NUT-Monitor: Add automatic reconnection mechanism for Qt5 and Qt6

[developersorli]

Summary

This PR adds an automatic reconnection mechanism to NUT-Monitor-py3qt5 and NUT-Monitor-py3qt6. When the connection to the NUT server is lost (e.g., after system suspend/resume, network interruption, or NUT server restart), the client will automatically attempt to reconnect.

Changes

1. Reconnection logic in gui_updater.__update():

   • Detects connection errors via specific exceptions
   • Stops the polling timer
   • Schedules reconnection attempt after 2 seconds

2. New __attempt_reconnect() method:

   • Gets connection parameters from the current GUI state
   • Properly tears down old PyNUTClient instance (closes telnet connection)
   • Creates new connection and verifies UPS availability
   • Restores GUI state and resumes polling

3. __reconnect_attempted flag - prevents multiple simultaneous attempts

4. Proper exception handling:

   • socket.error - network/socket errors
   • ConnectionError - Python 3 built-in connection errors
   • EOFError - end of file / connection closed
   • OSError - operating system errors
   • PyNUT.PyNUTError - NUT protocol errors

5. Proper teardown of old connection handler:

   • Closes underlying telnet connection
   • Deletes old handler reference

6. was_online flag - tracks connection state changes

Testing

• Qt5: ✅ Fully tested and confirmed working

  • System suspend/resume cycles
  • Network disconnection/reconnection
  • NUT server restart
  • Authentication and non-authentication modes

• Qt6: ✅ Fully tested and confirmed working

  • Same tests performed as Qt5
  • All functionality verified

Files Modified

• scripts/python/app/NUT-Monitor-py3qt5.in
• scripts/python/app/NUT-Monitor-py3qt6.in

Related Issues

Closes #3509

Notes for Reviewers

• Both versions use the same reconnection logic
• Both versions have been tested and confirmed working
• The implementation only affects the Qt versions, not GTK2 (Python 2, obsolete)
• The PyNUTClient instance is properly cleaned up before reconnection

[github-actions]

A ZIP file with standard source tarball and another tarball with pre-built docs for commit 1d157f9 is temporarily available: NUT-tarballs-PR-3523.zip.

[AppVeyorBot]

✅ Build nut 2.8.5.4914-master completed (commit 572b34b736 by @developersorli)

• artifacts
• A 7-zip archive with built NUT for Windows debug binaries is temporarily available: NUT-for-Windows-x86_64-SNAPSHOT-2.8.5.4914-master.7z