[BuildImage] Fix reboot failures on Ubuntu by preventing snap auto-refresh from interfering with build

CHANGELOG.md

@@ -28,6 +28,7 @@ CHANGELOG
 - Add GetFunction and GetPolicy permissions to PClusterBuildImageCleanupRole to prevent AccessDenied errors during build image stack deletion.
 - Fix validation error messages when `DevSettings` is null or `DevSettings/InstanceTypesData` is missing required fields.
 - Fix an issue where cfn-hup enters an endless loop on the head node after a rollback to a cluster state older than 24 hours, caused by cfn-signal failing to signal an expired wait condition handle.
+- Disable snap auto-refresh on Ubuntu during image building to prevent intermittent reboot failures.
 
 3.14.0
 ------

cli/src/pcluster/resources/imagebuilder/parallelcluster.yaml

@@ -34,6 +34,19 @@ phases:
                 *) echo "Unsupported OS: $RELEASE" && exit {{ FailExitCode }} ;;
               esac
 
+      # Disable snap refresh to prevent background updates during build (Ubuntu only)
+      - name: DisableSnapRefresh
+        action: ExecuteBash
+        inputs:
+          commands:
+            - |
+              set -x
+              OS="{{ build.OperatingSystemName.outputs.stdout }}"
+              if [[ $OS =~ ^ubuntu ]]; then
+                snap watch --last=auto-refresh 2>/dev/null || true
+                snap refresh --hold
+              fi
+
       # Initialize system information and URLs
       - name: SystemInfo
         action: ExecuteBash
@@ -260,6 +273,10 @@ phases:
                  echo "SSM agent is installed by default"
               fi
               
+              if [[ $OS =~ ^ubuntu ]]; then
+                snap refresh --unhold
+              fi
+              
               # Final cleanup
               rm -f /opt/parallelcluster/system_info
               /usr/local/sbin/ami_cleanup.sh "${CfnParamIsOfficialAmiBuild}"