ci: experiment with codecov "merge context"

.github/workflows/ci.yml

@@ -439,10 +439,10 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4
-        with:
-          # Use the PR head, not the merge commit, because the head commit (and the base)
-          # is what codecov uses to calculate diffs.
-          ref: ${{ github.event.pull_request.head.sha }}
+        # with:
+        #   # Use the PR head, not the merge commit, because the head commit (and the base)
+        #   # is what codecov uses to calculate diffs.
+        #   ref: ${{ github.event.pull_request.head.sha }}
       - uses: actions/setup-python@v5
         with:
           python-version: '3.12'
@@ -454,6 +454,30 @@ jobs:
           components: llvm-tools-preview,rust-src
       - name: Install cargo-llvm-cov
         uses: taiki-e/install-action@cargo-llvm-cov
+      - name: Set PR base on codecov
+        if: ${{ github.event_name == 'pull_request' && matrix.os == 'ubuntu-latest' }}
+        run: |
+          # fetch the merge commit between the PR base and head
+          BASE_REF=refs/heads/${{ github.event.pull_request.base.ref }}
+          MERGE_REF=refs/pull/${{ github.event.pull_request.number }}/merge
+
+          git fetch --progress --depth=1 origin "+$BASE_REF:$BASE_REF" "+$MERGE_REF:$MERGE_REF"
+          while [ -z "$(git merge-base "$BASE_REF" "$MERGE_REF")" ]; do
+            git fetch -q --deepen="10" origin "$BASE_REF" "$MERGE_REF";
+          done
+
+          MERGE_BASE=$(git merge-base "$BASE_REF" "$MERGE_REF")
+          echo "Merge base: $MERGE_BASE"
+
+          # inform codecov about the merge base
+          curl -Os https://cli.codecov.io/latest/linux/codecov
+          sudo chmod +x codecov
+          ./codecov pr-base-picking \
+            --base-sha $MERGE_BASE \
+            --pr ${{ github.event.number }} \
+            --slug PyO3/pyo3 \
+            --token ${{ secrets.CODECOV_TOKEN }} \
+            --service github
       - run: python -m pip install --upgrade pip && pip install nox
       - run: nox -s coverage
       - uses: codecov/codecov-action@v4

codecov.yml

@@ -1,4 +1,4 @@
-comment: off
+comment: false
 
 coverage:
   status: