Comment 7 for bug 1781169

apt 1.6.6 from bionic: I verified that lock-frontend was opened and locked at the beginning, that lock was locked after it, lock released before dpkg is called, and that lock-frontend was not released until the end.

[...]
openat(AT_FDCWD, "/var/lib/dpkg/lock-frontend", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 4
fcntl(4, F_SETFD, FD_CLOEXEC) = 0
fcntl(4, F_SETLK, {l_type=F_WRLCK, l_whence=SEEK_SET, l_start=0, l_len=0}) = 0
openat(AT_FDCWD, "/var/lib/dpkg/lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 5
fcntl(5, F_SETFD, FD_CLOEXEC) = 0
fcntl(5, F_SETLK, {l_type=F_WRLCK, l_whence=SEEK_SET, l_start=0, l_len=0}) = 0
[ ... Reading files ... ]
close(5) = 0
[... running dpkg and rebuilding cache; no close(4) ]
close(4) = 0
close(5) = -1 EBADF (Bad file descriptor)
close(3) = 0
+++ exited with 0 +++