Interlock for mailbox.

Thread.cc

@@ -91,6 +91,92 @@ namespace Alepha::Hydrogen::detail::Thread_m
 		state()->interruption_point();
 	}
 
+	namespace
+	{
+		enum State
+		{
+			Open,
+			Waiting,
+			Synchronizing,
+			Opening,
+		};
+	}
+
+	struct Interlock::Impl
+	{
+		State state= Open;
+
+		ConditionVariable var;
+	};
+
+	Interlock::Interlock() : pimpl( std::make_unique< Impl >() ) {}
+	Interlock::~Interlock()= default;
+
+	bool
+	Interlock::waiter( unique_lock< mutex > &lock )
+	{
+		// Defensively, if a thread called `checkpoint`, we
+		// might encounter that thread here, and need
+		// that thread to terminate the previous interlock
+		// phase.
+		while( impl().state == Opening )
+		{
+			impl().var.wait( lock );
+		}
+
+		if( impl().state == Open )
+		{
+			impl().state= Waiting;
+			return false;
+		}
+		else if( impl().state == Waiting )
+		{
+			impl().state= Synchronizing;
+			return true;
+		}
+		else { abort(); }
+	}
+
+	void
+	Interlock::noblock( unique_lock< mutex > &lock )
+	{
+		if( impl().state == Waiting )
+		{
+			// Nolock form will just leave the bit set.
+		}
+		else if( impl().state == Synchronizing )
+		{
+			impl().state= Opening;
+			impl().var.notify_one();
+		}
+		else { abort(); }
+	}
+
+	void
+	Interlock::interlock( unique_lock< mutex > &lock )
+	{
+		if( impl().state == Waiting )
+		{
+			while( impl().state == Waiting )
+			{
+				impl().var.wait( lock );
+			}
+			impl().state= Open;
+			impl().var.notify_one();
+		}
+		else if( impl().state == Synchronizing )
+		{
+			impl().state= Opening;
+			impl().var.notify_one();
+
+			while( impl().state == Opening )
+			{
+				impl().var.wait( lock );
+			}
+		}
+		else{ abort(); }
+	}
+
 	static auto init= Utility::enroll <=[]
 	{
 		state()= std::make_shared< ThreadState >();