Bug#992647: backuppc: stores wrong path to ping6 if built on merged-/usr system

[Simon McVittie]

Source: backuppc
Version: 4.4.0-4
Severity: important
Tags: patch bookworm sid
User: reproducible-builds at lists.alioth.debian.org
Usertags: usrmerge
X-Debbugs-Cc: reproducible-bugs at lists.alioth.debian.org

If backuppc is built on a merged-/usr system (as created by new installations
of Debian >= 10, debootstrap --merged-usr, or installing the usrmerge
package into an existing installation), the path to ping6 is recorded in the
binary as /usr/bin/ping6.

This can be seen on the reproducible-builds.org infra:
https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/diffoscope-results/backuppc.html
(search for "ping6" to see the difference I'm concerned about).

If you have sbuild available, an easy way to reproduce this is to build
twice, once with --add-depends-arch=usrmerge and once without.

The problematic situation is if the package is *built* on a merged-/usr
system, but *used* on a non-merged-/usr system. In this situation,
/usr/bin/ping6 exists on the build system but not on the system where
backuppc will be used, resulting in the feature that uses ping6 not being
available.

Technical Committee resolution #978636 mandates heading towards a
transition to merged-/usr, and this will become a non-issue at the end of
that transition; but variation between merged-/usr and non-merged-/usr
builds is a problem while that transition is taking place, because it
can lead to partial upgrades behaving incorrectly. It is likely that
this class of bugs will become release-critical later in the bookworm
development cycle.

Some Debian developers advocate that instead of merged-/usr, we should
use a different strategy where /bin becomes a "symlink farm" with
individual symlinks such as /bin/ping6 -> /usr/bin/ping6. If that route is
taken instead of merged-/usr, then resolving bugs like this one will be
equally important as part of that transition, because it shares the
property that both /bin/ping6 and /usr/bin/ping6 exist after the transition,
but only /bin/ping6 exists on untransitioned systems.

The attached patch resolves this: with it applied, the package builds
identically with and without --add-depends-arch=usrmerge.

A side benefit of fixing this is that this change seems to be sufficient
to make the package reproducible (as recommended by Policy §4.15).

    smcv
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