Hop to the main thread only when something drains its queue

deno test runs JS off the main thread while Deno's main thread waits in its
own loop, so every call hopped to a queue nobody served and the Deno smoke
test hung on macOS.

Author: lijy91

bindings/js/src/napi_support.cc

@@ -1,8 +1,10 @@
 #include "napi_support.h"
 
 #include <atomic>
+#include <chrono>
 #include <condition_variable>
 #include <map>
+#include <memory>
 #include <mutex>
 #include <thread>
 #include <tuple>
@@ -475,10 +477,38 @@ void RunOnMainThreadSync(const std::function<void()>& work) {
   done_signal.wait(lock, [&] { return done; });
 }
 
+namespace {
+
+// Whether something on the UI thread drains its queue — a host running the
+// platform loop, as under `deno desktop`. `deno test` also runs JS off the main
+// thread, but its main thread only waits in Deno's own loop: hopping there
+// would block forever, so calls stay on the JS thread instead.
+bool MainThreadIsServiced() {
+  struct Probe {
+    std::mutex mutex;
+    std::condition_variable signal;
+    bool ran = false;
+  };
+  // Shared, because a late run can land after this returns.
+  auto probe = std::make_shared<Probe>();
+  bool posted = nativeapi::RunOnMainThread([probe] {
+    std::lock_guard<std::mutex> lock(probe->mutex);
+    probe->ran = true;
+    probe->signal.notify_one();
+  });
+  if (!posted) {
+    return false;
+  }
+  std::unique_lock<std::mutex> lock(probe->mutex);
+  return probe->signal.wait_for(lock, std::chrono::seconds(1), [&] { return probe->ran; });
+}
+
+}  // namespace
+
 void InitRuntime(napi_env env) {
   g_js_thread = std::this_thread::get_id();
-  g_hop_to_main_thread =
-      !nativeapi::IsMainThread() && nativeapi::IsMainThreadDispatchSupported();
+  g_hop_to_main_thread = !nativeapi::IsMainThread() &&
+                         nativeapi::IsMainThreadDispatchSupported() && MainThreadIsServiced();
   g_env_alive.store(true);
   napi_add_env_cleanup_hook(
       env, [](void*) { g_env_alive.store(false); }, nullptr);