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/pacemaker-stack.nix

@@ -0,0 +1,94 @@
+# Pacemaker + Corosync HA stack for NixOS with known-good workarounds.
+#
+# Issues fixed here (confirmed through live testing on NixOS 25.11):
+#
+# 1. StateDirectory ownership reset: systemd's StateDirectory=pacemaker
+#    creates /var/lib/pacemaker owned root:root.  pacemaker-based (the CIB
+#    daemon) runs as the hacluster user and calls pcmk__daemon_can_write,
+#    which requires the CIB directory to be owned by hacluster or be
+#    group-writable by haclient.  Workaround: remove StateDirectory and let
+#    ExecStartPre create every required subdirectory with correct ownership.
+#
+# 2. HA_SBIN_DIR wrong path: ocf-shellfuncs sets HA_SBIN_DIR to the Nix
+#    store path of the resource-agents derivation's /sbin, which doesn't
+#    exist.  The DRBD OCF agent uses ${HA_SBIN_DIR}/crm_master, so it exits
+#    127 without this override.  Fix: export HA_SBIN_DIR=/run/current-system/sw/bin.
+#
+# 3. Broad PATH for OCF agents: the resource executor (pacemaker-execd) runs
+#    OCF agent scripts as children.  NixOS provides no implicit PATH for
+#    system services; without an explicit PATH the agents can't find ip, ss,
+#    mount, umount, drbdadm, etc.
+#
+# 4. FUSER=true: the Filesystem OCF agent calls check_binary $FUSER (default:
+#    fuser from psmisc), which is not installed.  Setting FUSER=true makes
+#    check_binary succeed (true is always in PATH) and the subsequent
+#    "$FUSER -km $mountpoint" becomes a no-op.  Pair with force_unmount=false
+#    on each Filesystem resource unless you want lazy unmount behaviour.
+{ lib, pkgs, ... }:
+
+let
+  ocfBinPath = lib.concatStringsSep ":" [
+    "${pkgs.iproute2}/bin"
+    "${pkgs.iproute2}/sbin"
+    "${pkgs.iputils}/bin"
+    "${pkgs.util-linux}/bin"
+    "${pkgs.util-linux}/sbin"
+    "${pkgs.gawk}/bin"
+    "${pkgs.gnugrep}/bin"
+    "${pkgs.gnused}/bin"
+    "${pkgs.coreutils}/bin"
+    "${pkgs.bash}/bin"
+    "${pkgs.procps}/bin"
+    "${pkgs.xfsprogs}/bin"
+    "${pkgs.drbd}/bin"
+    "${pkgs.python3}/bin"
+    "/run/current-system/sw/bin"
+    "/run/current-system/sw/sbin"
+    "/usr/local/sbin"
+    "/usr/local/bin"
+    "/usr/sbin"
+    "/usr/bin"
+    "/sbin"
+    "/bin"
+  ];
+
+  # Single pre-start script: schemas symlink + directory ownership.
+  # Runs before pacemakerd so pacemaker-based finds hacluster-owned dirs.
+  preStartCmd = "${pkgs.bash}/bin/bash -c '"
+    + "ln -sfn ${pkgs.pacemaker}/share/pacemaker /var/lib/pacemaker/schemas; "
+    + "for d in /var/lib/pacemaker /var/lib/pacemaker/cib /var/lib/pacemaker/cores "
+    + "/var/lib/pacemaker/pengine /var/lib/pacemaker/blackbox "
+    + "/var/lib/pacemaker/hostcache; do "
+    + "mkdir -p \"\\$d\" && chown hacluster:pacemaker \"\\$d\" && chmod 2770 \"\\$d\"; "
+    + "done'";
+
+  ocfEnv = {
+    PATH = lib.mkForce ocfBinPath;
+    OCF_ROOT = "${pkgs.ocf-resource-agents}/usr/lib/ocf";
+    HA_SBIN_DIR = "/run/current-system/sw/bin";
+    FUSER = "true";
+  };
+in
+{
+  users.groups.haclient = { };
+
+  services.corosync.enable = true;
+  services.pacemaker.enable = true;
+
+  systemd.services = {
+    pacemaker = {
+      serviceConfig = {
+        StateDirectory = lib.mkForce "";
+        ExecStartPre = lib.mkBefore [ preStartCmd ];
+      };
+      environment = ocfEnv;
+    };
+    pacemaker-execd.environment = ocfEnv;
+  };
+
+  environment.systemPackages = with pkgs; [
+    corosync
+    pacemaker
+    ocf-resource-agents
+  ];
+}