Add backport action

[gareth-ellis]

Original idea:

This adds a backport action that should do the following two things:

• Backport any PR to current latest branch (currently 8.15) that we tag with backport. We potentially could remove the requirement to add a label too - so absolutely everything is backported to 8.15
• Backport to other branches (e.g 8.13) by adding a label of v8.13.

The backport will work by creating a new PR(or PRs) against the relevant branch(es).

---

New simplified setup: #599 (comment)

[gbanasiak]

I left some comments.

I would add targetPRLabels option with a label that will allow us to find or filter out backport PRs quickly. We currently have backport label as a trigger. We could add backported label to indicate the PR was created by backport tool. For instance https://github.com/elastic/kibana is using auto-backport / backport label pair for this purpose.

[gbanasiak]

I did some testing, and it looks like PR labels are converted to branch list based on branchLabelMapping which is filtered through targetBranchChoices which in turn gets overridden by targetBranches. So current configuration will always backport to 8.15 regardless of the labels.

I think we need to choose between the following:

• configure targetBranches to the current latest branch which is closest to what we had earlier,
• configure branchLabelMapping and label each time (more work but more flexible).

WDYT?

[gareth-ellis]

hmm, I wonder if we can find a way to configure both - ideally I would like:

• (either tagged with backport or untagged, we can decide) -> backported to latest (currently 8.13)
• If we have a vX.XX tag, then we ALSO backport to those branches. Let me see if I can find a config that will allow that

[gareth-ellis]

@elasticmachine update branch

[gbanasiak]

@elasticmachine update branch

[gbanasiak]

We had quite a few backports recently so I think it's time to revisit this one. I've simplified setup in cf7d836 based on https://github.com/sorenlouv/backport-github-action documentation. The intended behavior is as follows:

• backport is triggered by backport-to-... label, e.g. backport-to-8.15,
• there is no automatic backport to the most recent non-master branch, see earlier testing in Add backport action #599 (comment),
• the backport PR is labeled with backport label.

[favilo]

This is really neat. Refactoring might be harder to ballet sometimes, but this i feel would make it actually approachable

[github-actions]

💚 All backports created successfully

Status	Branch	Result
✅	8.15

Note: Successful backport PRs will be merged automatically after passing CI.

Questions ?

Please refer to the Backport tool documentation and see the Github Action logs for details