backup(nas): fix parent-checkpoint recreation for incremental after VM restart

Found while testing on a real libvirt 10.0.0 / qemu 8.2 host: an incremental backup
taken after the VM has been (re)started since the last backup FAILED. CloudStack
rebuilds the domain XML on every VM start, wiping libvirt's checkpoint registry, but
the dirty bitmap persists on the qcow2 (QEMU reloads it). The old code rebuilt a
minimal checkpoint XML and did a fresh 'checkpoint-create', which QEMU rejects with
'Bitmap already exists'; the qemu-img --remove fallback then can't run because the
running VM holds a write lock on the image.

Fix: register the parent with 'checkpoint-create --redefine' using the FULL
checkpoint XML (checkpoint-dumpxml output), which adopts the existing bitmap. A
minimal/synthesized XML is rejected by libvirt's checkpoint RNG schema, so the full
dump is required:
  - dump-on-create: persist <bitmap>.checkpoint.xml next to each backup on the NAS
  - on-recreate: redefine from the parent backup's saved XML; if it's missing (a
    pre-fix backup) or redefine fails, fall back to a full so the chain restarts.

Verified on host xcbn-paix-host3 (libvirt 10.0.0): full->dirty->incremental, then
restart->redefine->incremental, all succeed; the previously-broken path now works.

Author: jmsperu

scripts/vm/hypervisor/kvm/nasbackup.sh

@@ -157,37 +157,28 @@ backup_running_vm() {
       ;;
   esac
 
-  # When incremental, verify the parent bitmap still exists on the running domain.
-  # CloudStack rebuilds the libvirt domain XML on every VM start, so libvirt's checkpoint
-  # registry is wiped — but the bitmap may still exist on the qcow2 itself (we pre-seed
-  # one on stopped-VM backups, see backup_stopped_vm). If the parent is missing from
-  # libvirt's view, recreate it. If it's missing entirely (qcow2 too), this falls through
-  # to a fresh-create which captures all writes since — slightly larger but correct.
+  # When incremental, make sure the parent checkpoint is registered with libvirt. CloudStack
+  # rebuilds the domain XML on every VM start, which wipes libvirt's in-memory checkpoint
+  # registry, while the dirty bitmap persists on the qcow2 (QEMU re-loads it on start). A
+  # fresh checkpoint-create cannot be used then — QEMU reports "Bitmap already exists" — and
+  # qemu-img cannot drop the bitmap on a running disk (the image is write-locked). The parent
+  # must instead be re-registered with --redefine, using the FULL checkpoint XML this script
+  # saved alongside the parent backup when it was taken (a minimal/synthesized XML is rejected
+  # by libvirt's checkpoint schema on redefine, so the full dump is required).
   if [[ "$effective_mode" == "incremental" ]]; then
     if ! virsh -c qemu:///system checkpoint-list "$VM" --name 2>/dev/null | grep -qx "$BITMAP_PARENT"; then
-      cat > $dest/recreate-checkpoint.xml <<XML
-<domaincheckpoint><name>$BITMAP_PARENT</name><disks>
-$(virsh -c qemu:///system domblklist "$VM" --details 2>/dev/null | awk '$2=="disk"{printf "<disk name=\"%s\"/>\n", $3}')
-</disks></domaincheckpoint>
-XML
-      if ! virsh -c qemu:///system checkpoint-create "$VM" --xmlfile $dest/recreate-checkpoint.xml > /dev/null 2>&1; then
-        # If a bitmap of the same name already lives on the qcow2 (pre-seeded by an
-        # offline backup) libvirt 7.2+ should reuse it during checkpoint-create. Older
-        # libvirt fails the create — clean up the orphan bitmap and retry as a fresh.
-        local retried_ok=1
-        for disk_path in $(virsh -c qemu:///system domblklist "$VM" --details 2>/dev/null | awk '$2=="disk"{print $4}'); do
-          [[ -f "$disk_path" ]] && qemu-img bitmap --remove "$disk_path" "$BITMAP_PARENT" 2>/dev/null || true
-        done
-        if ! virsh -c qemu:///system checkpoint-create "$VM" --xmlfile $dest/recreate-checkpoint.xml > /dev/null 2>&1; then
-          retried_ok=0
-        fi
-        if [[ "$retried_ok" == "0" ]]; then
-          echo "Failed to recreate parent bitmap $BITMAP_PARENT for $VM"
-          cleanup
-          exit 1
-        fi
+      parent_first="${PARENT_PATHS%%,*}"
+      parent_cp_xml="$(dirname "$parent_first")/$BITMAP_PARENT.checkpoint.xml"
+      if [[ -f "$parent_cp_xml" ]] && \
+         virsh -c qemu:///system checkpoint-create "$VM" --xmlfile "$parent_cp_xml" --redefine > /dev/null 2>&1; then
+        : # parent checkpoint re-registered; the incremental can proceed against it
+      else
+        # No saved checkpoint XML (e.g. a backup taken before this fix) or redefine failed.
+        # Fall back to a full so the chain restarts cleanly instead of failing the backup.
+        echo "INCREMENTAL_FALLBACK=full (parent checkpoint $BITMAP_PARENT could not be re-registered)"
+        effective_mode="full"
+        BITMAP_PARENT=""
       fi
-      rm -f $dest/recreate-checkpoint.xml
     fi
   fi
 
@@ -344,6 +335,10 @@ XML
   virsh -c qemu:///system domjobinfo $VM --completed
   du -sb $dest | cut -f1
   if [[ -n "$BITMAP_NEW" ]]; then
+    # Persist the FULL checkpoint XML next to this backup so a later incremental can
+    # re-register this checkpoint with --redefine after the VM restarts (which wipes
+    # libvirt's checkpoint registry but leaves the bitmap on the qcow2).
+    virsh -c qemu:///system checkpoint-dumpxml "$VM" "$BITMAP_NEW" > "$dest/$BITMAP_NEW.checkpoint.xml" 2>/dev/null || true
     # Echo the bitmap name on its own line so the Java caller can capture it for backup_details.
     echo "BITMAP_CREATED=$BITMAP_NEW"
   fi