core/unit: do not use unit path cache in unit_need_daemon_reload()

When unit_need_daemon_reload() calls unit_find_dropin_paths() to check
for new drop-in configs, the manager's unit path cache is used to limit
which directories are considered. If a new drop-in directory is created,
it may not be in the unit path cache, and hence unit_need_daemon_reload()
may return false, despite a new drop-in being present. However, if a
unit path cache is not given to unit_file_find_dropin_paths() at all,
then it behaves as if the target path was found in the unit path cache.

So, to fix this, adapt unit_find_dropin_paths() to take a boolean
argument indicating whether or not to pass along the unit path cache.
Set this to false in unit_need_daemon_reload().

Fixes #31752

src/core/load-dropin.c

@@ -102,7 +102,7 @@ int unit_load_dropin(Unit *u) {
                 return r;
 
         /* Load .conf dropins */
-        r = unit_find_dropin_paths(u, &l);
+        r = unit_find_dropin_paths(u, /* use_unit_path_cache = */ true, &l);
         if (r <= 0)
                 return 0;
 

src/core/load-dropin.h

@@ -6,12 +6,12 @@
 
 /* Read service data supplementary drop-in directories */
 
-static inline int unit_find_dropin_paths(Unit *u, char ***paths) {
+static inline int unit_find_dropin_paths(Unit *u, bool use_unit_path_cache, char ***paths) {
         assert(u);
 
         return unit_file_find_dropin_paths(NULL,
                                            u->manager->lookup_paths.search_path,
-                                           u->manager->unit_path_cache,
+                                           use_unit_path_cache ? u->manager->unit_path_cache : NULL,
                                            ".d", ".conf",
                                            u->id, u->aliases,
                                            paths);

src/core/unit.c

@@ -3809,7 +3809,7 @@ bool unit_need_daemon_reload(Unit *u) {
         if (u->load_state == UNIT_LOADED) {
                 _cleanup_strv_free_ char **dropins = NULL;
 
-                (void) unit_find_dropin_paths(u, &dropins);
+                (void) unit_find_dropin_paths(u, /* use_unit_path_cache = */ false, &dropins);
 
                 if (!strv_equal(u->dropin_paths, dropins))
                         return true;