[libc++] Properly extract the PR head and base from the Github event

.github/workflows/libcxx-run-benchmarks.yml

@@ -21,9 +21,6 @@ on:
 env:
   CC: clang-22
   CXX: clang++-22
-  COMMENT_BODY: ${{ github.event.comment.body }}
-  PULL_REQUEST_HEAD: ${{ github.event.issue.pull_request.head.sha }}
-  PULL_REQUEST_BASE: ${{ github.event.issue.pull_request.base.sha }}
 
 jobs:
   run-benchmarks:
@@ -33,12 +30,6 @@ jobs:
 
     runs-on: llvm-premerge-libcxx-next-runners # TODO: This should run on a dedicated set of machines
     steps:
-      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
-        with:
-          ref: ${PULL_REQUEST_HEAD}
-          fetch-depth: 0
-          fetch-tags: true # This job requires access to all the Git branches so it can diff against (usually) main
-
       - uses: actions/setup-python@v6
         with:
           python-version: '3.10'
@@ -48,18 +39,42 @@ jobs:
           python3 -m venv .venv
           source .venv/bin/activate
           python -m pip install -r libcxx/utils/requirements.txt
+          python -m pip install pygithub
+
+      - name: Extract information from the PR
+        id: vars
+        run: |
+          source .venv/bin/activate
+          cat <<EOF | python >> ${GITHUB_OUTPUT}
+          import github
+          repo = github.Github("${{ github.token }}").get_repo("${{ github.repository }}")
+          pr = repo.get_pull(${{ github.event.issue.number }})
+          print(f"pr_base={pr.base.sha}")
+          print(f"pr_head={pr.head.sha}")
+          EOF
+          BENCHMARKS=$(echo "${{ github.event.comment.body }}" | sed -nE 's/\/libcxx-bot benchmark (.+)/\1/p')
+          echo "benchmarks=${BENCHMARKS}" >> ${GITHUB_OUTPUT}
+
+      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
+        with:
+          ref: ${{ steps.vars.outputs.pr_head }}
+          fetch-depth: 0
+          fetch-tags: true # This job requires access to all the Git branches so it can diff against (usually) main
+          path: repo # Avoid nuking the workspace, where we have the Python virtualenv
 
       - name: Run baseline
         run: |
-          BENCHMARKS=$(echo "${COMMENT_BODY}" | sed -nE 's/\/libcxx-bot benchmark (.+)/\1/p')
-          baseline_commit=$(git merge-base ${PULL_REQUEST_BASE} ${PULL_REQUEST_SHA})
-          ./libcxx/utils/test-at-commit --commit ${baseline_commit} -B build/baseline -- -sv -j1 --param optimization=speed ${BENCHMARKS}
+          source .venv/bin/activate
+          baseline_commit=$(git -C repo merge-base ${{ steps.vars.outputs.pr_base }} ${{ steps.vars.outputs.pr_head }})
+          ./repo/libcxx/utils/test-at-commit --git-repo repo --commit ${baseline_commit} -B build/baseline -- -sv -j1 --param optimization=speed ${{ steps.vars.outputs.benchmarks }}
 
       - name: Run candidate
         run: |
-          BENCHMARKS=$(echo "${COMMENT_BODY}" | sed -nE 's/\/libcxx-bot benchmark (.+)/\1/p')
-          ./libcxx/utils/test-at-commit --commit ${PULL_REQUEST_SHA} -B build/candidate -- -sv -j1 --param optimization=speed ${BENCHMARKS}
+          source .venv/bin/activate
+          ./repo/libcxx/utils/test-at-commit --git-repo repo --commit ${{ steps.vars.outputs.pr_head }} -B build/candidate -- -sv -j1 --param optimization=speed ${{ steps.vars.outputs.benchmarks }}
 
       - name: Compare baseline and candidate runs
-        run: ./libcxx/utils/compare-benchmarks <(./libcxx/utils/consolidate-benchmarks build/baseline) \
-                                               <(./libcxx/utils/consolidate-benchmarks build/candidate)
+        run: |
+          source .venv/bin/activate
+          ./repo/libcxx/utils/compare-benchmarks <(./repo/libcxx/utils/consolidate-benchmarks build/baseline) \
+                                                 <(./repo/libcxx/utils/consolidate-benchmarks build/candidate)