Improved support for commenting when course timings change. (#1797)

This
* Works with the GitHub permissions model (at least in local testing on
another temporary repo)
 * Only comments when the timings actually change

.github/workflows/course-schedule-comment.yml

@@ -0,0 +1,70 @@
+# Based on https://securitylab.github.com/research/github-actions-preventing-pwn-requests/
+#
+# This runs with the full repo permissions, but checks out the upstream branch, not the PR.
+# Do not run arbitrary code from the PR here!
+name: Comment PR with Course Schedule
+on:
+  workflow_run:
+    workflows: ["Generate Course Schedule"]
+    types: [completed]
+
+jobs:
+  upload:
+    runs-on: ubuntu-latest
+    if: >
+      github.event.workflow_run.event == 'pull_request' &&
+      github.event.workflow_run.conclusion == 'success'
+    steps:
+      - name: "Checkout"
+        uses: actions/checkout@v4
+
+      - name: "Setup Rust cache"
+        uses: ./.github/workflows/setup-rust-cache
+
+      - name: "Generate Schedule on upstream branch"
+        run: |
+          cargo run -p mdbook-course --bin course-schedule > upstream-schedule
+
+      - name: "Download artifact from PR workflow"
+        # actions/download-artifact@v4 cannot do this without being given a PAT, although that
+        # is not required for public forked repositories.
+        uses: actions/github-script@v7.0.1
+        with:
+          script: |
+            var artifacts = await github.rest.actions.listWorkflowRunArtifacts({
+               owner: context.repo.owner,
+               repo: context.repo.repo,
+               run_id: ${{github.event.workflow_run.id }},
+            });
+            var matchArtifact = artifacts.data.artifacts.filter((artifact) => {
+              return artifact.name == "course-schedule"
+            })[0];
+            var download = await github.rest.actions.downloadArtifact({
+               owner: context.repo.owner,
+               repo: context.repo.repo,
+               artifact_id: matchArtifact.id,
+               archive_format: 'zip',
+            });
+            var fs = require('fs');
+            fs.writeFileSync('${{github.workspace}}/course-schedule.zip', Buffer.from(download.data));
+
+      - name: "Unzip artifact"
+        run: unzip course-schedule.zip
+
+      - name: "Comment on PR if schedules differ"
+        uses: actions/github-script@v7.0.1
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          script: |
+            var fs = require('fs');
+            var pr_number = Number(fs.readFileSync('pr-number'));
+            var upstream = fs.readFileSync('upstream-schedule').toString();
+            var schedule = fs.readFileSync('schedule').toString();
+            if (upstream != schedule) {
+              await github.rest.issues.createComment({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                issue_number: pr_number,
+                body: schedule,
+              });
+            }

.github/workflows/course-schedule.yml

@@ -0,0 +1,35 @@
+# Based on https://securitylab.github.com/research/github-actions-preventing-pwn-requests/
+name: Generate Course Schedule
+
+on:
+  pull_request:
+    paths:
+      - "src/**.md"
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Rust cache
+        uses: ./.github/workflows/setup-rust-cache
+
+      - name: Generate Schedule
+        run: |
+          mkdir -p ./course-schedule
+          cargo run -p mdbook-course --bin course-schedule > course-schedule/schedule
+
+          # GitHub does not provide a reliable way to determine the PR number from which
+          # a workflow_run was triggered (https://github.com/orgs/community/discussions/25220),
+          # so we'll do the slightly awkward thing and put that in the artifact. This means
+          # schedules could potentially be spammed to any PR in the repository, but that is
+          # not an awful outcome (and clear, reportable evidence of abuse).
+          echo ${{ github.event.number }} > ./course-schedule/pr-number
+
+      - uses: actions/upload-artifact@v4
+        with:
+          name: course-schedule
+          path: course-schedule/