Fixing windows singleton executors (#580)

The static singleton executors were being created in a member function templated on the task type. This was causing unnecessary thread pools to get constructed. Moved instances to the .cpp file to make it simpler to share across dlls (open issue is to normalize the various implementations).

Author: sean-parent

include/stlab/concurrency/default_executor.hpp

@@ -451,17 +451,16 @@ priority_task_system& pts();
 
 #if STLAB_TASK_SYSTEM(WINDOWS)
 
+template <executor_priority P>
+extern task_system<P>& single_task_system();
+
 template <executor_priority P = executor_priority::medium>
 struct executor_type {
     using result_type = void;
 
     template <class F>
     auto operator()(F&& f) const -> std::enable_if_t<std::is_nothrow_invocable_v<std::decay_t<F>>> {
-        static task_system<P> only_task_system{[] {
-            at_pre_exit([]() noexcept { only_task_system.join(); });
-            return task_system<P>{};
-        }()};
-        only_task_system(std::forward<F>(f));
+        single_task_system<P>()(std::forward<F>(f));
     }
 };
 

src/concurrency/default_executor.cpp

@@ -51,6 +51,24 @@ priority_task_system& pts() {
     }()};
     return only_task_system;
 }
+
+#elif STLAB_TASK_SYSTEM(WINDOWS)
+
+/// Returns the singleton instance of the task system for the given priority.
+template <executor_priority P>
+task_system<P>& single_task_system() {
+    static task_system<P> _task_system{[] {
+        at_pre_exit([]() noexcept { single_task_system<P>().join(); });
+        return task_system<P>{};
+    }()};
+    return _task_system;
+}
+
+/// Instantiations of single_task_system for each priority level.
+template task_system<executor_priority::high>& single_task_system<executor_priority::high>();
+template task_system<executor_priority::medium>& single_task_system<executor_priority::medium>();
+template task_system<executor_priority::low>& single_task_system<executor_priority::low>();
+
 #endif
 
 } // namespace detail