fix(updater): decide "no update needed" before deleting or downloading

The check exists already, but it sits after the pull, and the pull needs
the room that deleting the standby slot frees - so an update that turns
out to be unnecessary had already discarded the system this one would
roll back to, and on an ARMv6 board had spent half an hour doing it.

Asking the registry first costs one round trip. The digest has to come
from "buildx imagetools", which reports the manifest list's digest, the
one docker records in RepoDigests after a pull and the one /etc/image
holds; "docker manifest inspect" reports the per-platform digests and
would compare the wrong values. Advisory: an empty answer falls through
to the pull.

Signed-off-by: Simon Walz <contact@simonwalz.de>

Author: simonwalz

updater/perform_update.sh

@@ -110,6 +110,30 @@ CONTAINER_ROOTFS_FS="${TARGET_ROOT}/system-${SLOT}"
 # Consumed by the install-kernel-*.sh scripts inside the new rootfs:
 export CONTAINER_ROOTFS="@os/system-${SLOT}"
 
+# Ask the registry whether there is anything new, before anything is downloaded
+# or deleted. The check below at "No OS update available" is the same question,
+# but it can only be asked once the image has been pulled - and the pull needs
+# the room that deleting the standby slot frees, so by then the slot this system
+# would roll back to is already gone. On a small ARM board that costs half an
+# hour and the rollback target for an update that turns out to be unnecessary.
+#
+# imagetools reports the digest of the manifest *list*, which is what docker
+# records in RepoDigests after a pull and therefore what /etc/image holds.
+# "docker manifest inspect -v" is not a substitute: it reports the digests of
+# the individual platform manifests, which are different values (measured on a
+# board, 2026-09-06).
+#
+# Advisory only. An empty answer - no buildx, an unreachable or a
+# non-conforming registry - falls through to the pull and changes nothing.
+if [[ "${INSTALLIMAGE}" != "true" ]]; then
+  REMOTE_DIGEST="$(docker buildx imagetools inspect \
+    --format '{{.Manifest.Digest}}' "${IMAGE}" 2>/dev/null)"
+  if [[ -n "${REMOTE_DIGEST}" \
+      && "${IMAGE}@${REMOTE_DIGEST}" == "$(cat /etc/image 2>/dev/null)" ]]; then
+    raise_info 102 "No OS update available"
+  fi
+fi
+
 # Remove old slot:
 OLD_IMAGE_PATH="$(grep -oE '^[^@]+' "${CONTAINER_ROOTFS_FS}/etc/image" 2>/dev/null)"
 if [[ -d "${CONTAINER_ROOTFS_FS}" ]]; then