tty: Kill prepare_shared_tty

The routine in question just sets up the mutex to access
/dev/ptmx. This initialization can be done when we collect
a single tty.

✓ travis-ci: success for Sanitize initialization bits
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/cr-restore.c

@@ -1505,11 +1505,9 @@ static int restore_task_with_children(void *_arg)
 	if (current->parent == NULL) {
 		int i;
 
-		if (prepare_shared_tty())
-			goto err;
-
 		if (fdstore_init())
 			goto err;
+
 		if (join_namespaces()) {
 			pr_perror("Join namespaces failed");
 			goto err;

criu/include/tty.h

@@ -27,7 +27,6 @@ extern int dump_verify_tty_sids(void);
 extern struct collect_image_info tty_info_cinfo;
 extern struct collect_image_info tty_cinfo;
 extern struct collect_image_info tty_cdata;
-extern int prepare_shared_tty(void);
 
 struct mount_info;
 extern int devpts_restore(struct mount_info *pm);

criu/tty.c

@@ -326,8 +326,11 @@ static mutex_t *tty_mutex;
 
 static bool tty_is_master(struct tty_info *info);
 
-int prepare_shared_tty(void)
+static int init_tty_mutex(void)
 {
+	if (tty_mutex)
+		return 0;
+
 	tty_mutex = shmalloc(sizeof(*tty_mutex));
 	if (!tty_mutex) {
 		pr_err("Can't create ptmx index mutex\n");
@@ -1638,6 +1641,13 @@ static int collect_one_tty(void *obj, ProtobufCMessage *msg, struct cr_img *i)
 	if (add_post_prepare_cb_once(prep_tty_restore, NULL))
 		return -1;
 
+	/*
+	 * Call it explicitly. Post-callbacks will be called after
+	 * namespaces preparation, while the latter needs this mutex.
+	 */
+	if (init_tty_mutex())
+		return -1;
+
 	info->fdstore_id = -1;
 	list_add(&info->list, &all_ttys);
 	return file_desc_add(&info->d, info->tfe->id, &tty_desc_ops);