Interlock for mailbox.

Thread.h

@@ -27,11 +27,56 @@ namespace Alepha::Hydrogen  ::detail::  Thread_m
 		class ConditionVariable;
 		using condition_variable= ConditionVariable;
 
+		/*!
+		 * An `Interlock` is kind of like a condition variable, for two cooperating threads.
+		 *
+		 * An `Interlock` object represents the state which permits two cooperating threads to
+		 * perform a synchronization action.  Each side enters `Interlock::interlock`, and
+		 * when both of them have entered, the synchronization function will be executed.
+		 *
+		 * In some sense it is similar to a `std::barrier` with a capacity to be interrupted
+		 * and accepts parameter to pass as the interlock-synchronization action.
+		 */
+		class Interlock;
+
 		using std::mutex;
 		using std::lock_guard;
 		using std::unique_lock;
 	}
 
+	class exports::Interlock
+	{
+		private:
+			struct Impl;
+			std::unique_ptr< Impl > pimpl;
+
+			auto &impl() { return *pimpl; }
+			const auto &impl() const { return *pimpl; }
+
+			bool waiter( unique_lock< mutex > & );
+
+			void interlock( unique_lock< mutex > &lock );
+			void noblock( unique_lock< mutex > &lock );
+
+		public:
+			~Interlock();
+			Interlock();
+
+			void
+			wait( unique_lock< mutex > &lock, auto &&synchronize )
+			{
+				if( waiter( lock ) ) synchronize();
+				interlock( lock );
+			}
+
+			void
+			checkpoint( unique_lock< mutex > &lock, auto &&synchronize )
+			{
+				if( waiter( lock ) ) synchronize();
+				noblock( lock );
+			}
+	};
+
 	class exports::ConditionVariable
 	{
 		private: