apt-cacher-ng is completely broken and corrupts data

[Matias N. Goldberg]

After upgrading to Ubuntu 20.04 apt-cacher-ng started to behave erratically.

Trying to do `apt update` would randomly throw errors such as:

Err:20 https://repo.skype.com/deb stable InRelease
  Reading from proxy failed - select (115: Operation now in progress) [IP: 192.168.1.13 4687]

But before the error appears, the connection would stay frozen for a minute or so.

But please note that it is NOT related to HTTPS protocol, and this would be *random*. Trying to execute the command again could succeed for this website but fail for another, regardless of http/https protocol. Sometimes 6 URLs would fail, other times it would be only 1 or 2.

But that's not the only broken thing.

Going to the web interface (via http://192.168.1.13:4687/acng-report.html) I could go to the maintenance sections and I would see the same errors:

 - The cleanup section would stall forever
 - The cleanup section would throw random "500 internal error, SELECT returned bad file descriptor. Aborting"

Purging the package and installing again did not fix it.

The only solution was to install version 3.6.4-1_amd from http://ftp.debian.org/debian/pool/main/a/apt-cacher-ng/apt-cacher-ng_3.6.4-1_amd64.deb

which works with Ubuntu 20.04.

That fixed all the problems.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: apt-cacher-ng 3.3.1-2build1
Uname: Linux 5.13.7+ x86_64
ApportVersion: 2.20.11-0ubuntu27.21
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: XFCE
Date: Thu Apr 7 20:47:45 2022
InstallationDate: Installed on 2017-12-22 (1567 days ago)
InstallationMedia: Xubuntu 17.04 "Zesty Zapus" - Release amd64 (20170412)
SourcePackage: apt-cacher-ng
UpgradeStatus: Upgraded to focal on 2022-04-07 (0 days ago)
modified.conffile..etc.apt-cacher-ng.security.conf: [inaccessible: [Errno 13] Permission denied: '/etc/apt-cacher-ng/security.conf']
mtime.conffile..etc.apt-cacher-ng.acng.conf: 2022-04-07T20:47:42.966343