Same for xenial (1.1.0~beta1ubuntu0.16.04.3).

Also, strace logs look like this, in case we want to see how we can be sure it's kept locked all the time:

[...]
open("/var/lib/dpkg/lock-frontend", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 39
fcntl(39, F_SETFD, FD_CLOEXEC)          = 0
fcntl(39, F_SETLK, {l_type=F_WRLCK, l_whence=SEEK_SET, l_start=0, l_len=0}) = 0
open("/var/lib/dpkg/lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 40
fcntl(40, F_SETFD, FD_CLOEXEC)          = 0
fcntl(40, F_SETLK, {l_type=F_WRLCK, l_whence=SEEK_SET, l_start=0, l_len=0}) = 0
open("/var/lib/dpkg/updates/", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 41
close(41)                               = 0
close(41)                               = 0
close(41)                               = 0
open("/etc/nsswitch.conf", O_RDONLY|O_CLOEXEC) = 41
close(41)                               = 0
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 41
close(41)                               = 0
open("/lib/x86_64-linux-gnu/libnss_compat.so.2", O_RDONLY|O_CLOEXEC) = 41
close(41)                               = 0
open("/lib/x86_64-linux-gnu/libnsl.so.1", O_RDONLY|O_CLOEXEC) = 41
close(41)                               = 0
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 41
close(41)                               = 0
open("/lib/x86_64-linux-gnu/libnss_nis.so.2", O_RDONLY|O_CLOEXEC) = 41
close(41)                               = 0
open("/lib/x86_64-linux-gnu/libnss_files.so.2", O_RDONLY|O_CLOEXEC) = 41
close(41)                               = 0
open("/etc/passwd", O_RDONLY|O_CLOEXEC) = 41
close(41)                               = 0
close(41)                               = 0
close(41)                               = 0
open("/etc/group", O_RDONLY|O_CLOEXEC)  = 41
close(41)                               = 0
open("/etc/passwd", O_RDONLY|O_CLOEXEC) = 41
close(41)                               = 0
open("/etc/group", O_RDONLY|O_CLOEXEC)  = 41
close(41)                               = 0
open("/var/lib/apt/lists/partial", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 41
close(41)                               = 0
open("/etc/passwd", O_RDONLY|O_CLOEXEC) = 41
close(41)                               = 0
open("/etc/group", O_RDONLY|O_CLOEXEC)  = 41
close(41)                               = 0
open("/var/cache/apt/archives/partial", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 41
close(41)                               = 0
open("/etc/passwd", O_RDONLY|O_CLOEXEC) = 41
close(41)                               = 0
open("/etc/group", O_RDONLY|O_CLOEXEC)  = 41
close(41)                               = 0
open("/var/cache/apt/archives/partial", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 41
close(41)                               = 0
open("/var/cache/apt/archives/lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 41
fcntl(41, F_SETFD, FD_CLOEXEC)          = 0
fcntl(41, F_SETLK, {l_type=F_WRLCK, l_whence=SEEK_SET, l_start=0, l_len=0}) = 0
open("/etc/passwd", O_RDONLY|O_CLOEXEC) = 42
close(42)                               = 0
open("/proc/sys/kernel/ngroups_max", O_RDONLY) = 42
close(42)                               = 0
close(40)                               = 0
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=6098, si_uid=0, si_status=0, si_utime=6, si_stime=3} ---
open("/var/lib/dpkg/lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 40
fcntl(40, F_SETFD, FD_CLOEXEC)          = 0
fcntl(40, F_SETLK, {l_type=F_WRLCK, l_whence=SEEK_SET, l_start=0, l_len=0}) = 0
close(39)                               = 0
[ ... more closes ... ]
+++ exited with 0 +++