PAR2 backend: If par2 file creation fails, duplicity silently continues without warning - Patch inside

I have:

• ([x] when completed)
• searched https://gitlab.com/duplicity/duplicity/-/issues for similar issues. If you find a similar issue and the issue is still open, add a comment to the existing issue instead of opening a new one. If you find a Closed issue that seems like it is the same thing that you're experiencing, open a new issue and include a link to the original issue in the body of your new one.
• searched https://bugs.launchpad.net/duplicity for similar issues. If you find a similar issue, open a new issue on here and include a link to the original issue in the body of your new one.
• tested that this issue still occurs on the latest stable snap (install instructions: https://snapcraft.io/duplicity), please include the snap version (snap info duplicity | grep installed) output: installed: x.xx.xx (xx)
• ideally, tested that this issue still occurs on the latest edge snap, if you can test without risking your data. Please include the snap version output: installed: x.xx.xx (xx)

Summary

When par2 creation fails for whatever reasons (for example mistyping options passed to par2 with --par2-options), duplicity does not warn the user and silently continues the backup without par2 files.

This had been discussed here https://bugs.launchpad.net/duplicity/+bug/1642098, but there's no corresponding warning message in current master.

Environment

Debian sid

Duplicity latest master

duplicity a/b/c/ par2+file:///x/y/z/ --no-encryption --par2-options -xyz123

Steps to reproduce

Simply use above command line with garbage --par2-options.

What is the current bug behaviour?

See above.

What is the expected correct behaviour?

At least a warning to the user, if not abortion of backup.

Possible fixes

I added a simple patch, which logs an Error message with the PAR2 returncode in case of failure. So now the user is at least aware of PAR2 file creation failure.

duplicity-warn-on-par2-failure.patch