Add switchThreadCoro(). (#1501)

lib/inc/drogon/utils/coroutine.h

@@ -697,6 +697,19 @@ struct [[nodiscard]] LoopAwaiter : CallbackAwaiter<void>
     std::function<void()> taskFunc_;
 };
 
+struct [[nodiscard]] SwitchThreadAwaiter : CallbackAwaiter<void>
+{
+    explicit SwitchThreadAwaiter(trantor::EventLoop *loop) : loop_(loop)
+    {
+    }
+    void await_suspend(std::coroutine_handle<> handle)
+    {
+        loop_->runInLoop([handle]() { handle.resume(); });
+    }
+
+  private:
+    trantor::EventLoop *loop_;
+};
 struct [[nodiscard]] EndAwaiter : CallbackAwaiter<void>
 {
     EndAwaiter(trantor::EventLoop *loop) : loop_(loop)
@@ -738,6 +751,13 @@ inline internal::LoopAwaiter queueInLoopCoro(
     return {workLoop, std::move(taskFunc), resumeLoop};
 }
 
+inline internal::SwitchThreadAwaiter switchThreadCoro(
+    trantor::EventLoop *loop) noexcept
+{
+    assert(loop);
+    return internal::SwitchThreadAwaiter{loop};
+}
+
 inline internal::EndAwaiter untilQuit(trantor::EventLoop *loop)
 {
     assert(loop);

lib/tests/unittests/CoroutineTest.cc

@@ -1,6 +1,7 @@
 #include <drogon/drogon_test.h>
 #include <drogon/utils/coroutine.h>
 #include <drogon/HttpAppFramework.h>
+#include <trantor/net/EventLoopThread.h>
 #include <type_traits>
 
 using namespace drogon;
@@ -185,3 +186,20 @@ DROGON_TEST(AsyncWaitLifetime)
         CHECK(ptr2.use_count() == 1);
     }));
 }
+
+DROGON_TEST(SwitchThread)
+{
+    thread_local int num{0};
+    trantor::EventLoopThread thread;
+    thread.run();
+    thread.getLoop()->queueInLoop([]() { num = 100; });
+
+    auto switch_thread = [TEST_CTX, &thread]() -> Task<> {
+        CHECK(num == 0);
+        co_await switchThreadCoro(thread.getLoop());
+        CHECK(num == 100);
+        thread.getLoop()->quit();
+    };
+    sync_wait(switch_thread());
+    thread.wait();
+}