Bug#1021402: reproducible: Please force merged-/usr for build2

[Simon McVittie <smcv@debian.org>]

Package: jenkins.debian.org
Severity: normal
X-Debbugs-Cc: usrmerge@packages.debian.org, debian-qa@lists.debian.org

The reproducible builds infrastructure tries to vary the merged-/usr
status of the build system, like this:

- base tarball: explicitly disable merged-/usr
- build1: leave merged-/usr disabled
- build2: pass "--extrapackages usrmerge" to pbuilder, to convert the
  system to merged-/usr

This has been very useful for detecting bugs of the same class as
<https://bugs.debian.org/1015188>.

However, now that new chroots are merged-/usr by default, I don't think
this is working as intended. Disabling merged-/usr during base chroot
creation now creates a flag file /etc/unsupported-skip-usrmerge-conversion
representing "please don't merge /usr, contrary to the default", and
installing the usrmerge package is not sufficient to undo the effect of
that flag file.

I believe the procedure to convert a non-merged-/usr QA chroot to
merged-/usr is now something like this:

1. ensure the usrmerge package is installed
2. rm /etc/unsupported-skip-usrmerge-conversion
3. dpkg-reconfigure usrmerge

Please do that for reproducible builds in build2, reinstating the
previous setup where build1 is not merged-/usr but build2 is. I think
implementing this on reproducible builds will require adding a pbuilder
hook that does steps 2 and 3?

Thanks,
    smcv