Incomplete installation leaves package in unremovable state

Question

After a failed attempt to install PostgreSQL the package was left in an incomplete state:

root@server:~# dpkg -l | grep postgresql-14
iF  postgresql-14                  14.6-1.pgdg22.04+1  ...

Neither could the installation be forced:

root@server:~# apt-get install -f
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following package was automatically installed and is no longer required:
  postgresql-client-11
Use 'apt autoremove' to remove it.
0 upgraded, 0 newly installed, 0 to remove and 82 not upgraded.
1 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Setting up postgresql-14 (14.6-1.pgdg22.04+1) ...
Failed to connect to bus: No such file or directory
dpkg: error processing package postgresql-14 (--configure):
 installed postgresql-14 package post-installation script subprocess returned error exit status 1
Errors were encountered while processing:
 postgresql-14
E: Sub-process /usr/bin/dpkg returned an error code (1)

nor could the package be removed:

root@server:~# apt-get remove --auto-remove --purge postgresql-14
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following packages will be REMOVED:
  postgresql-14* postgresql-client-11*
0 upgraded, 0 newly installed, 2 to remove and 82 not upgraded.
1 not fully installed or removed.
After this operation, 58.1 MB disk space will be freed.
(Reading database ... 51465 files and directories currently installed.)
Removing postgresql-14 (14.6-1.pgdg22.04+1) ...
Failed to connect to bus: No such file or directory
dpkg: error processing package postgresql-14 (--remove):
 installed postgresql-14 package pre-removal script subprocess returned error exit status 1
dpkg: too many errors, stopping
Errors were encountered while processing:
 postgresql-14
Processing was halted because there were too many errors.
E: Sub-process /usr/bin/dpkg returned an error code (1)

How do I force removal of a broken package without re-installing the entire system?

Answer 1

First thing you could try (after normal removal fails) is to force removal with dpkg:

dpkg --remove --force-remove-reinstreq postgresql-14

If that also fails you have to dig deeper. According to the error messages the problem is with the maintainer scripts of the package. These (among other things) are unpacked to /var/lib/dpkg/info when a package is installed and get invoked from that directory.

If you want to debug the issue, check the content of the respective maintainer script (pre-removal script for your PostgreSQL package for instance would be postgresql-14.prerm) and follow the trail from there, i.e. check what commands the script invokes and/or what other files it sources to pinpoint what exactly is failing¹.

If you just want the package removed, simply replace the code after the shebang with an exit 0

#!/bin/sh
exit 0

and then re-run package removal

apt-get remove --auto-remove --purge postgresql-14

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¹ TWIMC: In our case the root cause turned out to be an artifact from replacing Systemd with a different init (s6). PostgreSQL maintainer scripts apparently distinguish between Systemd and other inits by the presence of a directory /run/systemd/system, which had not been removed after replacing the init on the system in question. After manually removing that directory the package could be (un)installed normally.