ci: resolve PR number from head repo/branch for fork PRs

.github/actions/comment-pull-request/action.yml

@@ -0,0 +1,85 @@
+name: Comment on Pull Request
+description: Create or update a comment on a pull request, optionally keyed by a tag
+
+inputs:
+  message:
+    description: Comment body
+    required: false
+  comment-tag:
+    description: A tag embedded in the comment that is used to find and update an existing comment instead of creating duplicates
+    required: false
+  pr-number:
+    description: Number of the pull request to comment on
+    required: false
+    default: ${{ github.event.pull_request.number || github.event.issue.number || github.event.workflow_run.pull_requests[0].number }}
+  github-token:
+    description: GitHub token used to authenticate with the API
+    required: false
+    default: ${{ github.token }}
+
+runs:
+  using: composite
+  steps:
+    - name: Create or update comment
+      uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9
+      env:
+        MESSAGE: ${{ inputs.message }}
+        COMMENT_TAG: ${{ inputs.comment-tag }}
+        PR_NUMBER: ${{ inputs.pr-number }}
+      with:
+        github-token: ${{ inputs.github-token }}
+        script: |
+          const message = process.env.MESSAGE;
+          const commentTag = process.env.COMMENT_TAG || '';
+          const prNumber = Number(process.env.PR_NUMBER);
+
+          if (!prNumber) {
+            core.setFailed('No pull request number provided (pr-number input) and none could be inferred from context.');
+            return;
+          }
+          if (!message) {
+            core.setFailed("The 'message' input must be provided.");
+            return;
+          }
+
+          const { owner, repo } = context.repo;
+
+          // Keep the same marker format as the previously used
+          // thollander/actions-comment-pull-request action so existing comments
+          // are updated instead of duplicated.
+          const tagPattern = commentTag
+            ? `<!-- thollander/actions-comment-pull-request "${commentTag}" -->`
+            : '';
+          const body = tagPattern ? `${message}\n${tagPattern}` : message;
+
+          let commentId;
+          if (tagPattern) {
+            const comments = await github.paginate(github.rest.issues.listComments, {
+              owner,
+              repo,
+              issue_number: prNumber,
+              per_page: 100,
+            });
+            const existing = comments.find((comment) => comment.body.includes(tagPattern));
+            if (existing) {
+              commentId = existing.id;
+            }
+          }
+
+          if (commentId) {
+            core.info(`Updating comment ${commentId} on PR #${prNumber}`);
+            await github.rest.issues.updateComment({
+              owner,
+              repo,
+              comment_id: commentId,
+              body,
+            });
+          } else {
+            core.info(`Creating comment on PR #${prNumber}`);
+            await github.rest.issues.createComment({
+              owner,
+              repo,
+              issue_number: prNumber,
+              body,
+            });
+          }

.github/workflows/leave-comment.yml

@@ -40,10 +40,26 @@ jobs:
             echo "EOF"
           } >> "$GITHUB_OUTPUT"
 
-      - name: Add Comment to PR
+      # `github.event.workflow_run.pull_requests` is only populated for PRs from
+      # branches in this repository; for fork PRs it is empty, so we resolve the
+      # PR number from the head repository and branch instead.
+      - name: Resolve PR number
+        id: pr
         if: steps.combine.outputs.combined
-        uses: thollander/actions-comment-pull-request@24bffb9b452ba05a4f3f77933840a6a841d1b32b # v3.0.1
+        env:
+          GH_TOKEN: ${{ github.token }}
+          HEAD_REPO: ${{ github.event.workflow_run.head_repository.login }}
+          HEAD_BRANCH: ${{ github.event.workflow_run.head_branch }}
+        run: |
+          PR_NUMBER=$(gh api \
+            "repos/${GITHUB_REPOSITORY}/pulls?head=${HEAD_REPO}:${HEAD_BRANCH}" \
+            --jq '[.[] | select(.state == "open")][0].number // .[0].number // empty')
+          echo "number=${PR_NUMBER:-}" >> "$GITHUB_OUTPUT"
+
+      - name: Add Comment to PR
+        if: steps.combine.outputs.combined && steps.pr.outputs.number
+        uses: ./.github/actions/comment-pull-request
         with:
           comment-tag: compared
           message: ${{ steps.combine.outputs.combined }}
-          pr-number: ${{ github.event.workflow_run.pull_requests[0].number }}
+          pr-number: ${{ steps.pr.outputs.number }}