Unexpected failure in GitHub Action run

In a recent pull request (https://github.com/InnerSourceCommons/InnerSourcePatterns/pull/579), one of our GitHub Action (GHA) workflow runs failed.

Overview of failed GHAs:

Now when looking at the details of that workflow run, I noticed something interesting:

• when you click on "workflow file", it takes you to the book.yml file from this branch/PR (which comes from a fork)
• however the file that really got executed is actually the one on the main branch of the upstream. I am so sure about this as the main branch contains the config for a matrix build for this GHA, while the branch from the PR does not have that matrix build yet)

So at the very least I am starting to wonder how GHAs really work. Seems like I might have some misunderstandings here.

My questions are:

• which GHAs are executed? the ones on the main branch, or the ones on the PR branch, or both?
• main and PR contain different versions of the same GHA. what happens in that case?

And lastly:
Why does this GHA run fail at all?

It seems like it is looking for a git ref hive-mind-pattern-1 and cannot find it? Possibly because that ref only exists on the fork but not on the upstream?

Do we have to configure the actions/checkout differently, so that it can find that git ref?
Right now we use ref: ${{ github.head_ref }}, which is defined as:

The head_ref or source branch of the pull request in a workflow run. This property is only available when the event that triggers a workflow run is either pull_request or pull_request_target.

I am suspecting that we are checking out the upstream repo InnerSourceCommons/InnerSourcePatterns but then trying to point to a ref that does not exist on that upsream.

Just not sure how one would configure this correctly?

Beginne mit .github/workflows/book.yml auf main und der Version im Pull Request #579. Untersuche anschließend den Workflow-Lauf 6003953262 und den Schritt actions/checkout, der github.head_ref verwendet. Vergleiche den Event-Kontext, das ausgecheckte Repository und die verfügbaren refs, um den Fehler zu erklären. Erledigt ist die Aufgabe, wenn dokumentiert ist, welche Workflow-Version und welcher ref verwendet werden, und die für eine erfolgreiche Ausführung des PR-Workflows erforderliche Konfiguration identifiziert wurde.