Fix potential deadlock in TestMdns (#6721)

* Fix locking scope for MDNS test.

std::condition_variable is supposed to be able to hold the lock mutex
when exiting after wait_for. If the lock on the mutex is held throughout
the 'RunMainLoop', the condition variable can never exit.

Updated the scope of the locks.

* Update the locking logic to make the test not racy at startup

* Fix unterminated comment

src/platform/tests/TestMdns.cpp

@@ -95,25 +95,57 @@ static const nlTest sTests[] = { NL_TEST_DEF("Test Mdns::PubSub", TestMdnsPubSub
 int TestMdns()
 {
     std::mutex mtx;
-    std::unique_lock<std::mutex> lock(mtx);
-    std::condition_variable done;
+
+    std::condition_variable readyCondition;
+    bool ready = false;
+
+    std::condition_variable doneCondition;
+    bool done = false;
+
     int retVal = EXIT_FAILURE;
 
-    std::thread t([&mtx, &done, &retVal]() {
+    std::thread t([&]() {
         {
-            std::lock_guard<std::mutex> localLock(mtx);
-            nlTestSuite theSuite = { "CHIP DeviceLayer mdns tests", &sTests[0], nullptr, nullptr };
+            std::lock_guard<std::mutex> lock(mtx);
+            ready = true;
+            readyCondition.notify_one();
+        }
+
+        nlTestSuite theSuite = { "CHIP DeviceLayer mdns tests", &sTests[0], nullptr, nullptr };
 
-            nlTestRunner(&theSuite, nullptr);
-            retVal = nlTestRunnerStats(&theSuite);
+        nlTestRunner(&theSuite, nullptr);
+        retVal = nlTestRunnerStats(&theSuite);
+
+        {
+            std::lock_guard<std::mutex> lock(mtx);
+            done = true;
+            doneCondition.notify_all();
         }
-        done.notify_all();
     });
 
-    if (done.wait_for(lock, std::chrono::seconds(5)) == std::cv_status::timeout)
     {
-        fprintf(stderr, "mDNS test timeout, is avahi daemon running?");
-        retVal = EXIT_FAILURE;
+        std::unique_lock<std::mutex> lock(mtx);
+        readyCondition.wait(lock, [&] { return ready; });
+
+        doneCondition.wait_for(lock, std::chrono::seconds(5));
+        if (!done)
+        {
+            fprintf(stderr, "mDNS test timeout, is avahi daemon running?\n");
+
+            chip::DeviceLayer::PlatformMgr().LockChipStack();
+            chip::DeviceLayer::PlatformMgr().Shutdown();
+            chip::DeviceLayer::SystemLayer.WakeSelect();
+            chip::DeviceLayer::PlatformMgr().UnlockChipStack();
+
+            // TODO: the above does not seem to actually reliably shut down the chip stack.
+            // Program will abort with core because chip thread will still run.
+            doneCondition.wait_for(lock, std::chrono::seconds(1));
+            if (!done)
+            {
+                fprintf(stderr, "Orderly shutdown of the platform main loop failed as well.\n");
+            }
+            retVal = EXIT_FAILURE;
+        }
     }
     return retVal;
 }