V6.1 continue beating on the backup

deploy/backup/create_postgres_backup.sh

@@ -16,6 +16,8 @@ logs_file="logs-${timestamp}.tar.gz"
 manifest_file="backup-${timestamp}.manifest"
 
 mkdir -p "${BACKUP_DIR}"
+mkdir -p "${APP_DATA_BACKUP_DIR}"
+app_data_backup_abs="$(cd "${APP_DATA_BACKUP_DIR}" && pwd -P)"
 
 # If SYNOLOGY_BACKUP_DIR wasn't exported in the shell, read it from ENV_FILE.
 if [ -z "${SYNOLOGY_BACKUP_DIR}" ] && [ -f "${ENV_FILE}" ]; then
@@ -68,10 +70,17 @@ find "${BACKUP_DIR}" -type f \( \
   -name 'backup-*.manifest' \
 \) -mtime +"${RETENTION_DAYS}" -delete
 
-# Always keep a rolling incremental mirror of uploaded files in BACKUP_DIR.
-mkdir -p "${APP_DATA_BACKUP_DIR}"
+# Always keep a rolling mirror of app data in BACKUP_DIR, unless that target
+# is nested under /app/data (which would recursively copy into itself).
 if [ -d /app/data ]; then
-  cp -a /app/data/. "${APP_DATA_BACKUP_DIR}/"
+  case "${app_data_backup_abs}" in
+    /app/data|/app/data/*)
+      echo "Skipping app data mirror because APP_DATA_BACKUP_DIR is nested under /app/data: ${APP_DATA_BACKUP_DIR}"
+      ;;
+    *)
+      cp -a /app/data/. "${APP_DATA_BACKUP_DIR}/"
+      ;;
+  esac
 fi
 
 mkdir -p "${UPLOADS_BACKUP_DIR}/documents" "${UPLOADS_BACKUP_DIR}/photos"