T6764: Fix unhandled exception on ethtool output parsing for Xen NICs

[sever-sever]

Change Summary

Not all NICs could provide ring-buffers info requested by ethtool in JSON format
For example 'vif' Xen/XCP-NG interfaces
Fix it

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T6764

Related PR(s)

Component(s) name

ethtool

Proposed changes

How to test

XEN/XCP-ng vif before the fix:

Traceback (most recent call last):
  File "/usr/libexec/vyos/conf_mode/interfaces_ethernet.py", line 354, in <module>
    verify(c)
  File "/usr/libexec/vyos/conf_mode/interfaces_ethernet.py", line 274, in verify
    verify_ethernet(ethernet)
  File "/usr/libexec/vyos/conf_mode/interfaces_ethernet.py", line 310, in verify_ethernet
    ethtool = Ethtool(ifname)
              ^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/vyos/ethtool.py", line 105, in __init__
    self._ring_buffer = loads(out)[0]
                        ~~~~~~~~~~^^^
IndexError: list index out of range

After the fix configuration applies without errors

Smoketest result

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[github-actions]

👍
No issues in PR Title / Commit Title

[dmbaturin]

My only concern here: can we differentiate between an expected failure for Xen NICs and similar and an unexpected failure where --show-ring is supposed to work? This approach may hide bugs.

I'm not against merging it as an immediate fix.

[zdc]

It depends on what is "error" from our point of view:

• unexpected ethtool output
• a feature is not supported by a NIC
• both

I have an internal feeling that for our purposes a reason why it failed does not matter, but input from someone deeply familiar with interface config logic is welcomed.

[github-actions]

CI integration 👍 passed!

Details

CI logs

• CLI Smoketests (no interfaces) 👍 passed
• CLI Smoketests (interfaces only) 👍 passed
• Config tests 👍 passed
• RAID1 tests 👍 passed
• TPM tests 👍 passed