VNCConn: avoid holding mutex_framebuffer for too long

...by introducing a 2nd front buffer the UI can read from while
libvncclient is busy writing to the client's back buffer.

Fixes unresponsive UI due to the VNC thread unfairly grabbing the mutex
in certain edge cases like when sending no input at all.

re #152 #282

Author: bk138

src/VNCConn.cpp

@@ -84,6 +84,7 @@ VNCConn::VNCConn(void* p) : condition_auth(mutex_auth)
   parent = p;
 
   cl = 0;
+  framebuffer_front = 0;
   multicastStatus = 0;
   latency = -1;
 
@@ -222,10 +223,17 @@ rfbBool VNCConn::thread_alloc_framebuffer(rfbClient* client)
   // alloc, zeroed
   client->frameBuffer = (uint8_t*)calloc(1, client->width*client->height*client->format.bitsPerPixel/8);
 
+  // free front framebuffer
+  if(conn->framebuffer_front)
+    free(conn->framebuffer_front);
+
+  // alloc, zeroed
+  conn->framebuffer_front = (uint8_t*)calloc(1, client->width*client->height*client->format.bitsPerPixel/8);
+
   // notify our parent
   conn->thread_post_fbresize_notify();
 
-  return client->frameBuffer ? TRUE : FALSE;
+  return (client->frameBuffer && conn->framebuffer_front) ? TRUE : FALSE;
 }
 
 
@@ -471,8 +479,6 @@ wxThread::ExitCode VNCConn::Entry()
 
 
           // request update and handle response
-          {
-          wxCriticalSectionLocker lock(mutex_framebuffer);
           if(!rfbProcessServerMessage(cl, 500))
 	    {
 	      if(errno == EINTR)
@@ -488,7 +494,7 @@ wxThread::ExitCode VNCConn::Entry()
               }
               return 0;
 	    }
-          }
+
 
 	  /*
 	    Compute nacked/loss ratio: We take a ratio sample every second and put it into a sample queue
@@ -983,6 +989,20 @@ void VNCConn::thread_update_finished(rfbClient* client)
   VNCConn* conn = (VNCConn*) rfbClientGetClientData(client, VNCCONN_OBJ_ID);
   if(! conn->GetThread()->TestDestroy())
     {
+      {
+          wxCriticalSectionLocker lock(conn->mutex_framebuffer);
+          if(client->frameBuffer && conn->framebuffer_front) {
+              // Copy framebuffer to front buffer for UI display
+#ifndef NDEBUG
+              wxStopWatch copy_timer;
+#endif
+              memcpy(conn->framebuffer_front, client->frameBuffer, client->width * client->height * client->format.bitsPerPixel / 8);
+#ifndef NDEBUG
+              wxLogDebug(wxT("VNCConn %p: copying %dx%d framebuffer took %ldms"), conn, client->width, client->height, copy_timer.Time());
+#endif
+          }
+      }
+
       if(!conn->updated_rect.IsEmpty())
 	conn->thread_post_update_notify(conn->updated_rect.x, conn->updated_rect.y, conn->updated_rect.width, conn->updated_rect.height);
 
@@ -1430,6 +1450,12 @@ void VNCConn::Shutdown()
       rfbClientCleanup(cl);
       cl = 0;
     }
+
+    // Clean up front framebuffer
+    if(framebuffer_front) {
+        free(framebuffer_front);
+        framebuffer_front = 0;
+    }
 }
 
 
@@ -1765,19 +1791,19 @@ wxBitmap VNCConn::getFrameBufferRegion(const wxRect& rect)
   if(rect.x < 0 || rect.x + rect.width > getFrameBufferWidth()
      || rect.y < 0 || rect.y + rect.height > getFrameBufferHeight()
      || !cl
-     || !cl->frameBuffer)
+     || !framebuffer_front)
        return wxBitmap();
 
   /*
-    copy directly from framebuffer into a new bitmap
+    copy directly from front framebuffer into a new bitmap
   */
 
   wxBitmap region(rect.width, rect.height, cl->format.bitsPerPixel);
   wxAlphaPixelData region_data(region);
   wxAlphaPixelData::Iterator region_it(region_data);
 
   int bytesPerPixel = cl->format.bitsPerPixel/8;
-  uint8_t *fbsub_it = cl->frameBuffer + rect.y*cl->width*bytesPerPixel + rect.x*bytesPerPixel;
+  uint8_t *fbsub_it = framebuffer_front + rect.y*cl->width*bytesPerPixel + rect.x*bytesPerPixel;
 
   for( int y = 0; y < rect.height; ++y )
     {

src/VNCConn.h

@@ -234,6 +234,7 @@ class VNCConn: public wxEvtHandler, public wxThreadHelper
 
   wxRect updated_rect;
 
+  uint8_t* framebuffer_front; // framebuffer for UI reading
   wxCriticalSection mutex_framebuffer;
 
   int multicastStatus;