cgroup-util: introduce generic cg_path_from_cgroupid() helper

Taken from nsresourced/userns-registry.c userns_destroy_cgroup()

src/basic/cgroup-util.c

@@ -68,6 +68,27 @@ int cg_cgroupid_open(int cgroupfs_fd, uint64_t id) {
         return RET_NERRNO(open_by_handle_at(cgroupfs_fd, &fh.file_handle, O_DIRECTORY|O_CLOEXEC));
 }
 
+int cg_path_from_cgroupid(int cgroupfs_fd, uint64_t id, char **ret) {
+        _cleanup_close_ int cgfd = -EBADF;
+        int r;
+
+        cgfd = cg_cgroupid_open(cgroupfs_fd, id);
+        if (cgfd < 0)
+                return cgfd;
+
+        _cleanup_free_ char *path = NULL;
+        r = fd_get_path(cgfd, &path);
+        if (r < 0)
+                return r;
+
+        if (!path_startswith(path, "/sys/fs/cgroup/"))
+                return -EXDEV; /* recognizable error */
+
+        if (ret)
+                *ret = TAKE_PTR(path);
+        return 0;
+}
+
 int cg_get_cgroupid_at(int dfd, const char *path, uint64_t *ret) {
         cg_file_handle fh = CG_FILE_HANDLE_INIT;
         int mnt_id;

src/basic/cgroup-util.h

@@ -183,6 +183,7 @@ typedef enum CGroupUnified {
 int cg_path_open(const char *controller, const char *path);
 int cg_cgroupid_open(int fsfd, uint64_t id);
 
+int cg_path_from_cgroupid(int cgroupfs_fd, uint64_t id, char **ret);
 int cg_get_cgroupid_at(int dfd, const char *path, uint64_t *ret);
 static inline int cg_path_get_cgroupid(const char *path, uint64_t *ret) {
         return cg_get_cgroupid_at(AT_FDCWD, path, ret);