feat(ha): add NixOS modules for DRBD+XFS+LIO+Corosync+Pacemaker HA stack

All 7 acceptance tests pass on live NixOS 25.11 VMs (VMIDs 200/201 on
pve1).  Failover completes in ~5 s with data integrity verified.

modules/ha/pacemaker-stack.nix — fixes four NixOS-specific breakages:
  - systemd StateDirectory resets /var/lib/pacemaker to root:root; removed
    and replaced with ExecStartPre to create/chown dirs as hacluster
  - HA_SBIN_DIR points to a non-existent Nix store path; overridden to
    /run/current-system/sw/bin so crm_master resolves correctly
  - OCF agents need an explicit broad PATH (iproute2, util-linux, xfsprogs,
    drbd, bash, etc.) — NixOS services have no implicit PATH
  - FUSER=true bypasses the psmisc fuser check_binary call in the
    Filesystem OCF agent (psmisc not installed on minimal hosts)

modules/ha/iscsi-target.nix — LIO iSCSI target via targetctl with a
Python/rtslib_fb ExecStop that explicitly clears the kernel LIO state
(not just saves JSON), so the XFS backing store's file descriptor is
released before umount — preventing EBUSY stop timeouts on failover.
Includes an empty-config guard so the secondary node never overwrites
the primary's saveconfig.json with an empty one.

test-lab/ha/common.nix — updated to import both modules, use fencing
dont-care (no STONITH in test lab), omit LVM handlers (non-existent on
NixOS paths), and merge repeated services/networking attr sets to satisfy
statix W20.  test-lab/ha/acceptance-tests.sh — final v4 with crm_standby
fix (pacemaker 3.x API).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HaH1cSGvhogRP5ExoF6nD8

modules/ha/iscsi-target.nix

@@ -0,0 +1,99 @@
+# LIO iSCSI target service (targetctl) for NixOS HA clusters.
+#
+# Provides the targetctl.service that saves/restores LIO configuration from
+# /etc/target/saveconfig.json.  Pacemaker manages this service via its
+# systemd resource agent (class="systemd" type="targetctl").
+#
+# Why ExecStop is not simply "targetctl save":
+#   targetctl save writes the LIO config to JSON but does NOT remove the LIO
+#   target from the kernel's configfs.  As a result, any fileio backing store
+#   that LIO has open (e.g. iscsi-lun.img on an XFS-over-DRBD filesystem)
+#   stays referenced in the kernel.  The subsequent XFS umount from the
+#   Filesystem OCF resource then returns EBUSY and either hangs for the full
+#   op-stop timeout or fails outright, blocking the entire failover.
+#
+#   The ExecStop script here additionally tears down the kernel LIO state
+#   via rtslib_fb after saving, so the backing-store file descriptor is
+#   released and umount succeeds immediately.
+#
+# Empty-config guard:
+#   The save step is skipped when no iSCSI targets are currently active.
+#   This prevents the secondary node (where LIO was never started) from
+#   overwriting a valid saveconfig.json with an empty one when Pacemaker
+#   stops the iscsi-target resource as part of a failover or cleanup.
+{ pkgs, ... }:
+
+let
+  python3 = pkgs.python3.withPackages (ps: [ ps.rtslib-fb ]);
+  targetctl = "${pkgs.targetcli-fb}/bin/targetctl";
+
+  targetctlStop = pkgs.writeScript "targetctl-stop" ''
+    #!${python3}/bin/python3
+    import subprocess, sys
+    import rtslib_fb
+
+    root = rtslib_fb.RTSRoot()
+    targets = list(root.targets)
+    if targets:
+        subprocess.run(
+            ["${targetctl}", "save", "/etc/target/saveconfig.json"],
+            capture_output=True,
+        )
+        print(f"saved {len(targets)} iSCSI target(s)")
+    else:
+        print("no active LIO targets — saveconfig.json unchanged")
+
+    for target in targets:
+        try:
+            for tpg in list(target.tpgs):
+                tpg.enable = False
+            target.delete()
+        except Exception as e:
+            print(f"warn (target): {e}", file=sys.stderr)
+    for so in list(root.storage_objects):
+        try:
+            so.delete()
+        except Exception as e:
+            print(f"warn (backstore): {e}", file=sys.stderr)
+    print("LIO kernel target cleared")
+  '';
+in
+{
+  boot.kernelModules = [
+    "target_core_mod"
+    "iscsi_target_mod"
+    "target_core_file"
+    "target_core_pscsi"
+    "target_core_user"
+    "configfs"
+  ];
+
+  systemd = {
+    mounts = [{
+      where = "/sys/kernel/config";
+      what = "configfs";
+      type = "configfs";
+      wantedBy = [ "multi-user.target" ];
+      before = [ "targetctl.service" ];
+    }];
+    services.targetctl = {
+      description = "LIO iSCSI target config save/restore";
+      wantedBy = [ "multi-user.target" ];
+      after = [ "sys-kernel-config.mount" "network.target" ];
+      requires = [ "sys-kernel-config.mount" ];
+      serviceConfig = {
+        Type = "oneshot";
+        RemainAfterExit = true;
+        ExecStart = "${targetctl} restore /etc/target/saveconfig.json";
+        ExecStop = "${targetctlStop}";
+      };
+      unitConfig.ConditionFileNotEmpty = "/etc/target/saveconfig.json";
+    };
+    tmpfiles.rules = [
+      "d /etc/target 0750 root root -"
+      "f /etc/target/saveconfig.json 0640 root root -"
+    ];
+  };
+
+  environment.systemPackages = [ pkgs.targetcli-fb ];
+}