fix: flush proxied event stream headers

apps/api/src/canary-gateway.test.ts

@@ -334,6 +334,31 @@ describe('same-origin canary gateway', () => {
     await reader?.cancel();
   });
 
+  it('flushes SSE response headers before the first upstream event', async () => {
+    const upstream = createServer((_request, response) => {
+      response.writeHead(200, {
+        'content-type': 'text/event-stream',
+        'cache-control': 'no-cache',
+      });
+      response.flushHeaders();
+    });
+    servers.push(upstream);
+    const gateway = await startGateway(await fixtureDist(), await listen(upstream));
+
+    const controller = new AbortController();
+    const timeout = setTimeout(() => controller.abort(), 500);
+    try {
+      const response = await fetch(`${gateway.origin}/api/v1/events`, {
+        signal: controller.signal,
+      });
+      expect(response.status).toBe(200);
+      expect(response.headers.get('content-type')).toBe('text/event-stream');
+      await response.body?.cancel();
+    } finally {
+      clearTimeout(timeout);
+    }
+  });
+
   it('has reversible, idempotent start/stop lifecycle', async () => {
     const gateway = createCanaryGateway({ distDir: await fixtureDist(), port: 0, upstreamPort: 1 });
     gateways.push(gateway);

apps/api/src/canary-gateway.ts

@@ -177,6 +177,7 @@ async function proxyRequest(
           upstreamResponse.statusMessage,
           withoutHopByHop(upstreamResponse.headers),
         );
+        response.flushHeaders();
         pipeline(upstreamResponse, response)
           .then(resolvePromise)
           .catch(() => {