permission error when installing ttf-mscorefonts-installer/Can't drop privileges as file couldn't be accessed by user '_apt'

I wanted to install the Microsoft fonts on 16.04 and ran the command below, but there are many errors (or only warnings?) about that the user _apt has no permissions on the downloaded .exe files.

Command:

sudo apt install ttf-mscorefont-installer

Error message (repeated for every downloaded font exe):

ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/andale32.exe
Get:1 http://downloads.sourceforge.net/corefonts/andale32.exe [198 kB]
Fetched 198 kB in 1s (180 kB/s)                                                              
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)

But in the end it still says that the installation was successful. So is this just a bug in the package's install scripts, or is anything wrong on my side here? Do I need to take any further actions?

Full output (the original output was the same as with --reinstall):

$ sudo apt install --reinstall ttf-mscorefonts-installer 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
0 to upgrade, 0 to newly install, 1 reinstalled, 0 to remove and 4 not to upgrade.
Need to get 29,5 kB of archives.
After this operation, 0 B of additional disk space will be used.
Get:1 http://ftp.uni-stuttgart.de/ubuntu xenial/multiverse amd64 ttf-mscorefonts-installer all 3.4+nmu1ubuntu2 [29,5 kB]
Fetched 29,5 kB in 0s (280 kB/s)                     
Preconfiguring packages ...
(Reading database ... 214933 files and directories currently installed.)
Preparing to unpack .../ttf-mscorefonts-installer_3.4+nmu1ubuntu2_all.deb ...
mscorefonts-eula license has already been accepted
Unpacking ttf-mscorefonts-installer (3.4+nmu1ubuntu2) over (3.4+nmu1ubuntu2) ...
Processing triggers for fontconfig (2.11.94-0ubuntu1) ...
Processing triggers for update-notifier-common (3.168) ...
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/andale32.exe
Get:1 http://downloads.sourceforge.net/corefonts/andale32.exe [198 kB]
Fetched 198 kB in 1s (180 kB/s)                                                              
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)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/arial32.exe
Get:1 http://downloads.sourceforge.net/corefonts/arial32.exe [554 kB]
Fetched 554 kB in 1s (466 kB/s)                                                             
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/arial32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/arialb32.exe
Get:1 http://downloads.sourceforge.net/corefonts/arialb32.exe [168 kB]
Fetched 168 kB in 0s (169 kB/s)                                                              
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/arialb32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/comic32.exe
Get:1 http://downloads.sourceforge.net/corefonts/comic32.exe [246 kB]
Fetched 246 kB in 1s (224 kB/s)                                                             
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/comic32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/courie32.exe
Get:1 http://downloads.sourceforge.net/corefonts/courie32.exe [646 kB]
Fetched 646 kB in 1s (472 kB/s)                                                               
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/courie32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/georgi32.exe
Get:1 http://downloads.sourceforge.net/corefonts/georgi32.exe [392 kB]
Fetched 392 kB in 1s (364 kB/s)                                                              
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/georgi32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/impact32.exe
Get:1 http://downloads.sourceforge.net/corefonts/impact32.exe [173 kB]
Fetched 173 kB in 0s (176 kB/s)                                                              
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/impact32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/times32.exe
Get:1 http://downloads.sourceforge.net/corefonts/times32.exe [662 kB]
Fetched 662 kB in 2s (235 kB/s)                                                              
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/times32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/trebuc32.exe
Get:1 http://downloads.sourceforge.net/corefonts/trebuc32.exe [357 kB]
Fetched 357 kB in 1s (307 kB/s)                                                              
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/trebuc32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/verdan32.exe
Get:1 http://downloads.sourceforge.net/corefonts/verdan32.exe [352 kB]
Fetched 352 kB in 4s (85,7 kB/s)                                                             
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/verdan32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/webdin32.exe
Get:1 http://downloads.sourceforge.net/corefonts/webdin32.exe [185 kB]
Fetched 185 kB in 0s (210 kB/s)                                                              
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/webdin32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)

These fonts were provided by Microsoft "in the interest of cross-
platform compatibility".  This is no longer the case, but they are
still available from third parties.

You are free to download these fonts and use them for your own use,
but you may not redistribute them in modified form, including changes
to the file name or packaging format.

Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/andale32.exe
  extracting fontinst.inf
  extracting andale.inf
  extracting fontinst.exe
  extracting AndaleMo.TTF
  extracting ADVPACK.DLL
  extracting W95INF32.DLL
  extracting W95INF16.DLL

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/arial32.exe
  extracting FONTINST.EXE
  extracting fontinst.inf
  extracting Ariali.TTF
  extracting Arialbd.TTF
  extracting Arialbi.TTF
  extracting Arial.TTF

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/arialb32.exe
  extracting fontinst.exe
  extracting fontinst.inf
  extracting AriBlk.TTF

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/comic32.exe
  extracting fontinst.inf
  extracting Comicbd.TTF
  extracting Comic.TTF
  extracting fontinst.exe

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/courie32.exe
  extracting cour.ttf
  extracting courbd.ttf
  extracting courbi.ttf
  extracting fontinst.inf
  extracting couri.ttf
  extracting fontinst.exe

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/georgi32.exe
  extracting fontinst.inf
  extracting Georgiaz.TTF
  extracting Georgiab.TTF
  extracting Georgiai.TTF
  extracting Georgia.TTF
  extracting fontinst.exe

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/impact32.exe
  extracting fontinst.exe
  extracting Impact.TTF
  extracting fontinst.inf

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/times32.exe
  extracting fontinst.inf
  extracting Times.TTF
  extracting Timesbd.TTF
  extracting Timesbi.TTF
  extracting Timesi.TTF
  extracting FONTINST.EXE

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/trebuc32.exe
  extracting FONTINST.EXE
  extracting trebuc.ttf
  extracting Trebucbd.ttf
  extracting trebucbi.ttf
  extracting trebucit.ttf
  extracting fontinst.inf

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/verdan32.exe
  extracting fontinst.exe
  extracting fontinst.inf
  extracting Verdanab.TTF
  extracting Verdanai.TTF
  extracting Verdanaz.TTF
  extracting Verdana.TTF

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/webdin32.exe
  extracting fontinst.exe
  extracting Webdings.TTF
  extracting fontinst.inf
  extracting Licen.TXT

All done, no errors.
All fonts downloaded and installed.
Setting up ttf-mscorefonts-installer (3.4+nmu1ubuntu2) ...

The underlying problem

Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/webdin32.exe' couldn't be accessed by user '_apt'.

This is indicating that the _apt user doesn't have access to write to the /var/lib/update-notifier/package-data-downloads/partial directory, or to any existing files there. As a consequence, the installation will not be able to extract font files to - in turn - update (or install for the first time), the indicated fonts.

I contend that the package is therefore NOT properly installed, unlike the currently accepted answer (https://askubuntu.com/a/812152/269638).

What should you do?

First, you'll want to verify that you have an _apt user as suggested here with the following command:

getent passwd _apt

If for some reason you do not have the _apt user and need instructions for installing it, please go to the link indicated above.

Next, you will want to assign the directory indicated, as well as all files and subdirectories contained within, to the _apt user. You'll want to retain the privileges associated with root superuser group and consequently the root user account. Following the reassignment of ownership of the directory, we should perform some directory clean-up and then finally ensure that the packages and their dependencies are properly installed.

The commands you need...

...after verifying that the _apt user does indeed exist:

sudo chown -R _apt:root /var/lib/update-notifier/package-data-downloads/partial/
sudo rm /var/lib/update-notifier/package-data-downloads/partial/*.FA‌​ILED
sudo apt install --reinstall update-notifier-common ttf-mscorefonts-installer 

This is the bug #1543280, though a rather harmless bug. The installation is completed successfully. The W: signifies that this is just a warning.

Also note that, This is not related with the package ttf-mscorefonts-installer solely, instead whenever apt access already downloaded files, you'll see this warning. It happens also for offline repositories. I have offline repositories which I use to update packages without Internet and I myself see this always in Xenial (16.04). Not a big problem at all.

For your convenience, I am showing the output of reinstalling xrdp from my local repository.

sudo apt-get --reinstall install xrdp 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
0 upgraded, 0 newly installed, 1 reinstalled, 0 to remove and 398 not upgraded.
Need to get 0 B/215 kB of archives.
After this operation, 0 B of additional disk space will be used.
Get:1 file:/media/Linux/linux_debs_repo/xenial_new  xrdp 0.6.1-2 [215 kB]
(Reading database ... 612447 files and directories currently installed.)
Preparing to unpack ..././debs/xrdp_0.6.1-2_amd64.deb ...
Unpacking xrdp (0.6.1-2) over (0.6.1-2) ...
Processing triggers for systemd (229-4ubuntu6) ...
Processing triggers for ureadahead (0.100.0-19) ...
ureadahead will be reprofiled on next reboot
Processing triggers for man-db (2.7.5-1) ...
Setting up xrdp (0.6.1-2) ...
N: Can't drop privileges for downloading as file '/media/Linux/linux_debs_repo/xenial_new/./debs/xrdp_0.6.1-2_amd64.deb' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)

Check this question about Apt's status in Unix and Linux SE apt-get update exit status

Another related bug is #1570141