workflows/get-merge-commit: return targetSha

We can fetch the targetSha directly with the mergedSha from the API.
This avoids a checkout with fetch-depth: 2 for a small performance
improvement.

.github/workflows/eval.yml

@@ -24,23 +24,13 @@ jobs:
     runs-on: ubuntu-24.04-arm
     needs: get-merge-commit
     if: needs.get-merge-commit.outputs.mergedSha
-    outputs:
-      targetSha: ${{ steps.targetSha.outputs.targetSha }}
     steps:
       - name: Check out the PR at the test merge commit
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
           ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
-          fetch-depth: 2
           path: nixpkgs
 
-      - name: Determine target commit
-        if: github.event_name == 'pull_request_target'
-        id: targetSha
-        run: |
-          targetSha=$(git -C nixpkgs rev-parse HEAD^1)
-          echo "targetSha=$targetSha" >> "$GITHUB_OUTPUT"
-
       - name: Install Nix
         uses: cachix/install-nix-action@526118121621777ccd86f79b04685a9319637641 # v31
         with:
@@ -143,7 +133,7 @@ jobs:
           path: prResult/*
 
       - name: Get target run id
-        if: needs.attrs.outputs.targetSha
+        if: needs.get-merge-commit.outputs.targetSha
         id: targetRunId
         run: |
           # Get the latest eval.yml workflow run for the PR's target commit
@@ -172,7 +162,7 @@ jobs:
           echo "targetRunId=$runId" >> "$GITHUB_OUTPUT"
         env:
           REPOSITORY: ${{ github.repository }}
-          TARGET_SHA: ${{ needs.attrs.outputs.targetSha }}
+          TARGET_SHA: ${{ needs.get-merge-commit.outputs.targetSha }}
           GH_TOKEN: ${{ github.token }}
 
       - uses: actions/download-artifact@v4
@@ -186,8 +176,8 @@ jobs:
       - name: Compare against the target branch
         if: steps.targetRunId.outputs.targetRunId
         run: |
-          git -C nixpkgs worktree add ../target ${{ needs.attrs.outputs.targetSha }}
-          git -C nixpkgs diff --name-only ${{ needs.attrs.outputs.targetSha }} \
+          git -C nixpkgs worktree add ../target ${{ needs.get-merge-commit.outputs.targetSha }}
+          git -C nixpkgs diff --name-only ${{ needs.get-merge-commit.outputs.targetSha }} \
             | jq --raw-input --slurp 'split("\n")[:-1]' > touched-files.json
 
           # Use the target branch to get accurate maintainer info
@@ -241,7 +231,7 @@ jobs:
       - name: Check out Nixpkgs at the base commit
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          ref: ${{ needs.attrs.outputs.targetSha }}
+          ref: ${{ needs.get-merge-commit.outputs.targetSha }}
           path: base
           sparse-checkout: ci
 

.github/workflows/get-merge-commit.yml

@@ -6,6 +6,9 @@ on:
       mergedSha:
         description: "The merge commit SHA"
         value: ${{ jobs.resolve-merge-commit.outputs.mergedSha }}
+      targetSha:
+        description: "The target commit SHA"
+        value: ${{ jobs.resolve-merge-commit.outputs.targetSha }}
       systems:
         description: "The supported systems"
         value: ${{ jobs.resolve-merge-commit.outputs.systems }}
@@ -17,6 +20,7 @@ jobs:
     runs-on: ubuntu-24.04-arm
     outputs:
       mergedSha: ${{ steps.merged.outputs.mergedSha }}
+      targetSha: ${{ steps.merged.outputs.targetSha }}
       systems: ${{ steps.systems.outputs.systems }}
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
@@ -35,9 +39,9 @@ jobs:
               echo "mergedSha=${{ github.sha }}" >> "$GITHUB_OUTPUT"
               ;;
             pull_request_target)
-              if mergedSha=$(base/ci/get-merge-commit.sh ${{ github.repository }} ${{ github.event.number }}); then
-                echo "Checking the merge commit $mergedSha"
-                echo "mergedSha=$mergedSha" >> "$GITHUB_OUTPUT"
+              if commits=$(base/ci/get-merge-commit.sh ${{ github.repository }} ${{ github.event.number }}); then
+                echo "Checking the commits:\n$commits"
+                echo "$commits" >> "$GITHUB_OUTPUT"
               else
                 # Skipping so that no notifications are sent
                 echo "Skipping the rest..."

ci/README.md

@@ -44,14 +44,14 @@ Why not just build the tooling right from the PRs Nixpkgs version?
 ## `get-merge-commit.sh GITHUB_REPO PR_NUMBER`
 
 Check whether a PR is mergeable and return the test merge commit as
-[computed by GitHub](https://docs.github.com/en/rest/guides/using-the-rest-api-to-interact-with-your-git-database?apiVersion=2022-11-28#checking-mergeability-of-pull-requests).
+[computed by GitHub](https://docs.github.com/en/rest/guides/using-the-rest-api-to-interact-with-your-git-database?apiVersion=2022-11-28#checking-mergeability-of-pull-requests) and its parent.
 
 Arguments:
 - `GITHUB_REPO`: The repository of the PR, e.g. `NixOS/nixpkgs`
 - `PR_NUMBER`: The PR number, e.g. `1234`
 
 Exit codes:
-- 0: The PR can be merged, the test merge commit hash is returned on stdout
+- 0: The PR can be merged, the hashes of the test merge commit and the target commit are returned on stdout
 - 1: The PR cannot be merged because it's not open anymore
 - 2: The PR cannot be merged because it has a merge conflict
 - 3: The merge commit isn't being computed, GitHub is likely having internal issues, unknown if the PR is mergeable

ci/get-merge-commit.sh

@@ -55,7 +55,10 @@ done
 
 if [[ "$mergeable" == "true" ]]; then
     log "The PR can be merged"
-    jq -r .merge_commit_sha <<< "$prInfo"
+    mergedSha="$(jq -r .merge_commit_sha <<< "$prInfo")"
+    echo "mergedSha=$mergedSha"
+    targetSha="$(gh api "/repos/$repo/commits/$mergedSha" --jq '.parents[0].sha')"
+    echo "targetSha=$targetSha"
 else
     log "The PR has a merge conflict"
     exit 2