core: handle lookup paths being symlinks

With a recent change paths leaving the statically known lookup paths would be
treated differently then those that remained within those. That was done
(AFAIK) to consistently handle alias names. Unfortunately that means that on
some distributions, especially those where /etc/ consists mostly of symlinks,
would trigger that new detection for every single unit in /etc/systemd/system.
The reason for that is that the units directory itself is already a symlink.

Rebased-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>

src/basic/unit-file.c

@@ -388,6 +388,7 @@ int unit_file_build_name_map(
 
         _cleanup_hashmap_free_ Hashmap *ids = NULL, *names = NULL;
         _cleanup_set_free_free_ Set *paths = NULL;
+        _cleanup_strv_free_ char **expanded_search_path = NULL;
         uint64_t timestamp_hash;
         int r;
 
@@ -406,6 +407,44 @@ int unit_file_build_name_map(
                         return log_oom();
         }
 
+        /* Go over all our search paths, chase their symlinks and store the result in the
+         * expanded_search_path list.
+         *
+         * This is important for cases where any of the unit directories itself are symlinks into other
+         * directories and would therefore cause all of the unit files to be recognized as linked units.
+         *
+         * This is important for distributions such as NixOS where most paths in /etc/ are symlinks to some
+         * other location on the filesystem (e.g.  into /nix/store/).
+         *
+         * Search paths are ordered by priority (highest first), and we need to maintain this order.
+         * If a resolved path is already in the list, we don't need to include.
+         *
+         * Note that we build a list that contains both the original paths and the resolved symlinks:
+         * we need the latter for the case where the directory is symlinked, as described above, and
+         * the former for the case where some unit file alias is a dangling symlink that points to one
+         * of the "original" directories (and can't be followed).
+         */
+        STRV_FOREACH(dir, lp->search_path) {
+                _cleanup_free_ char *resolved_dir = NULL;
+
+                r = strv_extend(&expanded_search_path, *dir);
+                if (r < 0)
+                        return log_oom();
+
+                r = chase_symlinks(*dir, NULL, 0, &resolved_dir, NULL);
+                if (r < 0) {
+                        if (r != -ENOENT)
+                                log_warning_errno(r, "Failed to resolve symlink %s, ignoring: %m", *dir);
+                        continue;
+                }
+
+                if (strv_contains(expanded_search_path, resolved_dir))
+                        continue;
+
+                if (strv_consume(&expanded_search_path, TAKE_PTR(resolved_dir)) < 0)
+                        return log_oom();
+        }
+
         STRV_FOREACH(dir, lp->search_path) {
                 _cleanup_closedir_ DIR *d = NULL;
 
@@ -504,7 +543,7 @@ int unit_file_build_name_map(
                                 /* We don't explicitly check for alias loops here. unit_ids_map_get() which
                                  * limits the number of hops should be used to access the map. */
 
-                                r = unit_file_resolve_symlink(lp->root_dir, lp->search_path,
+                                r = unit_file_resolve_symlink(lp->root_dir, expanded_search_path,
                                                               *dir, dirfd(d), de->d_name,
                                                               /* resolve_destination_target= */ false,
                                                               &dst);