tty: Restore controlling terminal once per session

There might be several same terminals opened (say
tty6 or whatever) which gonna look as separate
files but actually pointing to the same tty kernel
instance. Moreover if it's a controlling terminal
we will be trying to restore it as many times as
find non zero sid on a peer.

Instead lets do a simple trick first: choose a leader
from a terminal group and use it only for controlling
terminal restoration.

https://jira.sw.ru/browse/PSBM-40969Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Reviewed-by: Andrew Vagin <avagin@odin.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

tty.c

@@ -92,6 +92,8 @@ struct tty_info {
 
 	bool				create;
 	bool				inherit;
+
+	struct tty_info			*ctl_tty;
 };
 
 struct tty_dump_info {
@@ -1079,6 +1081,14 @@ static int tty_find_restoring_task(struct tty_info *info)
 				goto shell_job;
 		}
 
+		/*
+		 * Restoring via leader only. All files
+		 * opened over same real tty get propagated
+		 * automatically by kernel itself.
+		 */
+		if (info->ctl_tty != info)
+			return 0;
+
 		/*
 		 * Find out the task which is session leader
 		 * and it can restore the controlling terminal
@@ -1159,6 +1169,31 @@ int tty_setup_slavery(void)
 {
 	struct tty_info *info, *peer, *m;
 
+	/*
+	 * The image may carry several terminals opened
+	 * belonging to the same session, so choose the
+	 * leader which gonna be setting up the controlling
+	 * terminal.
+	 */
+	list_for_each_entry(info, &all_ttys, list) {
+		if (!info->tie->sid || info->ctl_tty ||
+		    info->driver->type == TTY_TYPE__CTTY)
+			continue;
+
+		info->ctl_tty = info;
+		pr_debug("ctl tty leader %x\n", info->tfe->id);
+		peer = info;
+		list_for_each_entry_safe_continue(peer, m, &all_ttys, list) {
+			if (!peer->tie->sid || peer->ctl_tty ||
+			    peer->driver->type == TTY_TYPE__CTTY)
+				continue;
+			if (peer->tie->sid == info->tie->sid) {
+				pr_debug(" `- slave %x\n", peer->tfe->id);
+				peer->ctl_tty = info;
+			}
+		}
+	}
+
 	list_for_each_entry(info, &all_ttys, list) {
 		if (tty_find_restoring_task(info))
 			return -1;
@@ -1316,6 +1351,7 @@ static int collect_one_tty(void *obj, ProtobufCMessage *msg)
 	info->driver = get_tty_driver(major(info->tie->rdev), minor(info->tie->rdev));
 	info->create = tty_is_master(info);
 	info->inherit = false;
+	info->ctl_tty = NULL;
 
 	if (verify_info(info))
 		return -1;