files: Implement find_unused_file_desc_id()

This function will be used to allocate id for fake files
(don't confuse with fake fds, e.g. fles).
Suggested-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/files.c

@@ -57,6 +57,8 @@ static struct hlist_head file_desc_hash[FDESC_HASH_SIZE];
 /* file_desc's, which fle is not owned by a process, that is able to open them */
 static LIST_HEAD(fake_master_head);
 
+static u32 max_file_desc_id = 0;
+
 static void init_fdesc_hash(void)
 {
 	int i;
@@ -79,6 +81,10 @@ int file_desc_add(struct file_desc *d, u32 id, struct file_desc_ops *ops)
 {
 	file_desc_init(d, id, ops);
 	hlist_add_head(&d->hash, &file_desc_hash[id % FDESC_HASH_SIZE]);
+
+	if (id > max_file_desc_id)
+		max_file_desc_id = id;
+
 	return 0; /* this is to make tail-calls in collect_one_foo look nice */
 }
 
@@ -108,6 +114,11 @@ static inline struct file_desc *find_file_desc(FdinfoEntry *fe)
 	return find_file_desc_raw(fe->type, fe->id);
 }
 
+u32 find_unused_file_desc_id(void)
+{
+	return max_file_desc_id + 1;
+}
+
 struct fdinfo_list_entry *find_used_fd(struct pstree_item *task, int fd)
 {
 	struct list_head *head;

criu/include/files.h

@@ -117,6 +117,7 @@ struct fdinfo_list_entry *collect_fd_to(int pid, FdinfoEntry *e,
 		struct rst_info *rst_info, struct file_desc *fdesc,
 		bool fake, bool force_master);
 
+u32 find_unused_file_desc_id(void);
 unsigned int find_unused_fd(struct pstree_item *, int hint_fd);
 struct fdinfo_list_entry *find_used_fd(struct pstree_item *, int fd);