Comment 26 for bug 1604750

Maybe this dbg output from updates_available helps reveal the issue:

___update_needed() entr 2017-04-19 17:57:22:332517641
___update_needed(): mycache = 2017-04-19 17:56:55:299087230
___update_needed(): /var/lib/apt = 2017-04-19 17:57:20:275457556
___update_needed(): /var/lib/apt/lists = 2017-04-19 17:56:48:442985575
------ mycache ^^^
1 0
------ mycache ###
___update_needed() exit 2017-04-19 17:57:22:335366171 (rc=0)
___update_cache() entr 2017-04-19 17:57:22:335890178
___update_cache() about to run update-notifier: 2017-04-19 17:56:55:299087230
--- mycache ^^^
1 0
--- mycache ###
flock -xn "/dev/shm/byobu-username-IPAtHq8s/cache.tmux/updates-available.lock" /usr/lib/update-notifier/apt-check 2>&1 | awk '-F;' 'END { print $1, $2 }' >"/dev/shm/byobu-username-IPAtHq8s/cache.tmux/updates-available" 2>/dev/null &
___update_cache() after running update-notifier (rc=0 pid=23482) in the background: 2017-04-19 17:57:22:331488041
---- mycache ^^^
---- mycache ###
___update_cache() exit 2017-04-19 17:57:22:343179552
___update_available() exit 2017-04-19 17:57:22:343674285
___update_available() entr 2017-04-19 17:57:29:349651054
___print_updates() entr 2017-04-19 17:57:29:350254352
----- mycache ^^^
1 0
----- mycache ###
___print_updates() exit 2017-04-19 17:57:29:351550685
___update_needed() entr 2017-04-19 17:57:29:352151300
___update_needed(): mycache = 2017-04-19 17:57:23:307502514
___update_needed(): /var/lib/apt = 2017-04-19 17:57:20:275457556
___update_needed(): /var/lib/apt/lists = 2017-04-19 17:56:48:442985575
------ mycache ^^^
1 0
------ mycache ###
___update_needed() exit 2017-04-19 17:57:29:356254254 (rc=1)
___update_available() exit 2017-04-19 17:57:29:356856277
___update_available() entr 2017-04-19 17:57:36:364345395
___print_updates() entr 2017-04-19 17:57:36:364951726
----- mycache ^^^
1 0
----- mycache ###