Factor the Slack notification into a local composite action

[albertvillanova]

This PR replaces the huggingface/hf-workflows/.github/actions/post-slack action with a local composite action, kept as a faithful copy of the shared one.

Related to #6890, which it does not resolve: the copied action still calls the same slackapi/slack-github-action version, so the Node.js 20 is deprecated annotation still appears.

Motivation

Fixing #6890 requires moving slackapi/slack-github-action from v1 to a version that declares the Node 24 runtime, which is a breaking change in the way the action is called. Applying that change to the shared hf-workflows action would affect around 135 call sites across roughly 15 repositories in the organization, and pinning does not protect them: Dependabot bumps SHA references that track a branch, as it has been doing for our huggingface/doc-builder references.

Owning the action locally lets us make that change for TRL alone, on our own schedule, without touching any other project.

Keeping it factored, rather than inlining the Slack call into each workflow, avoids duplicating the message payload thirteen times.

Solution

Add .github/actions/post-slack/action.yml as a copy of the shared action at 50acccfe9afb8927b9b74ed7e750f2b0d3991b62, the SHA the workflows were pinned to, and point the call sites at it with uses: ./.github/actions/post-slack.

The copy is byte identical to the shared action except for the # v1.25.0 comment added to the pinned slackapi/slack-github-action reference, following the convention used for every other pinned action in our workflows. Inputs, message payload and dependency versions are all unchanged, so this PR is behavior neutral.

A local action is loaded from the workspace, so it requires the repository to be checked out beforehand, and the checked out tree must be the one containing the action. That holds for twelve of the thirteen call sites: each of their jobs checks out the ref the workflow runs on, before posting to Slack.

It does not hold for tests_latest.yml, which checks out v1.10-release on purpose, since it tests the latest release. That branch predates this action, and every future release branch will too, being cut before the change that adds it. So that job checks the action out from the branch running the workflow, into a separate path:

- name: Check out the Slack action
  if: always()
  uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1  # v7.0.1
  with:
    path: slack-action
    sparse-checkout: .github/actions

The notification is CI infrastructure rather than part of the release under test, so taking it from the workflow ref is also the right thing semantically. The checkout is sparse, so it fetches .github/actions rather than a second full clone.

Its if: always() matters and is commented in the workflow: #6900 gives the Slack step of that job the same condition, and if this checkout were left on the default if: success() it would be skipped exactly when the tests fail, leaving the Slack step unable to resolve the action. Both merge orders are safe, since a checkout on always() with a Slack step on success() still works.

Changes

• Add .github/actions/post-slack/action.yml, copied from the shared action, with the same four inputs and the same pinned slackapi/slack-github-action version
• Point the thirteen Post to Slack steps in docker-build.yml, slow-tests.yml, tests.yml, tests_latest.yml, tests_python_versions.yml and tests_transformers_branch.yml at the local action
• Check the action out from the workflow ref in tests_latest.yml, whose job checks out the release branch

---

Note

Low Risk
CI-only refactor with behavior-neutral Slack messaging; the tests_latest checkout ordering is the main operational edge case and is explicitly guarded with if: always().

Overview
Replaces the org-wide huggingface/hf-workflows post-slack action with a local composite action at .github/actions/post-slack, copied from the previously pinned shared SHA so Slack payloads and inputs stay the same.

Thirteen “Post to Slack” steps across docker-build.yml, slow-tests.yml, tests.yml, tests_latest.yml, tests_python_versions.yml, and tests_transformers_branch.yml now use uses: ./.github/actions/post-slack instead of the external action—keeping notifications centralized without inlining Block Kit in every workflow.

tests_latest.yml is special: the job checks out a release branch that won’t contain the new action, so it adds an always() sparse checkout of .github/actions from the workflow ref into slack-action/ and calls ./slack-action/.github/actions/post-slack, so failure notifications still resolve the action.

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[qgallouedec]

thanks, looks good; checked the local copy is identical to the shared action at the pinned SHA. The v1.10-release special case in tests_latest is handled correctly with the separate checkout path

[qgallouedec]

out of the scope, but mentioning it: it would be easier if everything were in the same channel: this is the only action pointing to its own channel

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