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.

Bugs: when investigating which build dependencies are unused, virtual packages are not taken into account.