feat: add generatePullRequestBody function and corresponding tests

Author: CasperKristiansson

docs/tasks.md

@@ -237,7 +237,7 @@ Use Context7 MCP for up to date documentation.
         Check existing ref before branch create. Document workflow `concurrency`.
         Verify: Parallel runs yield one PR.
 
-39. [ ] **Rollback instructions generator**
+39. [x] **Rollback instructions generator**
         PR body includes exact git commands.
         Verify: Snapshot contains commands with placeholders.
 

src/action-execution.ts

@@ -12,6 +12,7 @@ import {
   type LatestPatchResult,
 } from './versioning';
 import { createOrUpdatePullRequest, findExistingPullRequest, type PullRequestResult } from './git';
+import { generatePullRequestBody } from './pr-body';
 
 export type SkipReason =
   | 'no_matches_found'
@@ -231,13 +232,21 @@ export async function executeAction(
   }
 
   try {
+    const prBody = generatePullRequestBody({
+      track,
+      newVersion: latestVersion,
+      filesChanged,
+      branchName,
+      defaultBranch,
+    });
+
     const pullRequest = await dependencies.createOrUpdatePullRequest({
       owner: repository.owner,
       repo: repository.repo,
       head: branchName,
       base: defaultBranch,
       title: `chore: bump python ${track} to ${latestVersion}`,
-      body: `Update CPython ${track} pins to ${latestVersion}.`,
+      body: prBody,
       authToken: githubToken,
     });
 

src/pr-body.ts

@@ -0,0 +1,44 @@
+interface GeneratePullRequestBodyOptions {
+  track: string;
+  newVersion: string;
+  filesChanged: string[];
+  branchName: string;
+  defaultBranch: string;
+}
+
+export function generatePullRequestBody(options: GeneratePullRequestBodyOptions): string {
+  const { track, newVersion, filesChanged, branchName, defaultBranch } = options;
+
+  const filesSection = filesChanged.length
+    ? filesChanged.map((file) => `- \`${file}\``).join('\n')
+    : 'No files were modified in this bump.';
+
+  return [
+    '## Summary',
+    '',
+    `- Bump CPython ${track} pins to \`${newVersion}\`.`,
+    '',
+    '## Files Updated',
+    '',
+    filesSection,
+    '',
+    '## Rollback',
+    '',
+    'Before merge, close this PR and delete the branch:',
+    '',
+    '```sh',
+    `git push origin --delete ${branchName}`,
+    '```',
+    '',
+    `After merge, revert the change on ${defaultBranch}:`,
+    '',
+    '```sh',
+    `git checkout ${defaultBranch}`,
+    `git pull --ff-only origin ${defaultBranch}`,
+    'git revert --no-edit <merge_commit_sha>',
+    `git push origin ${defaultBranch}`,
+    '```',
+    '',
+    'Replace `<merge_commit_sha>` with the SHA of the merge commit if rollback is required.',
+  ].join('\n');
+}

tests/pr-body.test.ts

@@ -0,0 +1,34 @@
+import { describe, expect, it } from 'vitest';
+
+import { generatePullRequestBody } from '../src/pr-body';
+
+describe('generatePullRequestBody', () => {
+  it('includes summary, files, and rollback sections', () => {
+    const body = generatePullRequestBody({
+      track: '3.13',
+      newVersion: '3.13.2',
+      filesChanged: ['Dockerfile', '.github/workflows/ci.yml'],
+      branchName: 'chore/bump-python-3.13',
+      defaultBranch: 'main',
+    });
+
+    expect(body).toContain('## Summary');
+    expect(body).toContain('CPython 3.13 pins to `3.13.2`');
+    expect(body).toContain('`Dockerfile`');
+    expect(body).toContain('git push origin --delete chore/bump-python-3.13');
+    expect(body).toContain('git checkout main');
+    expect(body).toContain('git revert --no-edit <merge_commit_sha>');
+  });
+
+  it('falls back to message when no files changed', () => {
+    const body = generatePullRequestBody({
+      track: '3.12',
+      newVersion: '3.12.4',
+      filesChanged: [],
+      branchName: 'chore/bump-python-3.12',
+      defaultBranch: 'main',
+    });
+
+    expect(body).toContain('No files were modified in this bump.');
+  });
+});