Use fatrace to record all file access during an sbuild run and find those build dependencies which have their files never needed. You need superuser privileges to run this script because of fatrace.

Run it like follows. In one terminal execute:

$ ./findunusedbd.sh

In another run sbuild like this:

$ sbuild \
	--chroot-setup-commands='/home/user/path/to/findunusedbd.sh chroot-setup' \
	--pre-realbuild-commands='/home/user/path/to/findunusedbd.sh pre-realbuild' \
	--post-realbuild-commands='/home/user/path/to/findunusedbd.sh post-realbuild'

This needs the --pre-realbuild-commands and --post-realbuild-commands to exist which can be added to sbuild by applying 0001-add-pre-realbuild-commands-and-post-realbuild-comman.patch to it.

Any unused dependencies can then be found by investigating the file unneededdepends.list.

The process can be automated for multiple packages by passing dsc files to run.sh:

$ ./run.sh ../mysources/*.dsc

This script will put the successful builds in buildsuccess.list and the found unused build dependencies as *.unusedbd for each dsc file in the current directory. A second pass on the successfully built dsc files will then check each of the found unused build dependencies for their validity by replacing them by an empty equivs package one after another. The results of that run are stored in *.unusedbd.real files for each dsc file in the current directory. The run.sh script expects to find findunusedbd.sh directly under /home. Best try this out in a chroot to not mess with the host system.

Customize the schroot:

$ sbuild-shell source:sid-amd64-sbuild
$ echo 'Acquire::Check-Valid-Until "false";' > /etc/apt/apt.conf.d/80-nocheckvaliduntil
$ apt-get install equivs --no-install-recommends

Bugs:

• when investigating which build dependencies are unused, virtual packages are not taken into account
• maybe the fake equivs package can be built outside the schroot to avoid the additional dependencies for installing equivs
• maybe equivs can be avoided altogether by finding a way to edit debian/control on the fly