Inspired in the solution applied in the Wazuh-manager and Wazuh-agent packages, the implementation consists in saving a temp file as reference to the previous status of the service at the pre-install stage (preinst), and a conditional that restores the running status of the Wazuh-indexer service at the post-install stage (postinst).

• Pre-install script additions

  ...
  # Reference to restore actual service status
  restart_service=/tmp/wazuh-indexer.restart
  
  # Stop existing service
  if command -v systemctl >/dev/null && systemctl is-active wazuh-indexer.service >/dev/null; then
      ...
      touch $restart_service
  fi
  ...

• Post-install script additions

  ...
  if [ -f $restart_service ]; then
      rm -f $restart_service
      echo "Restarting wazuh-indexer service..."
      if command -v systemctl > /dev/null; then
          systemctl restart wazuh-indexer.service > /dev/null 2>&1
      fi
      exit 0
  fi
  ...

Also the Pre-remove is updated to avoid running the service stop when the package is being upgraded

case "$1" in
    upgrade|deconfigure)
    ;;
    remove)
        echo "Running Wazuh Indexer Pre-Removal Script"
        # Stop existing service
        ...
    ;;
    failed-upgrade)
    ;;
    *)
        echo "prerm called with unknown argument \`$1'" >&2
        exit 0
    ;;
esac

Upgrading a running Wazuh-indexer using the package with the solution implemented results on the package running.

• Previous Wazuh-indexer status

  systemctl status wazuh-indexer
  
  ● wazuh-indexer.service - wazuh-indexer
       Loaded: loaded (/lib/systemd/system/wazuh-indexer.service; enabled; preset>
       Active: active (running) since Fri 2024-10-25 05:06:51 PDT; 13min ago
         Docs: https://documentation.wazuh.com
     Main PID: 3737 (java)
        Tasks: 76 (limit: 4571)
       Memory: 1.3G
          CPU: 27.348s
       CGroup: /system.slice/wazuh-indexer.service
               └─3737 /usr/share/wazuh-indexer/jdk/bin/java -Xshare:auto -Dopense>

• Upgrade Wazuh-indexer

  dpkg -i wazuh-indexer_5.0.0-0_arm64_b2d1265.deb 
  
  (Reading database ... 151060 files and directories currently installed.)
  Preparing to unpack wazuh-indexer_5.0.0-0_arm64_b2d1265.deb ...
  Running Wazuh Indexer Pre-Installation Script
  Stop existing wazuh-indexer.service
  Unpacking wazuh-indexer (5.0.0-0) over (5.0.0-0) ...
  Setting up wazuh-indexer (5.0.0-0) ...
  Running Wazuh Indexer Post-Installation Script
  Restarting wazuh-indexer service...

• Status after upgrade

  systemctl status wazuh-indexer
  
  ● wazuh-indexer.service - wazuh-indexer
       Loaded: loaded (/lib/systemd/system/wazuh-indexer.service; enabled; preset>
       Active: active (running) since Fri 2024-10-25 05:23:58 PDT; 1min 30s ago
         Docs: https://documentation.wazuh.com
     Main PID: 4051 (java)
        Tasks: 81 (limit: 4571)
       Memory: 1.3G
          CPU: 14.959s
       CGroup: /system.slice/wazuh-indexer.service
               └─4051 /usr/share/wazuh-indexer/jdk/bin/java -Xshare:auto -Dopense>