package systemd-resolved 253.5-1ubuntu7 failed to install/upgrade: installed systemd-resolved package post-installation script subprocess returned error exit status 1 [cp: '/etc/resolv.conf' and '/run/systemd/resolve/stub-resolv.conf' are the same file]

[CloudRoot AB]

upgraded from 22.04 -> 24.04

ProblemType: Package
DistroRelease: Ubuntu 24.04
Package: systemd-resolved 253.5-1ubuntu7
ProcVersionSignature: Ubuntu 6.2.0-39.40~22.04.1-generic 6.2.16
Uname: Linux 6.2.0-39-generic x86_64
ApportVersion: 2.27.0-0ubuntu6
Architecture: amd64
CasperMD5CheckResult: pass
CloudArchitecture: x86_64
CloudID: none
CloudName: none
CloudPlatform: none
CloudSubPlatform: config
Date: Wed Jan 3 21:06:55 2024
ErrorMessage: installed systemd-resolved package post-installation script subprocess returned error exit status 1
InstallationDate: Installed on 2023-02-27 (311 days ago)
InstallationMedia: Ubuntu-Server 22.04.1 LTS "Jammy Jellyfish" - Release amd64 (20220809)
Python3Details: /usr/bin/python3.11, Python 3.11.7, python3-minimal, 3.11.4-5
PythonDetails: N/A
RebootRequiredPkgs: Error: path contained symlinks.
RelatedPackageVersions:
 dpkg 1.22.1ubuntu5
 apt 2.7.6
SourcePackage: systemd
Title: package systemd-resolved 253.5-1ubuntu7 failed to install/upgrade: installed systemd-resolved package post-installation script subprocess returned error exit status 1
UpgradeStatus: Upgraded to noble on 2024-01-03 (0 days ago)
mtime.conffile..etc.systemd.resolved.conf: 2023-02-27T21:16:55.869158

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Nick Rosbrook (enr0n)]

I got some information from a user on a more recent duplicate (bug 2063533). The problem seems to be that some users, when disabling the stub-resolver, also create a symlink /etc/resolv.conf -> /run/systemd/resolve/resolv.conf. This is not actually necessary because systemd-resolved will make stub-resolv.conf a symlink to resolv.conf when DNSStubListener=no.

Hence, the command

cp "${DPKG_ROOT}/etc/resolv.conf" "${DPKG_ROOT}/run/systemd/resolve/stub-resolv.conf"

in systemd-resolved.postinst fails because the source and destination are *both* symlinks to /run/systemd/resolve/resolv.conf.