StandardNodeGadget : Fix lingering error badges

Errors are caused by evaluating nodes with bad input, either in the form of plug values or of context variables. We have always cleared error badges when plugs are dirtied, which takes care of the situation where the user has fixed the input in the form of plug values. But until now we have stuck our head in the sand completely when it comes to errors due to bad contexts.

But now we have the ContextTracker, we can associate errors with tracked contexts, and when the tracked context changes, clear any errors from previous contexts.

Fixes #3820.

Changes.md

@@ -19,6 +19,8 @@ Fixes
   - 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.
+- GraphEditor :
+  - Fixed lingering error badges (#3820).
 
 Breaking Changes
 ----------------

src/GafferUI/StandardNodeGadget.cpp

@@ -382,6 +382,10 @@ class StandardNodeGadget::ErrorGadget : public Gadget
 		{
 			PlugEntry &entry = m_errors[plug];
 			entry.error = error;
+			if( auto tracker = ContextTracker::acquireForFocus( plug.get() ) )
+			{
+				entry.trackedContext = tracker->context( plug.get() )->hash();
+			}
 			if( !entry.parentChangedConnection.connected() )
 			{
 				entry.parentChangedConnection = plug->parentChangedSignal().connect( boost::bind( &ErrorGadget::plugParentChanged, this, ::_1 ) );
@@ -395,6 +399,27 @@ class StandardNodeGadget::ErrorGadget : public Gadget
 			m_image->setVisible( m_errors.size() );
 		}
 
+		void removeStaleErrors( const ContextTracker *tracker )
+		{
+			// We call `removeError()` whenever a plug is dirtied, but that can
+			// still leave errors lingering if the original error was a problem
+			// of _context_ rather than plug values. So when the tracked context
+			// changes, we remove any errors which occurred within a different
+			// tracked context.
+			for( auto it = m_errors.begin(); it != m_errors.end(); )
+			{
+				if( it->second.trackedContext != tracker->context( it->first.get() )->hash() )
+				{
+					it = m_errors.erase( it );
+				}
+				else
+				{
+					++it;
+				}
+			}
+			m_image->setVisible( m_errors.size() );
+		}
+
 		std::string getToolTip( const IECore::LineSegment3f &position ) const override
 		{
 			std::string result = Gadget::getToolTip( position );
@@ -434,6 +459,7 @@ class StandardNodeGadget::ErrorGadget : public Gadget
 		struct PlugEntry
 		{
 			std::string error;
+			IECore::MurmurHash trackedContext;
 			Signals::ScopedConnection parentChangedConnection;
 		};
 
@@ -786,13 +812,16 @@ void StandardNodeGadget::updateFromContextTracker( const ContextTracker *context
 		{
 			m_nodeEnabledInContextTracker = true;
 		}
-		/// \todo Should we clear ErrorGadget when the context changes?
 	}
 	else
 	{
 		m_nodeEnabledInContextTracker = std::nullopt;
 	}
 	updateStrikeThroughVisibility();
+	if( auto g = errorGadget( /* createIfMissing = */ false ) )
+	{
+		g->removeStaleErrors( contextTracker );
+	}
 }
 
 void StandardNodeGadget::updateTextDimming()