Bug#935910: apt: Should be less tolerant of dpkg errors when forcing removal
Control: reassign -1 dpkg
On Tue, Aug 27, 2019 at 05:34:05PM +0100, Mark Hindley wrote:
> 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.
APT is the wrong place for that. We're trying to move to a future where
we only tell dpkg all the actions to do and then run a single dpkg, so
adding hacks like that feels counter-intuitive.
If we agree that this is a good thing to do, dpkg should do it.
--
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