Merge pull request #514 from omacom/empty-pr-fails

Fail the PR check when the diff against base is empty

.github/workflows/build-pr.yml

@@ -36,6 +36,7 @@ jobs:
       matrix: ${{ steps.list.outputs.matrix }}
       count: ${{ steps.gate.outputs.count }}
       trusted: ${{ steps.gate.outputs.trusted }}
+      empty: ${{ steps.list.outputs.empty }}
     steps:
       # Same rule as the build job: bin/build-matrix comes from base, the
       # package directories from the PR head.
@@ -78,6 +79,17 @@ jobs:
           echo "matrix=$matrix" >> "$GITHUB_OUTPUT"
           echo "planned=$(jq '.include | length' <<<"$matrix")" >> "$GITHUB_OUTPUT"
           jq -r '.include[] | "\(.package) \(.arch) -> \(.channels)"' <<<"$matrix"
+          # A PR whose diff against its base is empty changes nothing: its
+          # content already landed some other way (a sync PR beat it, or a
+          # merge from master swallowed it). Merging it would record a change
+          # that isn't one. Flag it so `result` fails rather than passes.
+          if [[ "${{ github.event_name }}" == pull_request ]]; then
+            total=$(git diff --name-only "${{ github.event.pull_request.base.sha }}" "${{ github.event.pull_request.head.sha }}" | wc -l)
+            echo "empty=$([[ $total -eq 0 ]] && echo true || echo false)" >> "$GITHUB_OUTPUT"
+            echo "files changed vs base: $total"
+          else
+            echo "empty=false" >> "$GITHUB_OUTPUT"
+          fi
       - id: gate
         env:
           STATUS: ${{ steps.vouch.outputs.status || 'dispatch' }}
@@ -170,4 +182,10 @@ jobs:
             echo "::error::Builds were not run: author is not vouched. Add to .github/VOUCHED.td or apply the 'build-approved' label."
             exit 1
           fi
+          # Nothing to merge: the PR's diff against its base is empty. Its
+          # change already landed elsewhere. Close it rather than merge it.
+          if [[ "${{ needs.changes.outputs.empty }}" == "true" ]]; then
+            echo "::error::This PR changes no files relative to its base. Its content is already on the target branch; close it instead of merging."
+            exit 1
+          fi
           [[ "${{ needs.build.result }}" == "success" || "${{ needs.build.result }}" == "skipped" ]]