Fail a drift only on the pull request that owns the vendored copy

Author: estebanzimanyi

.github/workflows/vendor-drift.yml

@@ -42,6 +42,25 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+        with:
+          # `git diff` against the base needs the history the two share.
+          fetch-depth: 0
+
+      # A pull request that changes the vendored copy, or the target that
+      # writes it, owns keeping it current. One that changes only this
+      # workflow does not, and failing it for a staleness it did not cause
+      # would block the very change that stops the staleness failing.
+      - name: Note whether this pull request owns the vendored copy
+        id: owns
+        if: github.event_name == 'pull_request'
+        run: |
+          git fetch --no-tags origin "${{ github.base_ref }}"
+          if git diff --name-only FETCH_HEAD...HEAD -- vendor/meos-api Makefile | grep -q .; then
+            echo "owns=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "owns=false" >> "$GITHUB_OUTPUT"
+            echo "::notice::This pull request changes neither vendor/meos-api nor the Makefile, so a drift is reported rather than failed."
+          fi
 
       # libclang (the Python wheel) bundles the .so but not the system C
       # headers MEOS depends on (json-c, gsl, proj, postgres). Without them,
@@ -60,8 +79,8 @@ jobs:
       - name: Refresh vendored MEOS-API artefacts from master
         run: make vendor-meos-api
 
-      # A pull request touching the vendored copy or the target that writes it
-      # must not leave it stale, and its author is there to act, so there the
+      # A pull request that changes the vendored copy or the target that writes
+      # it must not leave it stale, and its author is there to act, so there the
       # drift is an error.
       #
       # Nobody is there to act on a schedule or on a push already made, and
@@ -78,7 +97,7 @@ jobs:
           else
             echo "drift=true" >> "$GITHUB_OUTPUT"
             git diff --stat -- vendor/meos-api/
-            if [ "${{ github.event_name }}" = "pull_request" ]; then
+            if [ "${{ steps.owns.outputs.owns }}" = "true" ]; then
               echo "::error::vendor/meos-api/ is stale. Run \`make vendor-meos-api\` locally and include the result."
               exit 1
             fi