From: Simon Gardling <titaniumtown@proton.me>
Date: 2026-04-21
Subject: [PATCH] core/unit: reset freezer state when unit becomes inactive/failed

When a frozen unit is killed via anything that delivers SIGKILL outside
systemd's normal deactivation path (`systemctl kill --signal=KILL`,
`kill -9`, the OOM killer, a segfault), the service transitions to
failed or inactive correctly, but `u->freezer_state` is never reset.
This leaves the unit permanently stuck at FreezerState=frozen:

  - `systemctl thaw` refuses with "Unit is not active".
  - `systemctl start`/`restart` refuses with
    "Cannot perform operation on frozen unit".
  - `reset-failed`, `daemon-reload`, `daemon-reexec`, and `revert` do
    not clear the state.

The only way to recover a persistent unit in this state is to reboot.

Reset the freezer state to FREEZER_RUNNING whenever a unit transitions
to an inactive or failed state, and cancel any pending freeze/thaw dbus
reply that will never complete. The cgroup is pruned immediately after
this point, so there is nothing left to freeze; the "running" state is
the correct neutral state for a dead unit. On restart, the unit will
pick up its freezer state from the cgroup hierarchy again.

This complements PR #38528 (stop/reset watchdog on freeze/thaw) by
addressing the general SIGKILL path instead of just the watchdog-
initiated kill.
---
--- a/src/core/unit.c
+++ b/src/core/unit.c
@@ -2754,6 +2754,16 @@
                 SET_FLAG(u->markers,
                          (1u << UNIT_MARKER_NEEDS_RELOAD)|(1u << UNIT_MARKER_NEEDS_RESTART),
                          false);
+
+                /* If the unit was frozen (or in a freezer transition) when it became inactive/failed,
+                 * reset the freezer state to running. The cgroup is about to be pruned; leaving the
+                 * state stuck at frozen would prevent the unit from ever being started, thawed, or
+                 * cleared without rebooting PID 1. Also cancel any pending freeze/thaw dbus reply. */
+                if (u->freezer_state != FREEZER_RUNNING) {
+                        unit_set_freezer_state(u, FREEZER_RUNNING);
+                        (void) bus_unit_send_pending_freezer_message(u, /* canceled= */ true);
+                }
+
                 unit_prune_cgroup(u);
                 unit_unlink_state_files(u);
         } else if (ns != os && ns == UNIT_RELOADING)