[vector tile] Fix issue with disappearing features (fixes #36982)

The issue was that renderer/labeling were only keeping "their" required fields,
but the final fields may have been extended by the other, making the field indices
cached in expressions invalid.

The fix is to keep the final QgsFields around and avoid creating those QgsFields on-the-fly
from previously requested fields.

python/core/auto_generated/vectortile/qgsvectortilerenderer.sip.in

@@ -42,6 +42,15 @@ Sets polygon of the tile
     QPolygon tilePolygon() const;
 %Docstring
 Returns polygon (made out of four corners of the tile) in screen coordinates calculated from render context
+%End
+
+    void setFields( const QMap<QString, QgsFields> &fields );
+%Docstring
+Sets per-layer fields
+%End
+    QMap<QString, QgsFields> fields() const;
+%Docstring
+Returns per-layer fields
 %End
 
     QStringList layers() const;

src/core/vectortile/qgsvectortilebasiclabeling.cpp

@@ -143,9 +143,9 @@ QMap<QString, QSet<QString> > QgsVectorTileBasicLabelProvider::usedAttributes( c
   return requiredFields;
 }
 
-void QgsVectorTileBasicLabelProvider::setFields( const QMap<QString, QSet<QString> > &requiredFields )
+void QgsVectorTileBasicLabelProvider::setFields( const QMap<QString, QgsFields> &perLayerFields )
 {
-  mRequiredFields = requiredFields;
+  mPerLayerFields = perLayerFields;
 }
 
 QList<QgsAbstractLabelProvider *> QgsVectorTileBasicLabelProvider::subProviders()
@@ -167,7 +167,7 @@ bool QgsVectorTileBasicLabelProvider::prepare( QgsRenderContext &context, QSet<Q
   // populate sub-providers
   for ( int i = 0; i < mSubProviders.count(); ++i )
   {
-    QgsFields fields = QgsVectorTileUtils::makeQgisFields( mRequiredFields[mStyles[i].layerName()] );
+    QgsFields fields = mPerLayerFields[mStyles[i].layerName()];
 
     QgsExpressionContextScope *scope = new QgsExpressionContextScope( QObject::tr( "Layer" ) ); // will be deleted by popper
     scope->setFields( fields );
@@ -194,7 +194,7 @@ void QgsVectorTileBasicLabelProvider::registerTileFeatures( const QgsVectorTileR
     if ( !layerStyle.isActive( zoomLevel ) )
       continue;
 
-    QgsFields fields = QgsVectorTileUtils::makeQgisFields( mRequiredFields[layerStyle.layerName()] );
+    QgsFields fields = mPerLayerFields[layerStyle.layerName()];
 
     QgsExpressionContextScope *scope = new QgsExpressionContextScope( QObject::tr( "Layer" ) ); // will be deleted by popper
     scope->setFields( fields );

src/core/vectortile/qgsvectortilebasiclabeling.h

@@ -150,7 +150,7 @@ class QgsVectorTileBasicLabelProvider : public QgsVectorTileLabelProvider
     // virtual functions from QgsVectorTileLabelProvider
     void registerTileFeatures( const QgsVectorTileRendererData &tile, QgsRenderContext &context ) override;
     QMap<QString, QSet<QString> > usedAttributes( const QgsRenderContext &context, int tileZoom ) const override;
-    void setFields( const QMap<QString, QSet<QString>> &requiredFields ) override;
+    void setFields( const QMap<QString, QgsFields> &perLayerFields ) override;
 
   private:
     QList<QgsVectorTileBasicLabelingStyle> mStyles;
@@ -160,7 +160,7 @@ class QgsVectorTileBasicLabelProvider : public QgsVectorTileLabelProvider
 
   public:
     //! Names of required fields for each sub-layer (only valid between startRender/stopRender calls)
-    QMap<QString, QSet<QString> > mRequiredFields;
+    QMap<QString, QgsFields> mPerLayerFields;
 };
 
 /// @endcond

src/core/vectortile/qgsvectortilebasicrenderer.cpp

@@ -151,10 +151,8 @@ void QgsVectorTileBasicRenderer::renderTile( const QgsVectorTileRendererData &ti
     if ( !layerStyle.isActive( zoomLevel ) )
       continue;
 
-    QgsFields fields = QgsVectorTileUtils::makeQgisFields( mRequiredFields[layerStyle.layerName()] );
-
     QgsExpressionContextScope *scope = new QgsExpressionContextScope( QObject::tr( "Layer" ) ); // will be deleted by popper
-    scope->setFields( fields );
+    scope->setFields( tile.fields()[layerStyle.layerName()] );
     QgsExpressionContextScopePopper popper( context.expressionContext(), scope );
 
     QgsExpression filterExpression( layerStyle.filterExpression() );

src/core/vectortile/qgsvectortilelabeling.h

@@ -39,8 +39,8 @@ class QgsVectorTileLabelProvider : public QgsVectorLayerLabelProvider
     //! Returns field names for each sub-layer that are required for labeling
     virtual QMap<QString, QSet<QString> > usedAttributes( const QgsRenderContext &context, int tileZoom ) const = 0;
 
-    //! Sets required fields
-    virtual void setFields( const QMap<QString, QSet<QString>> &requiredFields ) = 0;
+    //! Sets fields for each sub-layer
+    virtual void setFields( const QMap<QString, QgsFields> &perLayerFields ) = 0;
 
     //! Registers label features for given tile to the labeling engine
     virtual void registerTileFeatures( const QgsVectorTileRendererData &tile, QgsRenderContext &context ) = 0;

src/core/vectortile/qgsvectortilelayerrenderer.cpp

@@ -137,7 +137,7 @@ bool QgsVectorTileLayerRenderer::render()
 
   if ( mLabelProvider )
   {
-    mLabelProvider->setFields( requiredFields );
+    mLabelProvider->setFields( mPerLayerFields );
     QSet<QString> attributeNames;  // we don't need this - already got referenced columns in provider constructor
     if ( !mLabelProvider->prepare( ctx, attributeNames ) )
     {
@@ -197,6 +197,7 @@ void QgsVectorTileLayerRenderer::decodeAndDrawTile( const QgsVectorTileRawData &
   QgsCoordinateTransform ct = ctx.coordinateTransform();
 
   QgsVectorTileRendererData tile( rawTile.id );
+  tile.setFields( mPerLayerFields );
   tile.setFeatures( decoder.layerFeatures( mPerLayerFields, ct ) );
   tile.setTilePolygon( QgsVectorTileUtils::tilePolygon( rawTile.id, ct, mTileMatrix, ctx.mapToPixel() ) );
 

src/core/vectortile/qgsvectortilerenderer.h

@@ -48,6 +48,11 @@ class CORE_EXPORT QgsVectorTileRendererData
     //! Returns polygon (made out of four corners of the tile) in screen coordinates calculated from render context
     QPolygon tilePolygon() const { return mTilePolygon; }
 
+    //! Sets per-layer fields
+    void setFields( const QMap<QString, QgsFields> &fields ) { mFields = fields; }
+    //! Returns per-layer fields
+    QMap<QString, QgsFields> fields() const { return mFields; }
+
     //! Sets features of the tile
     void setFeatures( const QgsVectorTileFeatures &features ) SIP_SKIP { mFeatures = features; }
     //! Returns features of the tile grouped by sub-layer names
@@ -60,6 +65,8 @@ class CORE_EXPORT QgsVectorTileRendererData
   private:
     //! Position of the tile in the tile matrix set
     QgsTileXYZ mId;
+    //! Per-layer fields
+    QMap<QString, QgsFields> mFields;
     //! Features of the tile grouped into sub-layers
     QgsVectorTileFeatures mFeatures;
     //! Polygon (made out of four corners of the tile) in screen coordinates calculated from render context