package management creates user _apt locks var cache apt archives partial

[Galen Thurber]

Xubuntu 16.04.1LTS

Package management system has created a user _apt.
_apt:x:105:65534::/nonexistent:/bin/false
This user is only the only user allowed to write to directory
drwx------ 2 _apt root 4096 Jan 15 22:31 partial

while
var/lib/update-notifier/package-data-downloads
drwxr-xr-x 2 root root 4096 Jan 16 09:19 partial

This ownership situation causes permission errors for dpkg, synaptic, apt-get
for example
/var/log/apt/term.log

W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/adobe-flashplugin_20170110.1.orig.tar.gz' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
Installing from local file /var/lib/update-notifier/package-data-downloads/partial/adobe-flashplugin_20170110.1.orig.tar.gz
...
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/andale32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)

[Robert Ancell (robert-ancell)]

Re-assigning to apt as I'm not sure what has done this (but it's not gnome-software AFAICT).

[Julian Andres Klode (juliank)]

Warnings are not errors, and this is really a duplicate of bug 1522675