IXWebSocketTransport::setReadyState(): Run under lock

When setReadyState(CLOSED) is executed from two different threads (the
server's thread, detecting a close trying to receive and a separate
sending thread), there is a race where both see _readyState as non-CLOSED
and both execute the _onCloseCallback().  Worse, the server's thread might
be returning from ->run(), unsetting the callback, while the other thread
is still about to call the _onCloseCallback(), resulting in a crash
rather than "just" a duplicate invocation.

This change ensures that setReadyState() *and* the_onCloseCallback()
are executed by a single thread till completion.

I was tempted to remove the std::atomic<> for _readyState, but left it
assuming it's still good having for getReadyState().

I've first tried _readyState.exchange() which seemed promising, but still
crashed once in a while as racing threads do not wait for the
_onCloseCallback() to complete.

ixwebsocket/IXWebSocketTransport.cpp

@@ -205,6 +205,8 @@ namespace ix
 
     void WebSocketTransport::setReadyState(ReadyState readyState)
     {
+        std::lock_guard<std::mutex> lock(_setReadyStateMutex);
+
         // No state change, return
         if (_readyState == readyState) return;
 

ixwebsocket/IXWebSocketTransport.h

@@ -185,6 +185,8 @@ namespace ix
 
         // Hold the state of the connection (OPEN, CLOSED, etc...)
         std::atomic<ReadyState> _readyState;
+        // Mutex to prevent racing in setReadyState()
+        std::mutex _setReadyStateMutex;
 
         OnCloseCallback _onCloseCallback;
         std::string _closeReason;