Kernel/Threads: Implement an SleepClientThread function for HLERequestContext-based services to make performing async tasks on the host while in an HLE service function easier.

An HLE service function that wants to perform an async operation should put the caller guest thread to sleep using SleepClientThread, passing in a callback to execute when the thread is resumed.
SleepClientThread returns a Kernel::Event that should be signaled to resume the guest thread when the host async operation completes.

src/core/hle/service/service.cpp

@@ -181,8 +181,16 @@ void ServiceFrameworkBase::HandleSyncRequest(SharedPtr<ServerSession> server_ses
     LOG_TRACE(Service, "%s",
               MakeFunctionString(info->name, GetServiceName().c_str(), cmd_buf).c_str());
     handler_invoker(this, info->handler_callback, context);
-    context.WriteToOutgoingCommandBuffer(cmd_buf, *Kernel::g_current_process,
-                                         Kernel::g_handle_table);
+
+    auto thread = Kernel::GetCurrentThread();
+    ASSERT(thread->status == THREADSTATUS_RUNNING || thread->status == THREADSTATUS_WAIT_HLE_EVENT);
+    // Only write the response immediately if the thread is still running. If the HLE handler put
+    // the thread to sleep then the writing of the command buffer will be deferred to the wakeup
+    // callback.
+    if (thread->status == THREADSTATUS_RUNNING) {
+        context.WriteToOutgoingCommandBuffer(cmd_buf, *Kernel::g_current_process,
+                                             Kernel::g_handle_table);
+    }
 }
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////