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Bug#1126595: dbus-system-bus-common: fatal: The user `messagebus' already exists and has a password

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Control: tags -1 = forky sid ftbfs pending

On Thu, 29 Jan 2026 at 17:11:06 +0000, Simon McVittie wrote:
># apt update && apt upgrade
># apt install adduser
># adduser --system _bangstar
># echo '_bangstar:!*' | chpasswd -e
># adduser --system _bangstar
…
>Actual result: on sid, the last `adduser --system _bangstar` (after 
>setting the password) fails with the same error message shown in the 
>subject line.

This appears to have been fixed in adduser git already, by commit 
debian/3.154-13-gd5820ae "rework EXISTING_ and existing_user_status", so 
the next adduser upload can presumably close this bug report.

For the FTBFS seen by Santiago as a result of this, it seems that the 
problem scenario is:

1. install dbus-system-bus-common on a system that has
    systemd-sysusers(8), so that the messagebus user is created by that;
2. remove systemd-sysusers(8) or copy /etc/shadow to a system that does
    not have it;
3. install dbus-system-bus-common again, so that the messagebus user is
    (re)created by adduser (which is meant to be idempotent, but this bug
    makes it not idempotent in all cases)

For autobuilders, that can be avoided by using sbuild's unshare backend 
in preference to its schroot backend, because it is the schroot backend 
that copies /etc/shadow from the host system to the chroot.

Regarding Santiago's concerns about upgrades from one Debian release to 
another, the only problematic scenario for dbus would be if 
systemd-sysusers(8) was previously installed, but is subsequently 
removed. This seems like it would be an unusual thing to do: bootable 
systems (even with sysvinit) normally have udev, which has a hard 
dependency on systemd-sysusers(8), and minimal chroots/containers 
normally add packages as required but do not remove them (they might 
install systemd-standalone-sysusers or systemd, but if they do, they 
will most likely keep it installed).

If I switched dbus-system-bus-common to exclusively use 
systemd-sysusers(8) and no longer have an adduser fallback (making it 
more similar to for example udev, openssh-server, dhcpcd-base and 
polkitd) then that would prevent this from affecting dbus. Given that 
openssh-server now has this dependency, I think we're closer to 
consensus on it being a normal thing to do than we were last time I 
looked at this.

     smcv

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Bug#1126595: dbus-system-bus-common: Many packages FTBFS with the nocheck build profile due to weird dbus/adduser error Simon McVittie <smcv@debian.org> - 2026-01-29 15:00 +0100
  Bug#1126595: dbus-system-bus-common: fatal: The user `messagebus' already exists and has a password Santiago Vila <sanvila@debian.org> - 2026-01-29 15:20 +0100
  Bug#1126595: dbus-system-bus-common: Many packages FTBFS with the nocheck build profile due to weird dbus/adduser error Santiago Vila <sanvila@debian.org> - 2026-01-29 16:20 +0100
    Bug#1126595: dbus-system-bus-common: fatal: The user `messagebus' already exists and has a password Simon McVittie <smcv@debian.org> - 2026-01-29 18:20 +0100
      Bug#1126595: dbus-system-bus-common: fatal: The user `messagebus' already exists and has a password Santiago Vila <sanvila@debian.org> - 2026-01-29 19:50 +0100
      Bug#1126595: dbus-system-bus-common: fatal: The user `messagebus' already exists and has a password Simon McVittie <smcv@debian.org> - 2026-01-31 13:00 +0100
  Bug#1126595: dbus-system-bus-common: fatal: The user `messagebus' already exists and has a password Santiago Vila <sanvila@debian.org> - 2026-01-29 16:40 +0100
  Bug#1126595: dbus-system-bus-common: fatal: The user `messagebus' already exists and has a password Santiago Vila <sanvila@debian.org> - 2026-01-29 17:20 +0100

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