feat(ci): post a sticky PR comment with the selected tests and jobs

The selector already wrote its selection (which test projects and jobs
will run, and what is skipped) to the job step summary, but that is only
visible by opening the run. Surface it as a sticky PR comment so reviewers
see the selection up front.

SelectTests now also writes the same summary markdown to the file named by
SELECT_TESTS_COMMENT_FILE. tests.yml's setup_for_tests sets that env and,
on pull_request, posts/updates a marker-keyed comment via actions/github-script.
The step is continue-on-error so a read-only token (e.g. a fork PR) never
fails CI. tests.yml is granted pull-requests: write, and ci.yml grants that
scope on the tests job (reusable-workflow tokens are capped by the caller).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: radical

.github/workflows/ci.yml

@@ -73,6 +73,12 @@ jobs:
     uses: ./.github/workflows/tests.yml
     name: Tests
     needs: [prepare_for_ci]
+    # pull-requests: write lets tests.yml post the sticky "selected tests" PR comment; contents: read
+    # keeps the rest at least-privilege. Reusable-workflow tokens are capped by the caller, so this is
+    # where the elevated scope must be granted.
+    permissions:
+      contents: read
+      pull-requests: write
     if: ${{ github.repository_owner == 'microsoft' && needs.prepare_for_ci.outputs.skip_workflow != 'true' }}
     with:
       versionOverrideArg: ${{ needs.prepare_for_ci.outputs.VERSION_SUFFIX_OVERRIDE }}

.github/workflows/tests.yml

@@ -11,6 +11,12 @@ on:
         required: false
         type: string
 
+permissions:
+  contents: read
+  # Needed by the "Comment selected tests on PR" step below to post/update the sticky selection
+  # comment. The token is still capped by the caller (ci.yml grants this on the tests job).
+  pull-requests: write
+
 jobs:
   setup_for_tests:
     name: Setup for tests
@@ -84,6 +90,9 @@ jobs:
           # Absolute path so eng/Build.props ($(BeforeBuildPropsPath)) resolves it regardless of cwd.
           # Under artifacts/ (gitignored) so it is not picked up as a source change.
           BEFORE_BUILD_PROPS: ${{ github.workspace }}/artifacts/BeforeBuildProps.props
+          # Where the selector writes the human-readable selection summary so the step below can post
+          # it as a sticky PR comment (the same content also goes to the job step summary).
+          SELECT_TESTS_COMMENT_FILE: ${{ github.workspace }}/artifacts/select-tests-comment.md
           # The ONE audit/enforce knob. 'false' (audit): no restriction props are written, so the
           # enumerate-tests step below builds the FULL matrix and run_* are all true; the advisory
           # summary still reports what enforcing would have skipped. 'true' (enforce): a non-ALL
@@ -129,6 +138,37 @@ jobs:
           # the run rather than mask the bug behind a silent fallback.
           ./dotnet.sh run --project tools/SelectTests/SelectTests.csproj -- "${args[@]}"
 
+      # Post (or update) a sticky PR comment listing which test projects and jobs the selector chose,
+      # so reviewers see the selection without opening the run's job summary. Informational only -- a
+      # token/permission hiccup (e.g. a fork PR with a read-only token) must never fail CI.
+      - name: Comment selected tests on PR
+        if: ${{ github.event_name == 'pull_request' }}
+        continue-on-error: true
+        uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
+        env:
+          SELECT_TESTS_COMMENT_FILE: ${{ github.workspace }}/artifacts/select-tests-comment.md
+        with:
+          script: |
+            const fs = require('fs');
+            const file = process.env.SELECT_TESTS_COMMENT_FILE;
+            if (!file || !fs.existsSync(file)) {
+              core.info(`No selection summary at ${file}; skipping comment.`);
+              return;
+            }
+            const marker = '<!-- select-tests-comment -->';
+            const body = `${marker}\n${fs.readFileSync(file, 'utf8')}`;
+            const { owner, repo } = context.repo;
+            const issue_number = context.issue.number;
+            const comments = await github.paginate(github.rest.issues.listComments, { owner, repo, issue_number });
+            const existing = comments.find(c => c.body && c.body.includes(marker));
+            if (existing) {
+              await github.rest.issues.updateComment({ owner, repo, comment_id: existing.id, body });
+              core.info(`Updated selection comment ${existing.id}.`);
+            } else {
+              await github.rest.issues.createComment({ owner, repo, issue_number, body });
+              core.info('Created selection comment.');
+            }
+
       # enumerate-tests reuses this job's checkout + restore (checkout/restore=false) so the props
       # file the selector wrote survives -- a fresh checkout would git-clean it away. When a
       # restriction was written, beforeBuildPropsPath points enumerate at the selected subset; in

tools/SelectTests/Program.cs

@@ -461,14 +461,29 @@ static void AppendProjectList(StringBuilder builder, string title, IReadOnlyList
 
         static void WriteOut(StringBuilder builder)
         {
+            var markdown = builder.ToString();
             var summaryPath = Environment.GetEnvironmentVariable("GITHUB_STEP_SUMMARY");
             if (summaryPath is not null)
             {
-                File.AppendAllText(summaryPath, builder.ToString());
+                File.AppendAllText(summaryPath, markdown);
             }
             else
             {
-                Console.Error.Write(builder.ToString());
+                Console.Error.Write(markdown);
+            }
+
+            // Also emit the full markdown to a standalone file when requested, so the workflow can post
+            // it as a sticky PR comment. The job summary alone is not visible without opening the run,
+            // and the selection (which test projects + jobs run) is the thing reviewers want up front.
+            var commentPath = Environment.GetEnvironmentVariable("SELECT_TESTS_COMMENT_FILE");
+            if (!string.IsNullOrEmpty(commentPath))
+            {
+                var dir = Path.GetDirectoryName(Path.GetFullPath(commentPath));
+                if (!string.IsNullOrEmpty(dir))
+                {
+                    Directory.CreateDirectory(dir);
+                }
+                File.WriteAllText(commentPath, markdown);
             }
         }
     }