core: add cg_path_get_unit_path()

From a given cgroup path, cg_path_get_unit() allows to retrieve the
unit's name. Although, this removes the path to the unit's cgroup,
preventing the result to be used to fetch xattrs.

Introduce cg_path_get_unit_path() which provides the path to the unit's
cgroup. This function behave similarly to cg_path_get_unit() (checking
the validity and escaping the unit's name).

src/basic/cgroup-util.c

@@ -1198,6 +1198,28 @@ int cg_path_get_unit(const char *path, char **ret) {
         return 0;
 }
 
+int cg_path_get_unit_path(const char *path, char **ret) {
+        _cleanup_free_ char *path_copy = NULL;
+        char *unit_name;
+
+        assert(path);
+        assert(ret);
+
+        path_copy = strdup(path);
+        if (!path_copy)
+                return -ENOMEM;
+
+        unit_name = (char *)skip_slices(path_copy);
+        unit_name[strcspn(unit_name, "/")] = 0;
+
+        if (!unit_name_is_valid(cg_unescape(unit_name), UNIT_NAME_PLAIN|UNIT_NAME_INSTANCE))
+                return -ENXIO;
+
+        *ret = TAKE_PTR(path_copy);
+
+        return 0;
+}
+
 int cg_pid_get_unit(pid_t pid, char **unit) {
         _cleanup_free_ char *cgroup = NULL;
         int r;

src/basic/cgroup-util.h

@@ -261,6 +261,7 @@ int cg_path_get_cgroupid(const char *path, uint64_t *ret);
 int cg_path_get_session(const char *path, char **session);
 int cg_path_get_owner_uid(const char *path, uid_t *uid);
 int cg_path_get_unit(const char *path, char **unit);
+int cg_path_get_unit_path(const char *path, char **unit);
 int cg_path_get_user_unit(const char *path, char **unit);
 int cg_path_get_machine_name(const char *path, char **machine);
 int cg_path_get_slice(const char *path, char **slice);