Bug#1101672: dpkg-db-backup.service causes high CPU usage from systemd due to restart loop

Package: dpkg
Version: 1.21.22
Severity: important
X-Debbugs-Cc: [email protected]

Dear Maintainer,

On multiple Debian 12 (Bookworm) systems, the dpkg-db-backup.service unit begins running at midnight and enters a rapid restart loop. This causes systemd (PID 1) to consume excessive CPU (often 100%) until systemd's start-limit-hit is triggered.

The service logs show that it starts and deactivates "successfully", but systemd appears to consider it a failure and restarts it repeatedly. This creates rapid log entries, spawns processes, and can overwhelm lightweight systems (e.g., virtual machines).

Steps to reproduce:
1. Let a Debian 12 system idle until midnight with the default timer enabled
2. Observe CPU usage via `top` or `htop` (systemd will be at or near 100%)
3. Check `journalctl -u dpkg-db-backup.service` for repeated start/fail cycles

Workaround:
Disabling the timer and masking the service resolves the issue:

'sudo systemctl disable --now dpkg-db-backup.timer sudo systemctl mask dpkg-db-backup.service'


This behavior affects both virtual and physical Debian 12 machines that have not been modified beyond using Docker and standard updates. Please investigate whether the unit file needs an adjusted Restart= policy or if the script exits too quickly to be considered successful.

Thank you!


-- Package-specific info:
This system uses merged-usr-via-aliased-dirs, going behind dpkg's
back, breaking its core assumptions. This can cause silent file
overwrites and disappearances, and its general tools misbehavior.
See <https://wiki.debian.org/Teams/Dpkg/FAQ#broken-usrmerge>.

-- System Information:
Debian Release: 12.10
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable-security'), (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 6.1.0-32-cloud-amd64 (SMP w/4 CPU threads; PREEMPT)
Locale: LANG=C.UTF-8, LC_CTYPE=C.UTF-8 (charmap=UTF-8), LANGUAGE not set
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages dpkg depends on:
ii  libbz2-1.0   1.0.8-5+b1
ii  libc6        2.36-9+deb12u10
ii  liblzma5     5.4.1-0.2
ii  libmd0       1.0.4-2
ii  libselinux1  3.4-1+b6
ii  libzstd1     1.5.4+dfsg2-5
ii  tar          1.34+dfsg-1.2+deb12u1
ii  zlib1g       1:1.2.13.dfsg-1

dpkg recommends no packages.

Versions of packages dpkg suggests:
ii  apt            2.6.1
pn  debsig-verify  <none>

-- no debconf information