Package: apt Version: 1.8.3 Severity: important Dear apt maintainers,
This report is a result of my investigation of #934491 which shows that when trying to migrate a sid install from systemd to sysvinit and elogind by the most obvious route it is possible to break the system in a way that is difficult to recover with the fairly simple command: apt install libpam-elogind sysvinit-core I think the root cause of the issue is the way apt invokes dpkg when forcing removals which is not sufficiently responsive to errors. In this case apt's dpkg commands are: echo 'libpam-systemd:amd64 deinstall' | /usr/bin/dpkg --set-selections echo 'systemd-sysv:amd64 deinstall' | /usr/bin/dpkg --set-selections echo 'systemd:amd64 deinstall' | /usr/bin/dpkg --set-selections echo 'libsystemd0:amd64 deinstall' | /usr/bin/dpkg --set-selections /usr/bin/dpkg --status-fd 18 --no-triggers --force-depends --remove libpam-systemd:amd64 /usr/bin/dpkg --status-fd 18 --no-triggers --unpack --auto-deconfigure /var/cache/apt/archives/libpam-elogind_241.3-1+debian1_amd64.deb /usr/bin/dpkg --status-fd 18 --no-triggers --force-depends --remove systemd-sysv:amd64 /usr/bin/dpkg --status-fd 18 --no-triggers --unpack --auto-deconfigure /var/cache/apt/archives/sysvinit-core_2.96~beta-1_amd64.deb /usr/bin/dpkg --status-fd 18 --no-triggers --force-depends --remove systemd:amd64 libsystemd0:amd64 ^^^ The line above causes the problem ^^^ /usr/bin/dpkg --status-fd 18 --no-triggers --unpack --auto-deconfigure /var/cache/apt/archives/libelogind0_241.3-1+debian1_amd64.deb /usr/bin/dpkg --status-fd 18 --no-triggers --configure libelogind0:amd64 /usr/bin/dpkg --status-fd 18 --no-triggers --unpack --auto-deconfigure /var/cache/apt/archives/elogind_241.3-1+debian1_amd64.deb /usr/bin/dpkg --status-fd 18 --configure --pending The problem is caused by the invocation forcing the removal of systemd and libsystemd0. If systemd is pid 1 then its prerm will fail to prevent removal. However, dpkg still continues to remove libsystemd0 as requested *before* exiting and signalling an error. apt then stops in response to the dpkg error. The system is then broken as there is no libsystemd.so.0 installed which apt itself requires. My solution is to invoke dpkg with --abort-after=1. The command line apt install -o DPkg::Options::="--abort-after=1" install libpam-elogind sysvinit-core no longer breaks a sid systemd system. This means that dpkg exits immediately after the systemd prerm fails and libsystemd0 remains installed. The dependencies can then be fixed with apt. I suggest that --abort-after=1 should be apt's default when forcing package removal. Thanks Mark
