systemd-sysext allows to overlay another image (or multiple) ontop of
a "base-image" = the current rootfs, via the use of overlayfs; to add
tools and features meant for development purposes.
To quote the documentation on systemd-sysext:
" ...addition in order to make debugging/development easier). System
extension images should not be misunderstood as a generic software
packaging framework, ..."
To build a lean image, that only holds packages that are not already
part of the base-image, a snapshot of the package-database is taken
after the installation of the base-rootfs is done, and picked up again
when collecting the rootfs of such a extension image.
with all this in place an example usage could look like this:
some-core-image.bb
inherit core-image
IMAGE_GEN_PKGDBFS = "1"
extending-image.bb
inherit image-sysext
IMAGE_FSTYPES = "squashfs"
IMAGE_BASE_PKGDB = "some-core-image"
# the above pointing at a package-db similar to:
#
build/deploy/images/$MACHINE/some-core-image-$MACHINE-20240210172305-pkgdb.rootfs.tar.gz
then on the device, running some-core-image, with the extension image placed at
FN:
$> ln -s "$FN" /run/extensions/$(basename $FN).raw
$> systemd-sysext list
$> SYSTEMD_LOG_LEVEL=debug systemd-sysext merge
As long as the VERSION_ID of the extension image matches the os-release
in the base image, the above commands return sucessfully;
for details on the compativility check see the docs for systemd-sysext.
=========
changes with v2:
rebase from 'kirkstone' onto 'master'
changes with v3;
incorporate review suggestions for simplification
add task dependency handling
add oe-selftest for the pkgdb handling
add variable documentation and
some more comments, and examples in the commit-msg
changes with v4;
rebase onto 'master' => no functional changes
fixed patchtest findings
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