VNCServerST: Add a timeout to pointer button ownership

When one clients holds down a button on the pointer device (probably
dragging something), other clients' attempts at pointer operations
are denied. This yields a sane user experience, but with limits.

When one clients starts dragging, and then his network connection fails,
other clients are denied access to the pointer until the VNC server
finally discovers that the connection is dead and closes it. This can
take about 15 minutes.

Add a timeout to this policy: If we don't hear from the client for 3
seconds, other clients are allowed to control the pointer once more.

This solves the problem that one failing network could make the server
completely deaf to other clients for a long time.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>

common/rfb/VNCServerST.cxx

   : blHosts(&blacklist), desktop(desktop_), desktopStarted(false),
     blockCounter(0), pb(0), ledState(ledUnknown),
     name(name_), pointerClient(0), clipboardClient(0),
+    pointerClientTime(0),
     comparer(0), cursor(new Cursor(0, 0, Point(), NULL)),
     renderedCursorInvalid(false),
     keyRemapper(&KeyRemapper::defInstance),
 void VNCServerST::pointerEvent(VNCSConnectionST* client,
                                const Point& pos, int buttonMask)
 {
+  time_t now = time(0);
   if (rfb::Server::maxIdleTime)
     idleTimer.start(secsToMillis(rfb::Server::maxIdleTime));
 
   // Let one client own the cursor whilst buttons are pressed in order
-  // to provide a bit more sane user experience
-  if ((pointerClient != NULL) && (pointerClient != client))
+  // to provide a bit more sane user experience. But limit the time to prevent
+  // locking out all others when e.g. the network is down.
+  if ((pointerClient != NULL) && (pointerClient != client) && ((now - pointerClientTime) < 3))
     return;
 
+  pointerClientTime = now;
   if (buttonMask)
     pointerClient = client;
   else

common/rfb/VNCServerST.h

     std::list<VNCSConnectionST*> clipboardRequestors;
     std::list<network::Socket*> closingSockets;
 
+    time_t pointerClientTime;
+
     ComparingUpdateTracker* comparer;
 
     Point cursorPos;