Fix a race condition in WindowsThread (port::Thread)

Summary:
Fix a race condition when we create a thread and immediately destroy
 This case should be supported.
  What happens is that the thread function needs the Data instance
  to actually run but has no shared ownership and must rely on the
  WindowsThread instance to continue existing.
  To address this we change unique_ptr to shared_ptr and then
  acquire an additional refcount for the threadproc which destroys it
  just before the thread exit.
  We choose to allocate shared_ptr instance on the heap as this allows
  the original thread to continue w/o waiting for the new thread to start
  running.
Closes facebook#3240

Differential Revision: D6511324

Pulled By: yiwu-arbug

fbshipit-source-id: 4633ff7996daf4d287a9fe34f60c1dd28cf4ff36

port/win/win_thread.cc

@@ -39,12 +39,17 @@ struct WindowsThread::Data {
 
 void WindowsThread::Init(std::function<void()>&& func) {
 
-  data_.reset(new Data(std::move(func)));
+  data_ = std::make_shared<Data>(std::move(func));
+  // We create another instance of shared_ptr to get an additional ref
+  // since we may detach and destroy this instance before the threadproc
+  // may start to run. We choose to allocate this additional ref on the heap
+  // so we do not need to synchronize and allow this thread to proceed
+  std::unique_ptr<std::shared_ptr<Data>> th_data(new std::shared_ptr<Data>(data_));
 
   data_->handle_ = _beginthreadex(NULL,
     0,    // stack size
     &Data::ThreadProc,
-    data_.get(),
+    th_data.get(),
     0,   // init flag
     &th_id_);
 
@@ -53,6 +58,7 @@ void WindowsThread::Init(std::function<void()>&& func) {
       std::errc::resource_unavailable_try_again),
       "Unable to create a thread");
   }
+  th_data.release();
 }
 
 WindowsThread::WindowsThread() :
@@ -129,7 +135,7 @@ void WindowsThread::join() {
     assert(false);
     throw std::system_error(static_cast<int>(lastError),
       std::system_category(),
-      "WaitForSingleObjectFailed");
+      "WaitForSingleObjectFailed: thread join");
   }
 
   CloseHandle(reinterpret_cast<HANDLE>(data_->handle_));
@@ -157,8 +163,9 @@ void  WindowsThread::swap(WindowsThread& o) {
 }
 
 unsigned int __stdcall  WindowsThread::Data::ThreadProc(void* arg) {
-  auto data = reinterpret_cast<WindowsThread::Data*>(arg);
-  data->func_();
+  auto ptr = reinterpret_cast<std::shared_ptr<Data>*>(arg);
+  std::unique_ptr<std::shared_ptr<Data>> data(ptr);
+  (*data)->func_();
   _endthreadex(0);
   return 0;
 }

port/win/win_thread.h

@@ -28,7 +28,7 @@ class WindowsThread {
 
   struct Data;
 
-  std::unique_ptr<Data>  data_;
+  std::shared_ptr<Data>  data_;
   unsigned int           th_id_;
 
   void Init(std::function<void()>&&);