core/cgroup: Properly handle aborting a pending freeze operation

We must thaw the cgroup even if cgroup.events/frozen=0 if a freeze
operation is in flight as it means the cgroup is already partially
frozen.

Fixes #37590.
Fixes #38337.

(cherry picked from commit 85d00912c0)
(cherry picked from commit 4bf37df99c)

src/core/cgroup.c

@@ -5124,6 +5124,7 @@ static int unit_cgroup_freezer_kernel_state(Unit *u, FreezerState *ret) {
 int unit_cgroup_freezer_action(Unit *u, FreezerAction action) {
         _cleanup_free_ char *path = NULL;
         FreezerState current, next, objective;
+        bool action_in_progress = false;
         int r;
 
         assert(u);
@@ -5138,14 +5139,21 @@ int unit_cgroup_freezer_action(Unit *u, FreezerAction action) {
         CGroupRuntime *crt = unit_get_cgroup_runtime(u);
         if (!crt || !crt->cgroup_path)
                 /* No realized cgroup = nothing to freeze */
-                goto skip;
+                goto finish;
 
         r = unit_cgroup_freezer_kernel_state(u, &current);
         if (r < 0)
                 return r;
 
-        if (current == objective)
-                goto skip;
+        if (current == objective) {
+                if (objective == FREEZER_FROZEN)
+                        goto finish;
+
+                /* Skip thaw only if no freeze operation was in flight */
+                if (IN_SET(u->freezer_state, FREEZER_RUNNING, FREEZER_THAWING))
+                        goto finish;
+        } else
+                action_in_progress = true;
 
         if (next == freezer_state_finish(next)) {
                 /* We're directly transitioning into a finished state, which in theory means that
@@ -5177,12 +5185,13 @@ int unit_cgroup_freezer_action(Unit *u, FreezerAction action) {
         if (r < 0)
                 return r;
 
-        unit_set_freezer_state(u, next);
-        return 1; /* Wait for cgroup event before replying */
+finish:
+        if (action_in_progress)
+                unit_set_freezer_state(u, next);
+        else
+                unit_set_freezer_state(u, freezer_state_finish(next));
 
-skip:
-        unit_set_freezer_state(u, freezer_state_finish(next));
-        return 0;
+        return action_in_progress;
 }
 
 int unit_get_cpuset(Unit *u, CPUSet *cpus, const char *name) {