Bug#813786: apt: Warning "Can't drop privileges" when running "apt-get source" as root

[Raphaël Hertzog <hertzog@debian.org>]

Package: apt
Version: 1.2.1
Severity: normal

Simple to reproduce:

$ LANG=C sudo apt-get source debian-installer
[sudo] password for rhertzog: 
Reading package lists... Done
Building dependency tree        
Reading state information... Done
NOTICE: 'debian-installer' packaging is maintained in the 'Git' version control system at:
git://git.debian.org/d-i/debian-installer.git
Please use:
git clone git://git.debian.org/d-i/debian-installer.git
to retrieve the latest (possibly unreleased) updates to the package.
Need to get 1 373 kB of source archives.
Get:1 http://nas/debian unstable/main debian-installer 20160106 (dsc) [3 434 B]
Get:2 http://nas/debian unstable/main debian-installer 20160106 (tar) [1 370 kB]
Fetched 1 373 kB in 0s (26,2 MB/s)          
dpkg-source: info: extracting debian-installer in debian-installer-20160106
dpkg-source: info: unpacking debian-installer_20160106.tar.gz
W: Can't drop privileges for downloading as file 'debian-installer_20160106.dsc' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)

$ ls -ald debian-installer*
drwxr-xr-x 5 root root    4096 janv.  1 16:27 debian-installer-20160106
-rw-r--r-- 1 root root    3434 janv.  6 06:12 debian-installer_20160106.dsc
-rw-r--r-- 1 root root 1370014 janv.  6 06:12 debian-installer_20160106.tar.gz

There's no such error when you run apt-get source as a normal user (which
is the usual case).

-- Package-specific info:

-- System Information:
Debian Release: stretch/sid
  APT prefers squeeze-lts
  APT policy: (500, 'squeeze-lts'), (500, 'oldoldstable'), (500, 'unstable'), (500, 'testing'), (500, 'stable'), (500, 'oldstable'), (1, 'experimental')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.3.0-1-amd64 (SMP w/4 CPU cores)
Locale: LANG=fr_FR.utf8, LC_CTYPE=fr_FR.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages apt depends on:
ii  adduser                 3.113+nmu3
ii  debian-archive-keyring  2014.3
ii  gnupg                   1.4.20-1
ii  gnupg2                  2.1.11-2
ii  gpgv                    1.4.20-1
ii  libapt-pkg5.0           1.2.1
ii  libc6                   2.21-7
ii  libgcc1                 1:5.3.1-7
ii  libstdc++6              5.3.1-7

apt recommends no packages.

Versions of packages apt suggests:
pn  apt-doc     <none>
ii  aptitude    0.7.5-3
ii  dpkg-dev    1.18.4
ii  python-apt  1.1.0~beta1
ii  synaptic    0.83+b1

-- no debconf information