Fix flaky CarbonRouterClient.basicUsageSameThreadClient test (#458)

Summary:
CarbonRouterClient.basicUsageSameThreadClient currently does not wait for code running on the proxy EVB to complete, which causes flakiness and errors like the following:

```
*** Aborted at 1734013374 (Unix time, try 'date -d 1734013374') ***
*** Signal 11 (SIGSEGV) (0x0) received by PID 72130 (pthread TID 0x7f4b45a096c0) (linux TID 72132) (code: 128), stack trace: ***
    @ 00000000007f0586 folly::symbolizer::(anonymous namespace)::signalHandler(int, siginfo_t*, void*)
                       /home/mszabo/Documents/fbcode-scratch/repos/gh.neting.cc-facebook-folly.git/folly/debugging/symbolizer/SignalHandler.cpp:453
    @ 0000000000019dcf (unknown)
    @ 000000000078cdb9 folly::TimeoutManager::clearCobTimeouts()
                       /usr/include/boost/intrusive/detail/list_node.hpp:60
                       -> /home/mszabo/Documents/fbcode-scratch/repos/gh.neting.cc-facebook-folly.git/folly/io/async/TimeoutManager.cpp
    @ 000000000078df70 folly::VirtualEventBase::destroyImpl()
                       /home/mszabo/Documents/fbcode-scratch/repos/gh.neting.cc-facebook-folly.git/folly/io/async/VirtualEventBase.cpp:38
    @ 00000000007739e6 bool folly::AtomicNotificationQueue<folly::Function<void ()> >::drive<folly::EventBase::FuncRunner&>(folly::EventBase::FuncRunner&)
                       /home/mszabo/Documents/fbcode-scratch/repos/gh.neting.cc-facebook-folly.git/folly/Function.h:370
                       -> /home/mszabo/Documents/fbcode-scratch/repos/gh.neting.cc-facebook-folly.git/folly/io/async/EventBase.cpp
    @ 0000000000775183 folly::EventBaseAtomicNotificationQueue<folly::Function<void ()>, folly::EventBase::FuncRunner>::execute()
                       /home/mszabo/Documents/fbcode-scratch/repos/gh.neting.cc-facebook-folly.git/folly/io/async/EventBaseAtomicNotificationQueue-inl.h:270
                       -> /home/mszabo/Documents/fbcode-scratch/repos/gh.neting.cc-facebook-folly.git/folly/io/async/EventBase.cpp
    @ 000000000077521c non-virtual thunk to folly::EventBaseAtomicNotificationQueue<folly::Function<void ()>, folly::EventBase::FuncRunner>::handlerReady(unsigned short)
                       /home/mszabo/Documents/fbcode-scratch/repos/gh.neting.cc-facebook-folly.git/folly/io/async/EventBaseAtomicNotificationQueue-inl.h:279
                       -> /home/mszabo/Documents/fbcode-scratch/repos/gh.neting.cc-facebook-folly.git/folly/io/async/EventBase.cpp
    @ 000000000077a783 folly::EventHandler::libeventCallback(int, short, void*)
                       /home/mszabo/Documents/fbcode-scratch/repos/gh.neting.cc-facebook-folly.git/folly/io/async/EventHandler.cpp:159
    @ 00000000000247f9 (unknown)
    @ 000000000002642e event_base_loop
    @ 000000000076e331 folly::EventBase::loopMain(int, folly::EventBase::LoopOptions)
                       /home/mszabo/Documents/fbcode-scratch/repos/gh.neting.cc-facebook-folly.git/folly/io/async/EventBase.cpp:99
    @ 000000000076e74d folly::EventBase::loopBody(int, folly::EventBase::LoopOptions)
                       /home/mszabo/Documents/fbcode-scratch/repos/gh.neting.cc-facebook-folly.git/folly/io/async/EventBase.cpp:516
    @ 000000000076e7e7 folly::EventBase::loop()
                       /home/mszabo/Documents/fbcode-scratch/repos/gh.neting.cc-facebook-folly.git/folly/io/async/EventBase.cpp:477
    @ 0000000000771336 folly::EventBase::loopForever()
                       /home/mszabo/Documents/fbcode-scratch/repos/gh.neting.cc-facebook-folly.git/folly/io/async/EventBase.cpp:784
    @ 00000000007221eb folly::IOThreadPoolExecutor::threadRun(std::shared_ptr<folly::ThreadPoolExecutor::Thread>)
                       /home/mszabo/Documents/fbcode-scratch/repos/gh.neting.cc-facebook-folly.git/folly/executors/IOThreadPoolExecutor.cpp:241
    @ 000000000072c557 void folly::detail::function::call_<std::_Bind<void (folly::ThreadPoolExecutor::*(folly::ThreadPoolExecutor*, std::shared_ptr<folly::ThreadPoolExecutor::Thread>))(std::shared_ptr<folly::ThreadPoolExecutor::Thread>)>, true, false, void>(, folly::detail::function::Data&)
                       /usr/include/c++/14/bits/invoke.h:74
                       -> /home/mszabo/Documents/fbcode-scratch/repos/gh.neting.cc-facebook-folly.git/folly/executors/ThreadPoolExecutor.cpp
    @ 000000000004b523 (unknown)
    @ 0000000000070cd6 start_thread
    @ 00000000000f4c8b __clone3
```

As a fix, add explicit synchronization to ensure the code scheduled on the EVB completes first before attempting to terminate the EVB.

Pull Request resolved: #458

Reviewed By: disylh

Differential Revision: D67521838

Pulled By: stuclar

fbshipit-source-id: 6f90bacea961a04054f31a9926a4fb2f9958a439

mcrouter/test/cpp_unit_tests/McrouterClientUsage.cpp

@@ -97,24 +97,29 @@ TEST(CarbonRouterClient, basicUsageSameThreadClient) {
   client->setProxyIndex(0);
 
   bool replyReceived = false;
-  eventBase.runInEventBaseThread([client = client.get(), &replyReceived]() {
-    // 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](
-            const McGetRequest&, McGetReply&& reply) {
-          EXPECT_EQ(carbon::Result::NOTFOUND, *reply.result_ref());
-          replyReceived = true;
-        });
-  });
+  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, &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);