lxc-stop (v4.0.12) seems not to work

[netITea]

I try to stop my application container with "lxc-stop -k -n up" and the program just hang.
The srtace-output shows following:

5278 10:44:41.007211 connect(4, {sa_family=AF_UNIX, sun_path=@"//var/lib/lxc/up/command"}, 27) = 0
5278 10:44:41.007278 getpid() = 5278
5278 10:44:41.007386 getuid() = 0
5278 10:44:41.007508 getgid() = 0
5278 10:44:41.007595 sendmsg(4, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="\4\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", iov_len=16}], msg_iovlen=1, msg_control=[{cmsg_len=28, cmsg_level=SOL_SOCKET, cmsg_type=SCM_CREDENTIALS, cmsg_data={pid=5278, uid=0, gid=0}}], msg_controllen=32, msg_flags=0}, MSG_NOSIGNAL) = 16
5278 10:44:41.007745 recvmsg(4, {msg_namelen=0}, MSG_CMSG_CLOEXEC) = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
5278 10:44:50.634810 --- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} ---
5278 10:44:50.635258 +++ killed by SIGINT +++

This matches the code of the function lxc_cmd() (it calls lxc_abstract_unix_send_credential()) witch try to send a STOP-command and waits for a response.
I see here two problems. The first one is that there no response-waiting-timeout, which is bad if I want to use C-API (c->stop or even c->shutdown with timeout) and it hangs.
The second one is, that I haven't seen the other side (strace-output of lxc-execute) receiving this STOP-command. May it have some relation to #4112?

[netITea]

This problem is related to #4112 indeed.
I found out that __lxc_start() does not achieves lxc_poll() because it stucks in lxc_spawn() which calls lxc_sync_barrier_child()!
And lxc_sync_barrier_child() waits for something (message or closing) on sync_sock[1].
I resume: the lxc app process is unable to receive any command via maincmd_fd.