test(nextjs): Assert single transaction for pages-router API routes

Guards against duplicate root transactions for pages-router API routes:
Next.js's own BaseServer.handleRequest transaction and the one created by
wrapApiHandlerWithSentry must never both be sent for a single request.

Author: logaretm

dev-packages/e2e-tests/test-applications/nextjs-pages-dir/tests/api-route-transaction.test.ts

@@ -0,0 +1,24 @@
+import { expect, test } from '@playwright/test';
+import { waitForTransaction } from '@sentry-internal/test-utils';
+
+// A pages-router API route sees both Next.js's own `BaseServer.handleRequest` OTEL transaction and the
+// transaction created by `wrapApiHandlerWithSentry`. Exactly one of them must be sent for a request, never
+// both. This guards against regressing back to duplicate root transactions for the same API route.
+test('Sends exactly one transaction for a pages-router API route', async ({ request }) => {
+  const apiRouteTransactions: string[] = [];
+
+  // Never resolves; we accumulate every matching transaction and assert on the total after a grace period.
+  const collectorPromise = waitForTransaction('nextjs-pages-dir', transactionEvent => {
+    if (transactionEvent?.transaction === 'GET /api/endpoint') {
+      apiRouteTransactions.push(transactionEvent.contexts?.trace?.trace_id ?? '<no-trace-id>');
+    }
+    return false;
+  });
+
+  const response = await request.get('/api/endpoint');
+  expect(await response.json()).toStrictEqual({ name: 'John Doe' });
+
+  await Promise.race([collectorPromise, new Promise(resolve => setTimeout(resolve, 6000))]);
+
+  expect(apiRouteTransactions).toHaveLength(1);
+});