In order to ensure that /usr/sbin/sendmail is present on the server we check for packages that provide the virtual package called "mail-transport-agent". Depending on the state of the cache, the list of providing packages is ordered differently. We always install the first package in that list. That means tl-setup installs different packages at random. When running 'sudo apt install mail-transport-agent' you always get the same order of possible packages: > Package mail-transport-agent is a virtual package provided by: > postfix 3.4.13-0ubuntu1 > exim4-daemon-light 4.93-13ubuntu1.5 > exim4-daemon-heavy 4.93-13ubuntu1.5 > ssmtp 2.64-8.1ubuntu1 > sendmail-bin 8.15.2-18 > qmail-run 2.0.2+nmu1 > opensmtpd 6.6.4p1-1 > nullmailer 1:2.2-3 > msmtp-mta 1.8.6-1 > masqmail 0.3.4-1build1 > esmtp-run 1.2-17 > dma 0.12-1 > courier-mta 1.0.6-1build2 > citadel-server 917-4 > lsb-invalid-mta 11.1.0ubuntu2 > You should explicitly select one to install. Debian has a github mirror of the apt code, and the relevant code that does the printing above can be found here: https://github.com/Debian/apt/blob/766b24b7f7484751950c76bc66d3d6cdeaf949a5/apt-private/private-cacheset.cc I can't see any obvious sorting being done however, perhaps it's arbitrary.
The 'python3-apt' package contains more low level information as part of the apt_pkg module. Here's documentation for that: https://apt-team.pages.debian.net/python-apt/library/apt_pkg.html Using that module we can find the same list of packages that 'apt install' finds (with some duplicates): >>> import apt_pkg >>> apt_pkg.init() >>> c = apt_pkg.Cache() Reading package lists... Done Building dependency tree Reading state information... Done >>> for (target, ver, pkg_ver) in c["mail-transport-agent"].provides_list: ... print(pkg_ver.parent_pkg.name) ... postfix exim4-daemon-light exim4-daemon-heavy ssmtp sendmail-bin qmail-run opensmtpd nullmailer msmtp-mta masqmail esmtp-run dma courier-mta citadel-server postfix lsb-invalid-mta exim4-daemon-light exim4-daemon-heavy
It's worth noting that we found this bug since one time when installing 'mail-transport-agent' the randomness fell on 'courier-mta' which apparently is broken: 2021-05-26 14:22:10,785: '/usr/sbin/sendmail' not found 2021-05-26 14:22:29,982: Selecting candidate "courier-mta" as provider for requested package "mail-transport-agent" 2021-05-26 14:23:31,281: Signing certificate... 2021-05-26 14:23:31,281: Job for courier-msa.service failed because the control process exited with error code. 2021-05-26 14:23:31,281: See "systemctl status courier-msa.service" and "journalctl -xe" for details. 2021-05-26 14:23:31,281: invoke-rc.d: initscript courier-msa, action "start" failed. 2021-05-26 14:23:31,281: ESC[0;1;31m●ESC[0m courier-msa.service - LSB: Courier MSA server 2021-05-26 14:23:31,281: Loaded: loaded (ESC]8;;file://ubuntu2004/etc/init.d/courier-msa^G/etc/init.d/courier-msaESC]8;;^G; generated) 2021-05-26 14:23:31,281: Active: ESC[0;1;31mfailedESC[0m (Result: exit-code) since Wed 2021-05-26 14:23:25 UTC; 12ms ago 2021-05-26 14:23:31,281: Docs: ESC]8;;man:systemd-sysv-generator(8)^Gman:systemd-sysv-generator(8)ESC]8;;^G 2021-05-26 14:23:31,281: Process: 4099 ExecStart=/etc/init.d/courier-msa start ESC[0;1;31m(code=exited, status=2)ESC[0m 2021-05-26 14:23:31,281: 2021-05-26 14:23:31,282: May 26 14:23:25 ubuntu2004 systemd[1]: Starting LSB: Courier MSA server... 2021-05-26 14:23:31,282: May 26 14:23:25 ubuntu2004 courier-msa[4099]: * Starting Courier MSA server esmtpd-msa 2021-05-26 14:23:31,282: May 26 14:23:25 ubuntu2004 systemd[1]: ESC[0;1;39mESC[0;1;31mESC[0;1;39mcourier-msa.service: Control process exited, code=exited, status=2/INVALIDARGUMENTESC[0m 2021-05-26 14:23:31,282: May 26 14:23:25 ubuntu2004 systemd[1]: ESC[0;1;38;5;185mESC[0;1;39mESC[0;1;38;5;185mcourier-msa.service: Failed with result 'exit-code'.ESC[0m 2021-05-26 14:23:31,282: May 26 14:23:25 ubuntu2004 systemd[1]: ESC[0;1;31mESC[0;1;39mESC[0;1;31mFailed to start LSB: Courier MSA server.ESC[0m 2021-05-26 14:23:31,282: dpkg: error processing package courier-mta (--configure): 2021-05-26 14:23:31,282: installed courier-mta package post-installation script subprocess returned error exit status 1 2021-05-26 14:23:31,282: Setting up expect (5.45.4-2build1) ... 2021-05-26 14:23:31,282: Processing triggers for systemd (245.4-4ubuntu3.6) ... 2021-05-26 14:23:31,282: Processing triggers for man-db (2.9.1-1) ... 2021-05-26 14:23:31,282: Processing triggers for libc-bin (2.31-0ubuntu9.2) ... 2021-05-26 14:23:31,282: Errors were encountered while processing: 2021-05-26 14:23:31,283: courier-mta 2021-05-26 14:23:31,288: failed to install packages with reason: installArchives() failed
We have found no evidence that the list of packages providing 'mail-transport-agent' is actually manually or intentionally sorted: https://www.debian.org/doc/debian-policy/ch-binary.html#s-virtual_pkg https://www.debian.org/doc/debian-policy/ch-relationships.html#s-virtual Looking at the list from the virtual package called 'x-terminal-emulator', it doesn't seem particularly sorted either: > Package x-terminal-emulator is a virtual package provided by: > xterm 353-1ubuntu1.20.04.2 > gnome-terminal 3.36.2-1ubuntu1~20.04 > xiterm+thai 1.10-2build1 > xfce4-terminal 0.8.9.1-1 > tilix 1.9.3-4build3 > termit 3.1-1 > terminology 1.6.0-2 > terminator 1.91-4ubuntu1 > stterm 0.8.2-1 > sakura 3.7.0-1 > rxvt-unicode 9.22-6build3 > qterminal 0.14.1-0ubuntu2 > pterm 0.73-2 > mlterm-tiny 3.8.9-1build1 > mlterm 3.8.9-1build1 > mate-terminal 1.24.0-2ubuntu1 > lxterminal 0.3.2-1 > lilyterm 0.9.9.4+git20150208.f600c0-5 > konsole 4:19.12.3-0ubuntu1 > kitty 0.15.0-1build1 > eterm 0.9.6-6build1 > deepin-terminal 5.0.0+ds1-1
But we can side-step the whole question of sorting the list - there is a different virtual package called 'default-mta'. One package chosen by the dist will provide 'default-mta'. On Ubuntu 18.04, 20.04 and 21.04 'default-mta' is provided by 'postfix'. On Debian 9 and 10 'default-mta' is provided by 'exim4-daemon-light'.
Fixed now by switching to default-mta.
Tested this on build 2113 on following: Default Postfix: ✔ Ubuntu 18.04 ✔ Ubuntu 20.04 Default Exim: ✔ Debian 9 ✔ Debian 10 Tested that each of these did not have any sendmail before, and after tl-setup we had sendmail provided by the default mta. Also verified that our tl-setup log stated that we installed it. Each of these were tested twice. Release notes looks good!
This got implemented with a flawed assumption that we always need to install sendmail. However if something is already installed then tl-setup will also try to install postfix, which is likely not what the admin wants.
Fixed again by virtue of bug 5980 being fixed.
