ImageGadget : Don't clear dirty flag after cancellation

In most cases this didn't matter, because we were only getting cancelled by a graph edit which would eventually dirty the input plug anyway. But cancellation could be due to an edit in an unrelated part of the graph, in which case our input plug won't be dirtied again, but we do want to restart the update.

Fixes #6043

Changes.md

@@ -15,6 +15,7 @@ Fixes
   - USD asset paths on Windows now use `/` as the path separator, instead of `\`. Set `IECOREUSD_FORCE_ASSET_PATH_FORWARD_SLASH=0` to keep the previous behaviour.
 - SceneWriter : Fixed crash when writing to a USD file that is already open elsewhere.
 - Viewer, ImageGadget :
+  - Fixed partial image updates when an unrelated InteractiveRender was running (#6043).
   - Fixed "colour tearing", where updates to some image channels became visible before updates to others.
   - Fixed unnecessary texture updates when specific image tiles don't change.
 - Constraint : The `target` browser now shows locations from the `targetScene` if it has an input connection. Before it always showed locations from the main input.

src/GafferImageUI/ImageGadget.cpp

@@ -53,6 +53,7 @@
 #include "Gaffer/BackgroundTask.h"
 #include "Gaffer/Context.h"
 #include "Gaffer/Node.h"
+#include "Gaffer/Process.h"
 #include "Gaffer/ScriptNode.h"
 
 #include "IECore/MessageHandler.h"
@@ -891,6 +892,7 @@ void ImageGadget::updateTiles()
 			{
 				m_tiles[TileIndex(tileOrigin, channelName)].resetActive();
 			}
+			throw;
 		}
 
 	};
@@ -902,8 +904,24 @@ void ImageGadget::updateTiles()
 		// OK to capture `this` via raw pointer, because ~ImageGadget waits for
 		// the background process to complete.
 		[ this, channelsToCompute, dataWindow, tileFunctor ] {
-			ImageAlgo::parallelProcessTiles( m_image.get(), tileFunctor, dataWindow );
-			m_dirtyFlags &= ~TilesDirty;
+
+			try
+			{
+				ImageAlgo::parallelProcessTiles( m_image.get(), tileFunctor, dataWindow );
+				m_dirtyFlags &= ~TilesDirty;
+			}
+			catch( const Gaffer::ProcessException & )
+			{
+				// No point starting a new compute if it's just
+				// going to error again.
+				m_dirtyFlags &= ~TilesDirty;
+			}
+			catch( const IECore::Cancelled & )
+			{
+				// Don't clear dirty flag, so that we restart
+				// on the next redraw.
+			}
+
 			if( refCount() )
 			{
 				ImageGadgetPtr thisRef = this;
@@ -913,6 +931,7 @@ void ImageGadget::updateTiles()
 					}
 				);
 			}
+
 		}
 	);