core/execute: warn when threaded mode is detected

Prompted by #22486.

src/core/execute.c

@@ -4250,6 +4250,12 @@ static int exec_child(
                 }
 
                 r = cg_attach_everywhere(params->cgroup_supported, p, 0, NULL, NULL);
+                if (r == -EUCLEAN) {
+                        *exit_status = EXIT_CGROUP;
+                        return log_unit_error_errno(unit, r, "Failed to attach process to cgroup %s "
+                                                    "because the cgroup or one of its parents or "
+                                                    "siblings is in the threaded mode: %m", p);
+                }
                 if (r < 0) {
                         *exit_status = EXIT_CGROUP;
                         return log_unit_error_errno(unit, r, "Failed to attach to cgroup %s: %m", p);

src/shared/cgroup-setup.c

@@ -346,6 +346,9 @@ int cg_attach(const char *controller, const char *path, pid_t pid) {
         xsprintf(c, PID_FMT "\n", pid);
 
         r = write_string_file(fs, c, WRITE_STRING_FILE_DISABLE_BUFFER);
+        if (r == -EOPNOTSUPP && cg_is_threaded(controller, path) > 0)
+                /* When the threaded mode is used, we cannot read/write the file. Let's return recognizable error. */
+                return -EUCLEAN;
         if (r < 0)
                 return r;