Fix flaky CarbonRouterClient.basicUsageSameThreadClient test

mcrouter/test/cpp_unit_tests/McrouterClientUsage.cpp

@@ -97,24 +97,28 @@ TEST(CarbonRouterClient, basicUsageSameThreadClient) {
   client->setProxyIndex(0);
 
   bool replyReceived = false;
-  eventBase.runInEventBaseThread([client = client.get(), &replyReceived]() {
+  folly::fibers::Baton baton;
+
+  eventBase.runInEventBaseThread([client = client.get(), &replyReceived, &baton]() {
     // We must ensure that req will remain alive all the way through the reply
     // callback given to client->send(). This demonstrates one way of ensuring
     // this.
     auto req = std::make_unique<McGetRequest>("key");
     auto reqRawPtr = req.get();
     client->send(
         *reqRawPtr,
-        [req = std::move(req), &replyReceived](
+        [req = std::move(req), &replyReceived, &baton](
             const McGetRequest&, McGetReply&& reply) {
           EXPECT_EQ(carbon::Result::NOTFOUND, *reply.result_ref());
           replyReceived = true;
+          baton.post();
         });
   });
 
   // Wait for proxy threads to complete outstanding requests and exit
   // gracefully. This ensures graceful destruction of the static
   // CarbonRouterInstance instance.
+  baton.wait();
   router->shutdown();
   ioThreadPool.reset();
   EXPECT_TRUE(replyReceived);