Merge feat/go-opaque-storage-profile into feat/json-tree-data-plan

Brings in the mutants gate fix from #1381, which reads the pull request's real base from the API and checks out the head, so the gate no longer diffs all of next and times out on this stack.

Claude-Session: https://claude.ai/code/session_01N7bCqa3ZVJMcEE99RzVqEu

Author: coderdan

.github/workflows/mutants.yml

@@ -73,6 +73,12 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
         with:
+          # On a pull request, the PR's head rather than GitHub's test merge
+          # commit: cargo-mutants applies `pr.diff` to the checked-out tree
+          # and refuses a hunk whose lines do not match it, and the diff
+          # below is taken between the base and this head. Empty on a
+          # manual dispatch, which checks out the default ref.
+          ref: ${{ github.event.pull_request.head.sha || '' }}
           # Full history, to diff the PR against its base for `--in-diff`.
           fetch-depth: 0
           persist-credentials: false
@@ -91,15 +97,34 @@ jobs:
 
       # PR only: a manual dispatch has no base to diff against, so it sweeps
       # both crates instead (see the next step).
+      #
+      # The base comes from the API at run time, not from the event payload.
+      # For a pull request whose base branch is itself the head of another
+      # open pull request (a stacked PR, or any PR into `next` while `next`
+      # has a PR into `main`), GitHub has been seen to build the test merge
+      # commit against the other PR's merge commit and to report the other
+      # PR's base in `github.base_ref`. The diff was then everything between
+      # `main` and `next`, and the gate ran for two hours on a Go-only
+      # change. `gh pr view` returns the base the PR page shows. The diff is
+      # taken between that base and the PR's head, which is what the checkout
+      # step put in the tree, so the hunks' line numbers are the tree's.
       - name: Compute the PR diff
         if: github.event_name == 'pull_request'
         env:
-          BASE_REF: ${{ github.base_ref }}
+          GH_TOKEN: ${{ github.token }}
+          PR_NUMBER: ${{ github.event.pull_request.number }}
+          HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+          PAYLOAD_BASE_REF: ${{ github.base_ref }}
         run: |
-          git fetch --no-tags origin "$BASE_REF"
-          git diff "origin/$BASE_REF" > pr.diff
-          echo "Changed lines under mutation:"
-          cat pr.diff
+          base=$(gh pr view "$PR_NUMBER" --repo "$GITHUB_REPOSITORY" --json baseRefName,baseRefOid --jq '"\(.baseRefName) \(.baseRefOid)"')
+          base_ref=${base% *}
+          base_sha=${base#* }
+          echo "Base: $base_ref at $base_sha (the event payload said '$PAYLOAD_BASE_REF'); head: $HEAD_SHA"
+          git fetch --no-tags origin "$base_sha"
+          test "$(git rev-parse HEAD)" = "$HEAD_SHA" || { echo "the checkout is not the PR head"; exit 1; }
+          git diff "$base_sha...HEAD" > pr.diff
+          echo "Files under mutation:"
+          grep '^diff --git' pr.diff | sed 's|^diff --git a/.* b/|  |' || echo "  (none)"
 
       # Never fails the job: the enforce step below is the gate, and the
       # comment must be posted first.