Fix unsafe access of map layer from background thread while running l…

…abel engine
qgis · Jun 28, 2021 · 731810c · 731810c
1 parent 849819c
commit 731810c
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Showing 2 changed files with 28 additions and 4 deletions.
diff --git a/src/core/labeling/qgslabelingengine.cpp b/src/core/labeling/qgslabelingengine.cpp
@@ -282,9 +282,9 @@ void QgsLabelingEngine::registerLabels( QgsRenderContext &context )
   for ( QgsAbstractLabelProvider *provider : std::as_const( mProviders ) )
   {
     std::unique_ptr< QgsExpressionContextScopePopper > layerScopePopper;
-    if ( QgsMapLayer *ml = provider->layer() )
+    if ( provider->layerExpressionContextScope() )
     {
-      layerScopePopper = std::make_unique< QgsExpressionContextScopePopper >( context.expressionContext(), QgsExpressionContextUtils::layerScope( ml ) );
+      layerScopePopper = std::make_unique< QgsExpressionContextScopePopper >( context.expressionContext(), new QgsExpressionContextScope( *provider->layerExpressionContextScope() ) );
     }
     processProvider( provider, context, *mPal );
   }
@@ -420,7 +420,7 @@ void QgsLabelingEngine::drawLabels( QgsRenderContext &context, const QString &la
 
     // provider will require the correct layer scope for expression preparation - at this stage, the existing expression context
     // only contains generic scopes
-    QgsExpressionContextScopePopper popper( context.expressionContext(), QgsExpressionContextUtils::layerScope( provider->layer() ) );
+    QgsExpressionContextScopePopper popper( context.expressionContext(), provider->layerExpressionContextScope() ? new QgsExpressionContextScope( *provider->layerExpressionContextScope() ) : new QgsExpressionContextScope() );
     provider->startRender( context );
   }
 
@@ -623,6 +623,10 @@ QgsAbstractLabelProvider::QgsAbstractLabelProvider( QgsMapLayer *layer, const QS
   , mPriority( 0.5 )
   , mUpsidedownLabels( QgsPalLayerSettings::Upright )
 {
+  if ( QgsVectorLayer *vl = qobject_cast< QgsVectorLayer * >( layer ) )
+  {
+    mLayerExpressionContextScope.reset( vl->createExpressionContextScope() );
+  }
 }
 
 void QgsAbstractLabelProvider::drawUnplacedLabel( QgsRenderContext &, pal::LabelPosition * ) const
@@ -653,6 +657,11 @@ void QgsAbstractLabelProvider::stopRender( QgsRenderContext &context )
   }
 }
 
+QgsExpressionContextScope *QgsAbstractLabelProvider::layerExpressionContextScope() const
+{
+  return mLayerExpressionContextScope.get();
+}
+
 //
 // QgsLabelingUtils
 //

diff --git a/src/core/labeling/qgslabelingengine.h b/src/core/labeling/qgslabelingengine.h
@@ -127,7 +127,11 @@ class CORE_EXPORT QgsAbstractLabelProvider
     //! Returns ID of associated layer, or empty string if no layer is associated with the provider.
     QString layerId() const { return mLayerId; }
 
-    //! Returns the associated layer, or NULLPTR if no layer is associated with the provider.
+    /**
+     * Returns the associated layer, or NULLPTR if no layer is associated with the provider.
+     *
+     * \warning Accessing the layer is not thread safe, and this should never be called while the labeling engine is running from a background thread!
+     */
     QgsMapLayer *layer() const { return mLayer.data(); }
 
     /**
@@ -152,6 +156,13 @@ class CORE_EXPORT QgsAbstractLabelProvider
     //! How to handle labels that would be upside down
     QgsPalLayerSettings::UpsideDownLabels upsidedownLabels() const { return mUpsidedownLabels; }
 
+    /**
+     * Returns the expression context scope created from the layer associated with this provider.
+     *
+     * \since QGIS 3.22
+     */
+    QgsExpressionContextScope *layerExpressionContextScope() const;
+
   protected:
     //! Associated labeling engine
     const QgsLabelingEngine *mEngine = nullptr;
@@ -174,6 +185,10 @@ class CORE_EXPORT QgsAbstractLabelProvider
     QgsLabelObstacleSettings::ObstacleType mObstacleType = QgsLabelObstacleSettings::PolygonBoundary;
     //! How to handle labels that would be upside down
     QgsPalLayerSettings::UpsideDownLabels mUpsidedownLabels;
+
+  private:
+
+    std::unique_ptr< QgsExpressionContextScope > mLayerExpressionContextScope;
 };
 
 Q_DECLARE_OPERATORS_FOR_FLAGS( QgsAbstractLabelProvider::Flags )