feat(comment-commands): auto suggest reviewers on pr open/update via git blame

.github/workflows/comment-commands.yml

@@ -24,9 +24,18 @@
 #
 # On pull requests, the author can request or cancel reviewer requests
 # via `/request-review @user [@user ...]` and `/unrequest-review @user
-# [@user ...]`. We avoid the `/review` namespace so it stays free for
+# [@user ...]`. We avoid the `/review` namespace so it stays free for 
 # future use (e.g. self-review).
 #
+# On PR open/update, the `suggest-reviewers` job automatically runs
+# `git blame` on every changed file and posts (or edits) a single
+# comment that splits candidates into two buckets:
+#   • Committers — collaborators who can be formally review-requested.
+#     The author uses `/request-review @login` to act on these.
+#   • Non-committer contributors — have context but cannot be review-
+#     requested; the author can @-mention them to notify.
+# The CI never sends a review request on its own.
+#
 # Sub-issue linking can be driven from either end of the relationship:
 # `/sub-issue #N [#M ...]` on a parent links those issues as children;
 # `/parent-issue #N` on a child sets #N as its parent. Unlinking mirrors
@@ -38,12 +47,171 @@ name: Comment commands
 on:
   issue_comment:
     types: [created]
+  pull_request_target:
+    types: [opened, synchronize, reopened]
 
 permissions:
+  contents: read
   issues: write
   pull-requests: write
 
 jobs:
+  suggest-reviewers:
+    if: github.event_name == 'pull_request'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+        with:
+          fetch-depth: 0
+      - uses: actions/github-script@v8
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          script: |
+            const { execFileSync } = require('node:child_process');
+            const pull_number = context.payload.pull_request.number;
+            const author = context.payload.pull_request.user.login;
+            const { owner, repo } = context.repo;
+
+            const { data: pull } = await github.rest.pulls.get({ owner, repo, pull_number });
+
+            try {
+              execFileSync('git', ['fetch', 'origin', pull.base.ref], { encoding: 'utf8' });
+            } catch (e) {
+              core.warning(`git fetch for base ref ${pull.base.ref} failed: ${e.message}`);
+            }
+
+            const files = await github.paginate(github.rest.pulls.listFiles, {
+              owner, repo, pull_number, per_page: 100,
+            });
+
+            // Parse `git blame -p` output to find the most-recent commit per file.
+            function latestBlameCommit(blameOutput) {
+              let latest = null;
+              let current = null;
+
+              function finalizeCurrent() {
+                if (!current || current.authorTime == null) return;
+                if (!latest || current.authorTime > latest.authorTime) latest = current;
+              }
+
+              for (const line of blameOutput.split(/\r?\n/)) {
+                const header = line.match(/^([0-9a-f^]+)\s+\d+\s+\d+\s+\d+$/);
+                if (header) {
+                  finalizeCurrent();
+                  current = { sha: header[1].replace(/^\^/, ''), authorTime: null };
+                  continue;
+                }
+                const authorTime = line.match(/^author-time\s+(\d+)$/);
+                if (authorTime && current) current.authorTime = Number(authorTime[1]);
+              }
+
+              finalizeCurrent();
+              return latest;
+            }
+
+            // Count changed files touched per login; track collaborator status.
+            const committerCounts = new Map();    // collaborators
+            const nonCommitterCounts = new Map(); // non-collaborators with a GitHub login
+
+            for (const { filename, status, previous_filename } of files) {
+              if (status === 'removed' || status === 'added') continue;
+              const blamePath = status === 'renamed' ? previous_filename : filename;
+
+              let blameOutput;
+              try {
+                blameOutput = execFileSync(
+                  'git', ['blame', '-p', pull.base.sha, '--', blamePath],
+                  { encoding: 'utf8' },
+                );
+              } catch (e) {
+                core.warning(`git blame on ${filename} failed: ${e.message}`);
+                continue;
+              }
+
+              const latest = latestBlameCommit(blameOutput);
+              if (!latest) continue;
+
+              let commit;
+              try {
+                ({ data: commit } = await github.rest.repos.getCommit({ owner, repo, ref: latest.sha }));
+              } catch (e) {
+                core.warning(`Commit lookup for ${latest.sha} failed: ${e.message}`);
+                continue;
+              }
+
+              const login = commit.author?.login ?? commit.committer?.login;
+              if (!login) continue;
+              if (login.toLowerCase() === author.toLowerCase()) continue;
+
+              const loginSource = commit.author?.login ? commit.author : commit.committer;
+              if (loginSource?.type === 'Bot') continue;
+
+              let isCollaborator = false;
+              try {
+                await github.rest.repos.checkCollaborator({ owner, repo, username: login });
+                isCollaborator = true;
+              } catch (_) { /* not a collaborator */ }
+
+              if (isCollaborator) {
+                committerCounts.set(login, (committerCounts.get(login) ?? 0) + 1);
+              } else {
+                nonCommitterCounts.set(login, (nonCommitterCounts.get(login) ?? 0) + 1);
+              }
+            }
+
+            const MAX_EACH = 3;
+
+            const committers = [...committerCounts.entries()]
+              .sort((a, b) => b[1] - a[1])
+              .slice(0, MAX_EACH)
+              .map(([l]) => l);
+
+            const nonCommitters = [...nonCommitterCounts.entries()]
+              .sort((a, b) => b[1] - a[1])
+              .slice(0, MAX_EACH)
+              .map(([l]) => l);
+
+            const MARKER = '<!-- texera-reviewer-suggestion -->';
+
+            let body = `${MARKER}\n`;
+            body += `**Suggested reviewers** (based on \`git blame\` of changed files):\n\n`;
+
+            if (committers.length) {
+              body += `**Committers** — can be formally requested: ${committers.map(l => `@${l}`).join(', ')}\n\n`;
+            } else {
+              body += `**Committers** — none identified\n\n`;
+            }
+
+            if (nonCommitters.length) {
+              body += `**Non-committer contributors** — cc to notify: ${nonCommitters.map(l => `@${l}`).join(', ')}\n\n`;
+            }
+
+            if (committers.length) {
+              body += `Use \`/request-review @${committers[0]}\` to request a review`;
+              if (nonCommitters.length) {
+                body += `, or cc ${nonCommitters.map(l => `@${l}`).join(' ')} to notify them`;
+              }
+              body += '.';
+            } else if (nonCommitters.length) {
+              body += `Cc ${nonCommitters.map(l => `@${l}`).join(' ')} to notify them.`;
+            } else {
+              body += `No candidates found from blame history.`;
+            }
+
+            // Update existing suggestion comment rather than posting a new one.
+            const allComments = await github.paginate(github.rest.issues.listComments, {
+              owner, repo, issue_number: pull_number, per_page: 100,
+            });
+            const existing = allComments.find(c => c.body?.includes(MARKER));
+
+            if (existing) {
+              await github.rest.issues.updateComment({ owner, repo, comment_id: existing.id, body });
+              core.info(`Updated reviewer suggestion comment on #${pull_number}`);
+            } else {
+              await github.rest.issues.createComment({ owner, repo, issue_number: pull_number, body });
+              core.info(`Posted reviewer suggestion comment on #${pull_number}`);
+            }
+
   take:
     # The startsWith filter at the job level keeps unrelated comments
     # from allocating a runner; the regex inside the script enforces an
@@ -153,7 +321,15 @@ jobs:
                 reviewers.push(h);
             }
             if (!reviewers.length && !team_reviewers.length) {
-              core.warning(`No valid @mentions in '${action}'; skipping.`);
+              // Explicit @mentions are required — the suggest-reviewers CI job
+              // already posted a suggestion comment on this PR, so the author
+              // has candidates to choose from.
+              await github.rest.issues.createComment({
+                owner, repo, issue_number: pull_number,
+                body:
+                  `Please specify at least one reviewer: \`/${action} @user\`.\n` +
+                  `Check the suggestion comment on this PR for candidates.`,
+              });
               return;
             }