core/service: properly handle freezer action -> watchdog propagation (#39222)

Follow-up for 25178aadb2bd04ef9e63f48c1ef42fb309f9332e

src/basic/unit-def.c

@@ -144,6 +144,16 @@ FreezerState freezer_state_finish(FreezerState state) {
         return freezer_state_finish_table[state];
 }
 
+FreezerState freezer_state_objective(FreezerState state) {
+        FreezerState objective;
+
+        objective = freezer_state_finish(state);
+        if (objective == FREEZER_FROZEN_BY_PARENT)
+                objective = FREEZER_FROZEN;
+
+        return objective;
+}
+
 static const char* const unit_marker_table[_UNIT_MARKER_MAX] = {
         [UNIT_MARKER_NEEDS_RELOAD]  = "needs-reload",
         [UNIT_MARKER_NEEDS_RESTART] = "needs-restart",

src/basic/unit-def.h

@@ -325,6 +325,7 @@ UnitActiveState unit_active_state_from_string(const char *s) _pure_;
 const char* freezer_state_to_string(FreezerState i) _const_;
 FreezerState freezer_state_from_string(const char *s) _pure_;
 FreezerState freezer_state_finish(FreezerState i) _const_;
+FreezerState freezer_state_objective(FreezerState state) _const_;
 
 const char* unit_marker_to_string(UnitMarker m) _const_;
 UnitMarker unit_marker_from_string(const char *s) _pure_;

src/core/service.c

@@ -414,7 +414,7 @@ static void service_extend_timeout(Service *s, usec_t extend_timeout_usec) {
 static void service_reset_watchdog(Service *s) {
         assert(s);
 
-        if (freezer_state_finish(UNIT(s)->freezer_state) != FREEZER_RUNNING) {
+        if (freezer_state_objective(UNIT(s)->freezer_state) != FREEZER_RUNNING) {
                 log_unit_debug(UNIT(s), "Service is currently %s, skipping resetting watchdog.",
                                freezer_state_to_string(UNIT(s)->freezer_state));
                 return;
@@ -1425,7 +1425,7 @@ static int service_coldplug(Unit *u) {
                 (void) unit_setup_exec_runtime(u);
 
         if (IN_SET(s->deserialized_state, SERVICE_START_POST, SERVICE_RUNNING, SERVICE_RELOAD, SERVICE_RELOAD_SIGNAL, SERVICE_RELOAD_NOTIFY, SERVICE_REFRESH_EXTENSIONS, SERVICE_MOUNTING) &&
-            freezer_state_finish(u->freezer_state) == FREEZER_RUNNING)
+            freezer_state_objective(u->freezer_state) == FREEZER_RUNNING)
                 service_start_watchdog(s);
 
         if (UNIT_ISSET(s->accept_socket)) {
@@ -5643,18 +5643,33 @@ int service_determine_exec_selinux_label(Service *s, char **ret) {
         return 0;
 }
 
-static int service_cgroup_freezer_action(Unit *u, FreezerAction action) {
+static int service_freezer_action(Unit *u, FreezerAction action) {
         Service *s = ASSERT_PTR(SERVICE(u));
+        FreezerState old_objective, new_objective;
         int r;
 
+        old_objective = freezer_state_objective(u->freezer_state);
+
         r = unit_cgroup_freezer_action(u, action);
-        if (r <= 0)
+        if (r < 0)
                 return r;
 
-        if (action == FREEZER_FREEZE)
-                service_stop_watchdog(s);
-        else if (action == FREEZER_THAW)
-                service_reset_watchdog(s);
+        new_objective = freezer_state_objective(u->freezer_state);
+
+        /* Note that we cannot trivially check the retval of unit_cgroup_freezer_action() here, since
+         * that signals whether the operation is ongoing from *kernel's PoV*. If the freeze operation
+         * is aborted, the frozen attribute of the cgroup would never have been flipped in kernel,
+         * and unit_cgroup_freezer_action() will happily return 0, yet the watchdog still needs to be reset;
+         * vice versa. */
+
+        if (old_objective != new_objective) {
+                if (new_objective == FREEZER_FROZEN)
+                        service_stop_watchdog(s);
+                else if (new_objective == FREEZER_RUNNING)
+                        service_reset_watchdog(s);
+                else
+                        assert_not_reached();
+        }
 
         return r;
 }
@@ -5796,7 +5811,7 @@ const UnitVTable service_vtable = {
         .live_mount = service_live_mount,
         .can_live_mount = service_can_live_mount,
 
-        .freezer_action = service_cgroup_freezer_action,
+        .freezer_action = service_freezer_action,
 
         .serialize = service_serialize,
         .deserialize_item = service_deserialize_item,

src/core/unit.c

@@ -6438,9 +6438,7 @@ void unit_next_freezer_state(Unit *u, FreezerAction action, FreezerState *ret_ne
                 assert_not_reached();
         }
 
-        objective = freezer_state_finish(next);
-        if (objective == FREEZER_FROZEN_BY_PARENT)
-                objective = FREEZER_FROZEN;
+        objective = freezer_state_objective(next);
         assert(IN_SET(objective, FREEZER_RUNNING, FREEZER_FROZEN));
 
         *ret_next = next;