dbus: wait for 'netplan try' to be ready (LP: #1949893)

[slyon]

Description

netplan-dbus now waits for the spawned 'netplan try' child to touch the /run/netplan/netplan-try.ready stamp file before it returns the DBus call. If no stamp file is detected within up to 5 sec it hits a timeout and returns an error.

Additionally, this PR places ephemeral netplan configs in /run/netplan/config-XXXXXX, to avoid potential exploits of tmpfiles, like https://lwn.net/Articles/250468/

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#1949893

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.09%. Comparing base (c6ad8e6) to head (5aa371b).
Report is 695 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #245   +/-   ##
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  Coverage   99.09%   99.09%           
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  Files          58       58           
  Lines        9913     9977   +64     
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+ Hits         9823     9887   +64     
  Misses         90       90           

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[mvo5]

Looks good, two nitpicks inside but given the constraints probably the best we can do :) [reading this again I really hope it does not sound negative - it's not meant this way]

[slyon]

Thank you @mvo5 for your comments! Indeed having a "stamp file" is a much more universal approach. I'm not sure why I didn't think about that earlier... I thought about passing signals from child to parent process (like we do from parent to child currently) and also about passing DBus signal directly from the Python code (but that would probably be overkill as we do not have proper DBus integration with the python code, currently and on the netplan-dbus daemon side it would have led to additional sd-bus interrupts and probably the need for async methods.)

Polling for a file is plain and simple solution.
I adopted your recommendation a bit, primarily by having a global /tmp/netplan-try.ready stamp file. As there can only every be one single netplan try call at a time (as it is touching system system config and (re-)starting services). This way we do not have to shuffle it around with the states and it can also be used in a non-DBus environment.

[mvo5]

Looks very nice! I have some comments/nitpicks/suggestions inline that are hopefully useful but nothing that would bock this. Thanks a lot for working on this!

[mvo5]

Looks very nice, thank you! One tiny nitpick inline but feel free to ignore :)

[slyon]

Thank you so much @mvo5 for all your input here. I think this is ready for merging \o/

Let me run some more test on it before merging it early next week.

[slyon]

Merged in e19441c