ci: Backport only the newly added label on labeled events (no-changelog) (#35695)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

.github/scripts/compute-backport-targets.mjs

@@ -1,6 +1,7 @@
 // Creates backport PR's according to labels on merged PR
 
 import {
+	getEventFromGithubEventPath,
 	getPullRequestById,
 	readPrLabels,
 	resolveRcBranchForTrack,
@@ -85,7 +86,19 @@ async function fetchPossiblePullRequestFromEnv() {
 
 export async function getLabels() {
 	const pullRequest = await fetchPossiblePullRequestFromEnv();
-	return new Set(readPrLabels(pullRequest));
+	if (pullRequest) {
+		return new Set(readPrLabels(pullRequest));
+	}
+
+	const event = getEventFromGithubEventPath();
+	// On a `labeled` event only the added label counts — other backport labels
+	// on the PR were already handled when they were added (or on merge), and
+	// reprocessing them would recreate those backport PRs.
+	if (event.action === 'labeled' && event.label?.name) {
+		return new Set([event.label.name]);
+	}
+
+	return new Set(readPrLabels(event.pull_request));
 }
 
 async function main() {

.github/scripts/compute-backport-targets.test.mjs

@@ -1,6 +1,6 @@
 import { describe, it, mock, before } from 'node:test';
 import assert from 'node:assert/strict';
-import { readPrLabels } from './github-helpers.mjs';
+import { getEventFromGithubEventPath, readPrLabels } from './github-helpers.mjs';
 
 /**
  * Run these tests by running
@@ -13,6 +13,7 @@ import { readPrLabels } from './github-helpers.mjs';
 mock.module('./github-helpers.mjs', {
 	namedExports: {
 		ensureEnvVar: () => {}, // no-op
+		getEventFromGithubEventPath: getEventFromGithubEventPath,
 		readPrLabels: readPrLabels,
 		resolveRcBranchForTrack: mockResolveRcBranchForTrack,
 		writeGithubOutput: () => {}, //no-op
@@ -77,6 +78,15 @@ describe('Compute backport targets', () => {
 		assert.ok(labels.has('release'));
 		assert.ok(labels.has('Backport to Stable'));
 	});
+	it('Should only use the added label on a labeled event', async () => {
+		process.env.GITHUB_EVENT_PATH = './fixtures/mock-github-event-labeled.json';
+		/** @type { Set<string> } */
+		const labels = await getLabels();
+
+		assert.equal(labels.size, 1);
+		assert.ok(labels.has('Backport to Beta'));
+	});
+
 	it('Should parse labels properly in manual workflow context', async () => {
 		process.env.PULL_REQUEST_ID = '123';
 		/** @type { Set<string> } */

.github/scripts/fixtures/mock-github-event-labeled.json

@@ -0,0 +1,7 @@
+{
+	"action": "labeled",
+	"label": { "name": "Backport to Beta" },
+	"pull_request": {
+		"labels": ["release", "Backport to Stable", "Backport to Beta"]
+	}
+}