feat(backup): on-demand bitmap recreation for incremental NAS backup

CloudStack rebuilds the libvirt domain XML on every VM start, which means
persistent QEMU dirty bitmaps don't survive a stop/start cycle. Rather
than hooking into the VM start lifecycle (intrusive across the
orchestration layer), this commit handles the missing bitmap *lazily* at
the next backup attempt:

  nasbackup.sh
    - When -M incremental is requested, the script first checks
      `virsh checkpoint-list` for the parent bitmap. If absent, it
      recreates the checkpoint on the running domain so libvirt accepts
      the <incremental> reference. The next incremental will be larger
      than usual (it captures all writes since recreate, not since the
      previous incremental) but is correct; subsequent ones return to
      normal size.
    - On recreation, emits BITMAP_RECREATED=<name> on stdout for the
      orchestrator to record.

  BackupAnswer
    + bitmapRecreated field surfaced from the agent.

  LibvirtTakeBackupCommandWrapper
    - Strips BITMAP_RECREATED= line from stdout before size parsing.
    - Sets answer.setBitmapRecreated(...).

  NASBackupChainKeys
    + BITMAP_RECREATED key for backup_details.

  NASBackupProvider
    - When the agent reports a recreated bitmap, persists it under
      backup_details and logs an info-level message so operators can
      correlate larger-than-usual incrementals with VM restarts.

This satisfies the bitmap-loss-on-VM-restart concern from the RFC review
without touching VirtualMachineManager / StartCommand / agent lifecycle.

Refs: #12899

Author: jmsperu

core/src/main/java/org/apache/cloudstack/backup/BackupAnswer.java

@@ -35,6 +35,11 @@ public class BackupAnswer extends Answer {
     // Set when an incremental was requested but the agent had to fall back to a full
     // (e.g. VM was stopped). Provider should record this backup as type=full.
     private Boolean incrementalFallback;
+    // Set when the agent had to recreate the parent bitmap before this incremental
+    // (e.g. CloudStack rebuilt the domain XML on the previous VM start, losing bitmaps).
+    // The first incremental after a recreate is larger than usual; subsequent
+    // incrementals return to normal size. Informational — recorded in backup_details.
+    private String bitmapRecreated;
 
     public BackupAnswer(final Command command, final boolean success, final String details) {
         super(command, success, details);
@@ -90,4 +95,12 @@ public Boolean getIncrementalFallback() {
     public void setIncrementalFallback(Boolean incrementalFallback) {
         this.incrementalFallback = incrementalFallback;
     }
+
+    public String getBitmapRecreated() {
+        return bitmapRecreated;
+    }
+
+    public void setBitmapRecreated(String bitmapRecreated) {
+        this.bitmapRecreated = bitmapRecreated;
+    }
 }

plugins/backup/nas/src/main/java/org/apache/cloudstack/backup/NASBackupChainKeys.java

@@ -42,6 +42,9 @@ public final class NASBackupChainKeys {
     public static final String TYPE_FULL = "full";
     public static final String TYPE_INCREMENTAL = "incremental";
 
+    /** Set to the bitmap name when this incremental had to recreate its parent bitmap on the host (informational; this incremental is larger than usual). */
+    public static final String BITMAP_RECREATED = "nas.bitmap_recreated";
+
     private NASBackupChainKeys() {
     }
 }

plugins/backup/nas/src/main/java/org/apache/cloudstack/backup/NASBackupProvider.java

@@ -434,6 +434,12 @@ public Pair<Boolean, Backup> takeBackup(final VirtualMachine vm, Boolean quiesce
             backupVO.setBackedUpVolumes(backupManager.createVolumeInfoFromVolumes(volumes));
             if (backupDao.update(backupVO.getId(), backupVO)) {
                 persistChainMetadata(backupVO, effective, answer.getBitmapCreated());
+                if (answer.getBitmapRecreated() != null) {
+                    backupDetailsDao.persist(new BackupDetailVO(backupVO.getId(),
+                            NASBackupChainKeys.BITMAP_RECREATED, answer.getBitmapRecreated(), true));
+                    logger.info("NAS incremental for VM {} recreated parent bitmap {} (likely VM was restarted since last backup)",
+                            vm.getInstanceName(), answer.getBitmapRecreated());
+                }
                 return new Pair<>(true, backupVO);
             } else {
                 throw new CloudRuntimeException("Failed to update backup");

plugins/hypervisors/kvm/src/main/java/com/cloud/hypervisor/kvm/resource/wrapper/LibvirtTakeBackupCommandWrapper.java

@@ -117,6 +117,7 @@ public Answer execute(TakeBackupCommand command, LibvirtComputingResource libvir
         // numeric-suffix parser keeps working.
         String stdout = result.second().trim();
         String bitmapCreated = null;
+        String bitmapRecreated = null;
         boolean incrementalFallback = false;
         StringBuilder filtered = new StringBuilder();
         for (String line : stdout.split("\n")) {
@@ -125,6 +126,10 @@ public Answer execute(TakeBackupCommand command, LibvirtComputingResource libvir
                 bitmapCreated = trimmed.substring("BITMAP_CREATED=".length());
                 continue;
             }
+            if (trimmed.startsWith("BITMAP_RECREATED=")) {
+                bitmapRecreated = trimmed.substring("BITMAP_RECREATED=".length());
+                continue;
+            }
             if (trimmed.startsWith("INCREMENTAL_FALLBACK=")) {
                 incrementalFallback = true;
                 continue;
@@ -152,6 +157,7 @@ public Answer execute(TakeBackupCommand command, LibvirtComputingResource libvir
         BackupAnswer answer = new BackupAnswer(command, true, stdout);
         answer.setSize(backupSize);
         answer.setBitmapCreated(bitmapCreated);
+        answer.setBitmapRecreated(bitmapRecreated);
         answer.setIncrementalFallback(incrementalFallback);
         return answer;
     }

scripts/vm/hypervisor/kvm/nasbackup.sh

@@ -150,6 +150,29 @@ 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 persistent bitmaps
+  # are lost across stop/start. If the parent is missing, recreate it as a fresh bitmap
+  # so libvirt accepts the <incremental> reference. The first backup after a recreate
+  # captures all writes since the recreate point — slightly larger than ideal, but correct.
+  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
+        echo "Failed to recreate parent bitmap $BITMAP_PARENT for $VM"
+        cleanup
+        exit 1
+      fi
+      # Marker for the orchestrator: this incremental is larger because the bitmap was rebuilt.
+      echo "BITMAP_RECREATED=$BITMAP_PARENT"
+      rm -f $dest/recreate-checkpoint.xml
+    fi
+  fi
+
   # Build backup XML (and matching checkpoint XML when applicable).
   name="root"
   echo "<domainbackup mode='push'>" > $dest/backup.xml