## Why
PR CI should test the exact commit that was pushed to the PR branch. By
default, GitHub's `pull_request` event checks out a synthetic merge
commit from `refs/pull/<number>/merge`, so the tested tree can include
an implicit merge with the current base branch instead of matching the
pushed head SHA.
Using the PR head SHA makes each check result correspond to a concrete
commit the author submitted. This also behaves better for stacked PR
workflows, including Sapling stacks and other Git stack tooling: a
middle PR's head commit already contains the lower stack changes in its
tree, without pulling in commits above it or GitHub's temporary merge
ref.
## What Changed
- Set every `actions/checkout` in `pull_request` workflows under
`.github/workflows` to use `github.event.pull_request.head.sha` on PR
events and `github.sha` otherwise.
- Updated `blob-size-policy` to compare
`github.event.pull_request.base.sha` and
`github.event.pull_request.head.sha`, since it no longer checks out
GitHub's merge commit where `HEAD^1`/`HEAD^2` represented the PR range.
## Verification
- Parsed the edited workflow YAML files with Ruby.
- Checked that every checkout block in the `pull_request` workflows has
the PR-head `ref`.
This does two things:
- We use `persist-credentials: false` everywhere now. This is
unfortunately not the default in GitHub Actions, but it prevents
`actions/checkout` from dropping `secrets.GITHUB_TOKEN` onto disk.
- We interpose (some) template expansions through environment variables.
I've limited this to contexts that have non-fixed values; contexts that
are fixed (like `*.result`) are not dangerous to expand directly inline
(but maybe we should clean those up in the future for consistency
anyways).
This is a medium-risk change in terms of CI breakage: I did a scan for
usage of `git push` and other commands that implicitly use the persisted
credential, but couldn't find any. Even still, some implicit usages of
the persisted credentials may be lurking. Please ping ww@ if any issues
arise.
This builds on top of https://github.com/openai/codex/pull/15828 by
ensuring that hash-pinned actions with version comments are fully
qualified, rather than referencing floating/mutable comments like "v7".
This makes actions management tools behave more consistently.
This shouldn't break anything, since it's comment only. But if it does,
ping ww@ 🙂
