Validate supplied branch and tag exist for RN gen

[donoghuc]

Release notes

[rn:skip]

What does this PR do?

Add a validation step to the action for generating release notes. Without input validation tags/branches that do not exist (due to typo, errors in workflow inputs) can go un-noticed with PRs that have empty commit ranges.

[github-actions]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)
• /run exhaustive tests : Run the exhaustive tests Buildkite pipeline.

[mergify]

This pull request does not have a backport label. Could you fix it @donoghuc? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• If no backport is necessary, please add the backport-skip label

[v1v]

nit: if you want to use the more native github approach, although https://github.com/actions/checkout#fetch-all-history-for-all-tags-and-branches.

but I think

 # Whether to fetch tags, even if fetch-depth > 0.
 # Default: false
 fetch-tags: ''

Suggested change

	fetch-depth: 0
	- name: Validate the supplied target branch and last release tag exist
	run: |
	git fetch --tags
	fetch-depth: 0
	fetch-tags: true
	- name: Validate the supplied target branch and last release tag exist
	run: |

[donoghuc]

ah, right. Good call.

[v1v]

Use intermediate env variables for inputs as explained in the security hardening:

• https://docs.github.com/en/actions/reference/security/secure-use#use-an-intermediate-environment-variable

Suggested change

	if ! git rev-parse --verify "origin/${{ github.event.inputs.branch }}" >/dev/null 2>&1; then
	echo "Error: Branch ${{ github.event.inputs.branch }} does not exist"
	exit 1
	fi
	if ! git rev-parse --verify "v${{ github.event.inputs.last_release }}" >/dev/null 2>&1; then
	echo "Error: Tag v${{ github.event.inputs.last_release }} does not exist"
	exit 1
	fi
	if ! git rev-parse --verify "origin/${BRANCH}" >/dev/null 2>&1; then
	echo "Error: Branch ${BRANCH} does not exist"
	exit 1
	fi
	if ! git rev-parse --verify "v${LAST_RELEASE}" >/dev/null 2>&1; then
	echo "Error: Tag v${LAST_RELEASE} does not exist"
	exit 1
	fi
	env:
	LAST_RELEASE: ${{ inputs.last_release }}
	BRANCH: ${{ inputs.branch }}

Maybe these env variables should be created a the job-level so it can be used in other steps

[donoghuc]

Updated with your last thought (ENV vars at job level).

[donoghuc]

Run showing validation is working (fails when tag does not exist): https://github.com/elastic/logstash/actions/runs/20114283936
Run showing happy path works: https://github.com/elastic/logstash/actions/runs/20114309562