of the re-execs wedges PID 1 (it never finishes
re-initialising after the re-exec), and every later `systemctl` call hangs or
returns 'Transport endpoint is not connected'. The identical loop on a QEMU
node survives indefinitely, so the test is nspawn-specific.

Assisted-by: Claude:claude-opus-4-8

nixos/tests/all-tests.nix

@@ -152,6 +152,7 @@ in
     ssh-backdoor = runTestOn [ "x86_64-linux" ] ./nixos-test-driver/ssh-backdoor.nix;
     console-log = runTest ./nixos-test-driver/console-log.nix;
     containers = runTest ./nixos-test-driver/containers.nix;
+    nspawn-daemon-reexec-dbus = runTest ./nspawn-daemon-reexec-dbus.nix;
     skip-typecheck = runTest ./nixos-test-driver/skip-typecheck.nix;
     options-doc-regression = import ./nixos-test-driver/options-doc-regression.nix { inherit pkgs; };
     driver-timeout =

nixos/tests/nspawn-daemon-reexec-dbus.nix

@@ -0,0 +1,79 @@
+# Regression test for an nspawn-only systemd re-exec failure that broke D-Bus.
+#
+# Demonstrates `systemctl show` keeps working on `daemon-reexec`.
+#
+# Trigger: `systemctl daemon-reexec` issues a D-Bus `Manager.Reexecute`, like
+# `switch-to-configuration-ng` on a systemd package change.
+#
+# Root cause: inside nspawn test containers PID 1 re-send `READY=1` on the
+# `NOTIFY_SOCKET` on re-exec. The test driver stopped draining that socket
+# after boot, so until drained its receive buffer filled and `systemctl` hung /
+# errored `Failed to connect to bus: Transport endpoint is not connected`.
+{ ... }:
+{
+  name = "nspawn-daemon-reexec-dbus";
+
+  # `containers.<name>` => systemd-nspawn machine (vs `nodes.<name>` => QEMU).
+  # An empty container boots full systemd + D-Bus, which is all we need.
+  containers.machine = { };
+
+  testScript = # python
+    ''
+      import re
+
+      BUS_BROKEN = re.compile(
+          r"Transport endpoint is not connected|Failed to connect to bus"
+      )
+
+      # Without the fix the notify socket's receive buffer fills after 10
+      # undrained `READY=1` resends, so PID 1 blocks then.
+      REEXECS = 10
+
+
+      def bus_broken():
+          """Whether the in-container D-Bus is unusable. A broken bus prints a
+          transport error or hangs until `timeout` kills it (status 124); both
+          count as broken."""
+          status, out = machine.execute(
+              "timeout 10 systemctl show -p ActiveState --value "
+              "multi-user.target 2>&1",
+              check_return=False,
+              timeout=20,
+          )
+          return status != 0 or bool(BUS_BROKEN.search(out)), status, out
+
+
+      machine.start()
+      machine.wait_for_unit("multi-user.target", timeout=120)
+
+      # Pre-reexec sanity: the bus works and shows no break.
+      broken, status, out = bus_broken()
+      assert not broken, (
+          f"bus already broken before any reexec: status={status} out={out!r}"
+      )
+      machine.log(f"pre-reexec sanity OK: {out.strip()!r}")
+
+      broke_at = None
+      for i in range(1, REEXECS + 1):
+          # The same D-Bus Manager.Reexecute that switch-to-configuration issues
+          # on a systemd change.
+          machine.execute(
+              "timeout 30 systemctl daemon-reexec",
+              check_return=False,
+              timeout=45,
+          )
+          broken, status, out = bus_broken()
+          machine.log(f"[reexec {i}] status={status} out={out.strip()!r}")
+          if broken:
+              broke_at = i
+              break
+
+      assert broke_at is None, (
+          f"nspawn D-Bus broke after daemon-reexec #{broke_at} of {REEXECS} "
+          "(systemctl hung or returned a bus transport error). The re-exec'd "
+          "PID 1 never finished re-initialising -- the test driver stopped "
+          "draining the notify socket, so PID 1's READY=1 resend blocked. "
+          "Never observed on QEMU."
+      )
+    '';
+}