[utility]: Change behavior of main thread verification utility (envoyproxy#14660)

Currently, the isMainThread function can only be called during the lifetime of thread local instance because the singleton that store main thread id is initialized in the constructor of tls instance and cleared in the destructor of tls instance. Change the utility so that outside the lifetime of tls instance, the function return true by default because everything is in main thread when threading is off.

Risk Level: low
Testing: change unit to reflect change of behavior.

Signed-off-by: chaoqin-li1123 <[email protected]>

Author: chaoqin-li1123

source/common/common/thread.h

@@ -177,7 +177,14 @@ struct MainThread {
     delete MainThreadSingleton::getExisting();
     MainThreadSingleton::clear();
   }
-  static bool isMainThread() { return MainThreadSingleton::get().inMainThread(); }
+  static bool isMainThread() {
+    // If threading is off, only main thread is running.
+    if (MainThreadSingleton::getExisting() == nullptr) {
+      return true;
+    }
+    // When threading is on, compare thread id with main thread id.
+    return MainThreadSingleton::get().inMainThread();
+  }
 
 private:
   std::thread::id main_thread_id_{std::this_thread::get_id()};

test/common/thread_local/thread_local_impl_test.cc

@@ -16,23 +16,18 @@ namespace Envoy {
 namespace ThreadLocal {
 
 TEST(MainThreadVerificationTest, All) {
-  // Main thread singleton is initialized in the constructor of tls instance. Call to main thread
-  // verification will fail before that.
-  EXPECT_DEATH(Thread::MainThread::isMainThread(),
-               "InjectableSingleton used prior to initialization");
+  // Before threading is on, assertion on main thread should be true.
+  EXPECT_TRUE(Thread::MainThread::isMainThread());
   {
-    EXPECT_DEATH(Thread::MainThread::isMainThread(),
-                 "InjectableSingleton used prior to initialization");
     InstanceImpl tls;
-    // Call to main thread verification should succeed after tls instance has been initialized.
-    ASSERT(Thread::MainThread::isMainThread());
+    // Tls instance has been initialized.
+    // Call to main thread verification should succeed in main thread.
+    EXPECT_TRUE(Thread::MainThread::isMainThread());
     tls.shutdownGlobalThreading();
     tls.shutdownThread();
   }
-  // Main thread singleton is cleared in the destructor of tls instance. Call to main thread
-  // verification will fail after that.
-  EXPECT_DEATH(Thread::MainThread::isMainThread(),
-               "InjectableSingleton used prior to initialization");
+  // After threading is off, assertion on main thread should be true.
+  EXPECT_TRUE(Thread::MainThread::isMainThread());
 }
 
 class TestThreadLocalObject : public ThreadLocalObject {
@@ -305,6 +300,8 @@ TEST(ThreadLocalInstanceImplDispatcherTest, Dispatcher) {
         thread_dispatcher->run(Event::Dispatcher::RunType::NonBlock);
         // Verify we have the expected dispatcher for the new thread thread.
         EXPECT_EQ(thread_dispatcher.get(), &tls.dispatcher());
+        // Verify that it is inside the worker thread.
+        EXPECT_FALSE(Thread::MainThread::isMainThread());
       });
   thread->join();