sysupdated: Split Update() into Acquire() and Install() D-Bus methods (#40628)

These mirror the `sysupdate acquire` and `sysupdate install` verbs,
which are themselves a split of `sysupdate update` into downloading and
installing stages.

I have dropped the existing `Update()` method in the final commit, as
per the suggestion on
https://gitlab.gnome.org/GNOME/gnome-software/-/merge_requests/2004#note_2145880.
If the D-Bus API for `sysupdated` is considered stable now, though, I
can drop that commit.

CC @AdrianVovk

Fixes https://github.com/systemd/systemd/issues/34814

test/units/TEST-72-SYSUPDATE.sh

@@ -21,6 +21,8 @@ if [[ ! -x "$SYSUPDATE" ]]; then
     exit 77
 fi
 
+have_updatectl=$([[ -x "$SYSUPDATED" ]] && command -v updatectl)
+
 # Loopback devices may not be supported. They are used because sfdisk cannot
 # change the sector size of a file, and we want to test both 512 and 4096 byte
 # sectors. If loopback devices are not supported, we can only test one sector
@@ -112,6 +114,16 @@ update_now() {
     elif [[ "$update_type" == "split" ]]; then
         "$SYSUPDATE" --verify=no acquire
         "$SYSUPDATE" --verify=no update
+    elif [[ "$update_type" == "updatectl" ]]; then
+        if $have_updatectl; then
+            systemctl start systemd-sysupdated
+            updatectl update
+        else
+            # Gracefully fall back to sysupdate
+            "$SYSUPDATE" --verify=no update
+        fi
+    else
+        exit 1
     fi
     (! "$SYSUPDATE" --verify=no check-new)
 }
@@ -150,8 +162,14 @@ verify_version_current() {
     cmp "$WORKDIR/source/dir-$version/bar.txt" "$WORKDIR/dirs/current/bar.txt"
 }
 
+verify_object_fields() {
+    local updatectl_output="${1:?}"
+
+    [[ "${updatectl_output}" != *"Unrecognized object field"* ]] || exit 1
+}
+
 for sector_size in "${SECTOR_SIZES[@]}"; do
-for update_type in monolithic split-offline split; do
+for update_type in monolithic split-offline split updatectl; do
     # Disk size of:
     # - 1MB for GPT
     # - 4 partitions of 2048 sectors each
@@ -352,7 +370,7 @@ EOF
     # Create sixth version, update using updatectl and verify it replaced the
     # correct version
     new_version "$sector_size" v6
-    if [[ -x "$SYSUPDATED" ]] && command -v updatectl; then
+    if $have_updatectl; then
         systemctl start systemd-sysupdated
         "$SYSUPDATE" --verify=no check-new
         updatectl update
@@ -370,12 +388,12 @@ EOF
     # testing for specific output, but this will at least catch obvious crashes
     # and allow updatectl to run under the various sanitizers. We create a
     # component so that updatectl has multiple targets to list.
-    if [[ -x "$SYSUPDATED" ]] && command -v updatectl; then
+    if $have_updatectl; then
         mkdir -p /run/sysupdate.test.d/
         cp "$CONFIGDIR/01-first.transfer" /run/sysupdate.test.d/01-first.transfer
-        updatectl list
-        updatectl list host
-        updatectl list host@v6
+        verify_object_fields "$(updatectl list 2>&1)"
+        verify_object_fields "$(updatectl list host 2>&1)"
+        verify_object_fields "$(updatectl list host@v6 2>&1)"
         updatectl check
         rm -r /run/sysupdate.test.d
     fi