core: delay adding target dependencies until all units are loaded and aliases resolved (#8381)

Currently we add target dependencies while we are loading units. This
can create ordering loops even if configuration doesn't contain any
loop. Take for example following configuration,

$ systemctl get-default
multi-user.target

$ cat /etc/systemd/system/test.service
[Unit]
After=default.target

[Service]
ExecStart=/bin/true

[Install]
WantedBy=multi-user.target

If we encounter such unit file early during manager start-up (e.g. load
queue is dispatched while enumerating devices due to SYSTEMD_WANTS in
udev rules) we would add stub unit default.target and we order it Before
test.service. At the same time we add implicit Before to
multi-user.target. Later we merge two units and we create ordering cycle
in the process.

To fix the issue we will now never add any target dependencies until we
loaded all the unit files and resolved all the aliases.

src/core/unit.c

@@ -649,6 +649,9 @@ void unit_free(Unit *u) {
         if (u->in_cleanup_queue)
                 LIST_REMOVE(cleanup_queue, u->manager->cleanup_queue, u);
 
+        if (u->in_target_deps_queue)
+                LIST_REMOVE(target_deps_queue, u->manager->target_deps_queue, u);
+
         safe_close(u->ip_accounting_ingress_map_fd);
         safe_close(u->ip_accounting_egress_map_fd);
 
@@ -1338,6 +1341,18 @@ int unit_load_fragment_and_dropin_optional(Unit *u) {
         return unit_load_dropin(unit_follow_merge(u));
 }
 
+void unit_add_to_target_deps_queue(Unit *u) {
+        Manager *m = u->manager;
+
+        assert(u);
+
+        if (u->in_target_deps_queue)
+                return;
+
+        LIST_PREPEND(target_deps_queue, m->target_deps_queue, u);
+        u->in_target_deps_queue = true;
+}
+
 int unit_add_default_target_dependency(Unit *u, Unit *target) {
         assert(u);
         assert(target);
@@ -1364,35 +1379,6 @@ int unit_add_default_target_dependency(Unit *u, Unit *target) {
         return unit_add_dependency(target, UNIT_AFTER, u, true, UNIT_DEPENDENCY_DEFAULT);
 }
 
-static int unit_add_target_dependencies(Unit *u) {
-
-        static const UnitDependency deps[] = {
-                UNIT_REQUIRED_BY,
-                UNIT_REQUISITE_OF,
-                UNIT_WANTED_BY,
-                UNIT_BOUND_BY
-        };
-
-        unsigned k;
-        int r = 0;
-
-        assert(u);
-
-        for (k = 0; k < ELEMENTSOF(deps); k++) {
-                Unit *target;
-                Iterator i;
-                void *v;
-
-                HASHMAP_FOREACH_KEY(v, target, u->dependencies[deps[k]], i) {
-                        r = unit_add_default_target_dependency(u, target);
-                        if (r < 0)
-                                return r;
-                }
-        }
-
-        return r;
-}
-
 static int unit_add_slice_dependencies(Unit *u) {
         UnitDependencyMask mask;
         assert(u);
@@ -1521,10 +1507,7 @@ int unit_load(Unit *u) {
         }
 
         if (u->load_state == UNIT_LOADED) {
-
-                r = unit_add_target_dependencies(u);
-                if (r < 0)
-                        goto fail;
+                unit_add_to_target_deps_queue(u);
 
                 r = unit_add_slice_dependencies(u);
                 if (r < 0)