github: check downgrade to v4

[kenhys]

github: check downgrade to v4

To downgrade correctly you must:

* Remove fluent-package in beforehand.
* Manually migrate buffer or logs
* Re-install td-agent v4.

NOTE: on debian/ubuntu, need to disable td-agent.service alias
beforehand because:
  * It conflict with v4 td-agent.service
  * If you didn't disable it, it causes broken symlink from td-agent.service to /lib/systemd/system/fluentd.service.

[daipom]

Thanks!
We need to add this new test here to run it in CI.

fluent-package-builder/fluent-package/yum/systemd-test/test.sh

Lines 21 to 25 in b4b7017

	test_filenames=(
	update-from-v4.sh
	update-to-next-version.sh
	update-to-next-version-with-backward-compat-for-v4.sh
	)

fluent-package-builder/fluent-package/apt/systemd-test/test.sh

Lines 26 to 30 in b4b7017

	# As no bookworm package for v4, so execute upgrade test for other code name.
	test_filenames+=(
	update-from-v4.sh
	update-to-next-version-with-backward-compat-for-v4.sh
	)

[kenhys]

checking failure on local vm.

[kenhys]

Technically package downgrade will work but, not same as before.
Thus, uninstall v5 && re install v4 is the recommended action to do now.

[daipom]

Thanks! I'm concerned about the following points.
I didn't think this step was necessary.
I'm checking the behavior.

* Disable td-agent.service alias beforehand.
  
  * It conflict with v4 td-agent.service
  * If you didn't disable it, it causes broken symlink from td-agent.service to /lib/systemd/system/fluentd.service.

[daipom]

In my local env Rocky Linux 8, I don't need to manually remove the td-agent.service alias.
It is automatically removed when uninstalling fluent-package.

I'm testing daipom@3471b31 on my fork:

• rpm: https://github.com/daipom/fluent-package-builder/actions/runs/7164750480
• apt: https://github.com/daipom/fluent-package-builder/actions/runs/7164750483

[daipom]

In my local env Rocky Linux 8, I don't need to manually remove the td-agent.service alias. It is automatically removed when uninstalling fluent-package.

I'm testing daipom@3471b31 on my fork:

* rpm: https://github.com/daipom/fluent-package-builder/actions/runs/7164750480

* apt: https://github.com/daipom/fluent-package-builder/actions/runs/7164750483

Let me sort out the necessary steps for downgrading while looking at these results.

[daipom]

To downgrade correctly you must:

* Disable td-agent.service alias beforehand.
  
  * It conflict with v4 td-agent.service
  * If you didn't disable it, it causes broken symlink from td-agent.service to /lib/systemd/system/fluentd.service.

* Purge v5 apt-line in fluent-lts-apt-source or fluent-apt-source.

In my conclusion, these are required only for deb.
For rpm, we don't need these steps because symlinks are automatically removed when uninstalling.

[daipom]

Thanks! LGTM!