keyfile: do not try to write out unvalidated YAML (LP: #1952967)

[slyon]

Description

netplan's keyfile parser can handle certain device types and settings from NM's keyfile format. But we should try to avoid writing out partial YAML where some (essential) fields are missing, as they cannot (yet) be properly parsed.

We need to check that during testing and ideally also at runtime, in order to fall back to the full passthrough mode, using the generic nm-devices type.

FR-1898

Checklist

• Runs make check successfully.
• Retains 100% code coverage (make check-coverage).
• New/changed keys in YAML format are documented.
• (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad. LP#1952967

[codecov-commenter]

Codecov Report

Merging #247 (008fdd2) into main (64e381e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #247   +/-   ##
=======================================
  Coverage   99.10%   99.10%           
=======================================
  Files          58       58           
  Lines       10038    10052   +14     
=======================================
+ Hits         9948     9962   +14     
  Misses         90       90           

Impacted Files	Coverage Δ
src/abi_compat.c	100.00% <100.00%> (ø)
src/parse-nm.c	100.00% <100.00%> (ø)
tests/parser/base.py	100.00% <100.00%> (ø)
tests/parser/test_keyfile.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 64e381e...008fdd2. Read the comment docs.

[schopin-pro]

LGTM, although I wouldn't have expected this many different changes from the PR description. But then, who am I to talk 😉

There's only a small nitpick, but you can disregard it and merge if you feel like it.