chore(prlint): add rule to reject pr contribution from main branch

tools/@aws-cdk/prlint/lint.ts

@@ -508,6 +508,9 @@ export class PullRequestLinter {
     validationCollector.validateRuleSet({
       testRuleSet: [{ test: validateTitleScope }],
     });
+    validationCollector.validateRuleSet({
+      testRuleSet: [{ test: validateBranch }],
+    })
 
     validationCollector.validateRuleSet({
       exemption: shouldExemptBreakingChange,
@@ -687,6 +690,22 @@ function validateTitleScope(pr: GitHubPr): TestResult {
   return result;
 }
 
+/**
+ * Check that the PR is not opened from main branch of author's fork
+ * 
+ * @param pr github pr
+ * @returns test result
+ */
+function validateBranch(pr: GitHubPr): TestResult {
+  const result = new TestResult();
+
+  if (pr.head && pr.head.ref) {
+    result.assessFailure(pr.head.ref === 'main', 'Pull requests from `main` branch of a fork cannot be accepted. Please reopen this contribution from another branch on your fork. For more information, see https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md#step-4-pull-request.')
+  }
+
+  return result;
+}
+
 function assertStability(pr: GitHubPr, _files: GitHubFile[]): TestResult {
   const title = pr.title;
   const body = pr.body;

tools/@aws-cdk/prlint/test/lint.test.ts

@@ -93,6 +93,24 @@ describe('breaking changes format', () => {
   });
 });
 
+test('disallow PRs from main branch of fork', async () => {
+  const issue: Subset<linter.GitHubPr> = {
+    number: 1,
+    title: 'chore: some title',
+    body: 'making a pr from main of my fork',
+    labels: [{ name: 'pr-linter/exempt-test' }, { name: 'pr-linter/exempt-readme' }],
+    user: {
+      login: 'author',
+    },
+    head: {
+      label: 'author:main',
+      ref: 'main'
+    }
+  };
+  const prLinter = configureMock(issue, undefined);
+  await expect(prLinter.validatePullRequestTarget(SHA)).rejects.toThrow(/Pull requests from `main` branch of a fork cannot be accepted. Please reopen this contribution from another branch on your fork./);
+});
+
 describe('commit message format', () => {
   test('valid scope', async () => {
     const issue = {