Fix thread_poll joinable should be atomic

Author: shuai132

asio/include/asio/impl/thread_pool.ipp

@@ -125,9 +125,9 @@ void thread_pool::attach()
 
 void thread_pool::join()
 {
-  if (joinable_)
+  bool expected = true;
+  if (joinable_.compare_exchange_strong(expected, false))
   {
-    joinable_ = false;
     scheduler_.work_finished();
     threads_.join();
   }

asio/include/asio/thread_pool.hpp

@@ -229,7 +229,7 @@ class thread_pool
   detail::atomic_count num_threads_;
 
   // Whether a join call will have any effect.
-  bool joinable_;
+  std::atomic<bool> joinable_;
 };
 
 /// Executor implementation type used to submit functions to a thread pool.