[pal] Fix invalid candidates created for parallel line labeling

mode when a closed linestring is too small for labels to fit
within feature

Fixes #18283

src/core/pal/feature.cpp

@@ -856,12 +856,17 @@ int FeaturePart::createCandidatesAlongLineNearMidpoint( QList<LabelPosition *> &
   {
     lineStepDistance = std::min( std::min( labelHeight, labelWidth ), lineStepDistance / mLF->layer()->pal->line_p );
   }
-  else // line length < label width => centering label position
+  else if ( !line->isClosed() ) // line length < label width => centering label position
   {
     currentDistanceAlongLine = - ( labelWidth - totalLineLength ) / 2.0;
     lineStepDistance = -1;
     totalLineLength = labelWidth;
   }
+  else
+  {
+    // closed line, not long enough for label => no candidates!
+    currentDistanceAlongLine = std::numeric_limits< double >::max();
+  }
 
   double candidateLength;
   double beta;

src/core/pal/pointset.cpp

@@ -846,3 +846,8 @@ double PointSet::length() const
     return -1;
   }
 }
+
+bool PointSet::isClosed() const
+{
+  return qgsDoubleNear( x[0], x[nbPoints - 1] ) && qgsDoubleNear( y[0], y[nbPoints - 1] );
+}

src/core/pal/pointset.h

@@ -156,13 +156,18 @@ namespace pal
        */
       double length() const;
 
+      /**
+       * Returns true if pointset is closed.
+       */
+      bool isClosed() const;
+
     protected:
       mutable GEOSGeometry *mGeos = nullptr;
       mutable bool mOwnsGeom = false;
 
       int nbPoints;
       double *x = nullptr;
-      double *y;   // points order is counterclockwise
+      double *y = nullptr;   // points order is counterclockwise
 
       int *cHull = nullptr;
       int cHullSize;