MDBF-1159 Libvirt - collect system logs as a step LogFile

[RazvanLiviuVarzaru]

When an error occurs, displaying system logs in standard output may not be beneficial if they do not provide additional information that could help with debugging the issue.

This was initially implemented as a trap, triggered whenever the scripts exit with a failure. See: MDBF-1130 (bbb5955)

Sometimes, the errors of interest are located at the beginning of the log, but they are obscured by a large number of system logs. Collecting system logs could be done into a file that is uploaded at the buildbot step level.

This can be implemented by defining the logfiles parameter at the step level. See: https://docs.buildbot.net/latest/manual/configuration/steps/shell_command.html

[RazvanLiviuVarzaru]

@grooverdan I assigned you for review because you were the original implementer of the collection mechanism.
I kept the original logic you developed intact, it works very well, I just added means to offload the logs from stdio to dedicated log entries under the step, so that is much easier to debug.

Thank you for your time!

(FYI, this example is https://jira.mariadb.org/browse/MDEV-38628)

---

Test: https://buildbot.dev.mariadb.org/#/builders/809/builds/4

Where a step failure is present, logs are collected.

[fauust]

Overall, this looks good!

See my comments about >/dev/null filtering, in my experience it's a bad idea since you can miss errors and it makes script much more complex to maintain and debug.

[RazvanLiviuVarzaru]

@fauust thank you for the review,
I addressed your review comments, the remaining /dev/null are for tee because we don't want to pollute the main log but instead write to the uploaded log files.

Please approve if there's nothing else to fix.