Fix labeling ignores "label per part" setting when geometry parts

are adjacent

Fixes #26763

src/core/qgspallabeling.cpp

@@ -60,6 +60,7 @@
 #include "qgsmaptopixelgeometrysimplifier.h"
 #include "qgscurvepolygon.h"
 #include "qgsmessagelog.h"
+#include "qgsgeometrycollection.h"
 #include <QMessageBox>
 
 using namespace pal;
@@ -3115,15 +3116,37 @@ QgsGeometry QgsPalLabeling::prepareGeometry( const QgsGeometry &geometry, QgsRen
   }
 
   // fix invalid polygons
-  if ( geom.type() == QgsWkbTypes::PolygonGeometry && !geom.isGeosValid() )
+  if ( geom.type() == QgsWkbTypes::PolygonGeometry )
   {
-    QgsGeometry bufferGeom = geom.buffer( 0, 0 );
-    if ( bufferGeom.isNull() )
+    if ( geom.isMultipart() )
     {
-      QgsDebugMsg( QStringLiteral( "Could not repair geometry: %1" ).arg( bufferGeom.lastError() ) );
-      return QgsGeometry();
+      // important -- we need to treat ever part in isolation here. We can't test the validity of the whole geometry
+      // at once, because touching parts would result in an invalid geometry, and buffering this "dissolves" the parts.
+      // because the actual label engine treats parts as separate entities, we aren't bound by the usual "touching parts are invalid" rule
+      // see https://github.com/qgis/QGIS/issues/26763
+      QVector< QgsGeometry> parts;
+      parts.reserve( qgsgeometry_cast< const QgsGeometryCollection * >( geom.constGet() )->numGeometries() );
+      for ( auto it = geom.const_parts_begin(); it != geom.const_parts_end(); ++it )
+      {
+        QgsGeometry partGeom( ( *it )->clone() );
+        if ( !partGeom.isGeosValid() )
+        {
+          partGeom = partGeom.buffer( 0, 0 );
+        }
+        parts.append( partGeom );
+      }
+      geom = QgsGeometry::collectGeometry( parts );
+    }
+    else if ( !geom.isGeosValid() )
+    {
+      QgsGeometry bufferGeom = geom.buffer( 0, 0 );
+      if ( bufferGeom.isNull() )
+      {
+        QgsDebugMsg( QStringLiteral( "Could not repair geometry: %1" ).arg( bufferGeom.lastError() ) );
+        return QgsGeometry();
+      }
+      geom = bufferGeom;
     }
-    geom = bufferGeom;
   }
 
   if ( !clipGeometry.isNull() &&

tests/src/core/testqgslabelingengine.cpp

@@ -52,6 +52,7 @@ class TestQgsLabelingEngine : public QObject
     void testRegisterFeatureUnprojectible();
     void testRotateHidePartial();
     void testParallelLabelSmallFeature();
+    void testAdjacentParts();
     void testLabelBoundary();
     void testLabelBlockingRegion();
 
@@ -807,6 +808,57 @@ void TestQgsLabelingEngine::testParallelLabelSmallFeature()
   //  QVERIFY( imageCheck( "label_rotate_hide_partial", img, 20 ) );
 }
 
+void TestQgsLabelingEngine::testAdjacentParts()
+{
+  // test combination of map rotation with reprojected layer
+  QgsPalLayerSettings settings;
+  setDefaultLabelParams( settings );
+
+  QgsTextFormat format = settings.format();
+  format.setSize( 20 );
+  format.setColor( QColor( 0, 0, 0 ) );
+  settings.setFormat( format );
+
+  settings.fieldName = QStringLiteral( "'X'" );
+  settings.isExpression = true;
+  settings.placement = QgsPalLayerSettings::OverPoint;
+  settings.labelPerPart = true;
+
+  std::unique_ptr< QgsVectorLayer> vl2( new QgsVectorLayer( QStringLiteral( "Polygon?crs=epsg:3946&field=id:integer" ), QStringLiteral( "vl" ), QStringLiteral( "memory" ) ) );
+  vl2->setRenderer( new QgsNullSymbolRenderer() );
+
+  QgsFeature f;
+  f.setAttributes( QgsAttributes() << 1 );
+  f.setGeometry( QgsGeometry::fromWkt( QStringLiteral( "MultiPolygon (((1967901.6872910603415221 5162590.11975561361759901, 1967905.31832842249423265 5162591.80023225769400597, 1967907.63076798897236586 5162586.43503414187580347, 1967903.84105980419553816 5162584.57283254805952311, 1967901.6872910603415221 5162590.11975561361759901)),((1967901.64785283687524498 5162598.3270823871716857, 1967904.82891705213114619 5162601.06552503909915686, 1967910.82140435534529388 5162587.99774718284606934, 1967907.63076798897236586 5162586.43503414187580347, 1967905.31832842249423265 5162591.80023225769400597, 1967901.6872910603415221 5162590.11975561361759901, 1967899.27472299290820956 5162596.28855143301188946, 1967901.64785283687524498 5162598.3270823871716857)),((1967904.82891705213114619 5162601.06552503909915686, 1967901.64785283687524498 5162598.3270823871716857, 1967884.28552994946949184 5162626.09785370342433453, 1967895.81538487318903208 5162633.84423183929175138, 1967901.64141261484473944 5162624.63927845563739538, 1967906.47453573765233159 5162616.87410452589392662, 1967913.7844126324634999 5162604.47178338281810284, 1967909.58057221467606723 5162602.89022256527096033, 1967904.82891705213114619 5162601.06552503909915686)))" ) ) );
+  QVERIFY( vl2->dataProvider()->addFeature( f ) );
+
+  vl2->setLabeling( new QgsVectorLayerSimpleLabeling( settings ) );  // TODO: this should not be necessary!
+  vl2->setLabelsEnabled( true );
+
+  // make a fake render context
+  QSize size( 640, 480 );
+  QgsMapSettings mapSettings;
+  mapSettings.setDestinationCrs( vl2->crs() );
+
+  mapSettings.setOutputSize( size );
+  mapSettings.setExtent( f.geometry().boundingBox() );
+  mapSettings.setLayers( QList<QgsMapLayer *>() << vl2.get() );
+  mapSettings.setOutputDpi( 96 );
+
+  QgsLabelingEngineSettings engineSettings = mapSettings.labelingEngineSettings();
+  engineSettings.setFlag( QgsLabelingEngineSettings::UsePartialCandidates, false );
+  engineSettings.setFlag( QgsLabelingEngineSettings::DrawLabelRectOnly, true );
+  //engineSettings.setFlag( QgsLabelingEngineSettings::DrawCandidates, true );
+  mapSettings.setLabelingEngineSettings( engineSettings );
+
+  QgsMapRendererSequentialJob job( mapSettings );
+  job.start();
+  job.waitForFinished();
+
+  QImage img = job.renderedImage();
+  QVERIFY( imageCheck( QStringLiteral( "label_adjacent_parts" ), img, 20 ) );
+}
+
 void TestQgsLabelingEngine::testLabelBoundary()
 {
   // test that no labels are drawn outside of the specified label boundary