[CRIU] [PATCH] restore: handle exit code of the unlock network script

[Andrey Vagin]

When we are migrating processes from one host to another host,
we need to know the moment, when processes can be killed on the source
host.
If a migration script is killed (segv, exception, etc), the process tree
must not live on both nodes and we need to reduce the chance of
killing processes.

In this patch I suggest to check exit code of network unlock scripts and
use the following scheme to restore processes:

1. The source host dumps processes and stops in the post-dump script.
2. Then it sends signal to restore process tree on the other side.
3. The destination host restores processes and stops in the
   network-unlock script.
4. Then it sends signal to the source host to kill the origin process
   tree.
5. At the final stage the destination host unlocks network and resumes
   the migrated process tree.

We have a small window between the 4-th and 5-th steps, because if a
migration script is killed in this moment, the process tree will be
killed on both nodes. And we have zero chance to leave two copies of
the process tree.

Signed-off-by: Andrey Vagin <avagin at openvz.org>
---
 cr-restore.c  | 4 +++-
 include/net.h | 2 +-
 net.c         | 8 +++++---
 3 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/cr-restore.c b/cr-restore.c
index b352daa..22396a2 100644
--- a/cr-restore.c
+++ b/cr-restore.c
@@ -1517,7 +1517,9 @@ static int restore_root_task(struct pstree_item *init)
 	}
 
 	/* Unlock network before disabling repair mode on sockets */
-	network_unlock();
+	ret = network_unlock();
+	if (ret < 0)
+		goto out_kill;
 
 	/*
 	 * -------------------------------------------------------------
diff --git a/include/net.h b/include/net.h
index a9f0d46..a02f98a 100644
--- a/include/net.h
+++ b/include/net.h
@@ -15,7 +15,7 @@ struct veth_pair {
 };
 
 extern int network_lock(void);
-extern void network_unlock(void);
+extern int network_unlock(void);
 
 extern struct ns_desc net_ns_desc;
 
diff --git a/net.c b/net.c
index ac66374..f7df130 100644
--- a/net.c
+++ b/net.c
@@ -594,15 +594,17 @@ int network_lock(void)
 	return run_scripts("network-lock");
 }
 
-void network_unlock(void)
+int network_unlock(void)
 {
 	pr_info("Unlock network\n");
 
+	if ((current_ns_mask & CLONE_NEWNET) && run_scripts("network-unlock"))
+		return -1;
+
 	cpt_unlock_tcp_connections();
 	rst_unlock_tcp_connections();
 
-	if (current_ns_mask & CLONE_NEWNET)
-		run_scripts("network-unlock");
+	return 0;
 }
 
 int veth_pair_add(char *in, char *out)
-- 
1.8.5.3