Gracefully handle topology errors when tracing (fixes #14447)

(cherry picked from commit 11e7140)

python/core/qgstracer.sip

@@ -49,6 +49,11 @@ class QgsTracer : QObject
     //! Whether the internal data structures have been initialized
     bool isInitialized() const;
 
+    //! Whether there was an error during graph creation due to noding exception,
+    //! indicating some input data topology problems
+    //! @note added in QGIS 2.14.2
+    bool hasTopologyProblem() const;
+
     //! Possible errors that may happen when calling findShortestPath()
     enum PathError
     {

src/core/qgstracer.cpp

@@ -17,6 +17,7 @@
 
 #include "qgsgeometry.h"
 #include "qgsgeometryutils.h"
+#include "qgsgeos.h"
 #include "qgslogger.h"
 #include "qgsvectorlayer.h"
 
@@ -439,6 +440,7 @@ QgsTracer::QgsTracer()
     : mGraph( 0 )
     , mReprojectionEnabled( false )
     , mMaxFeatureCount( 0 )
+    , mHasTopologyProblem( false )
 {
 }
 
@@ -448,6 +450,8 @@ bool QgsTracer::initGraph()
   if ( mGraph )
     return true; // already initialized
 
+  mHasTopologyProblem = false;
+
   QgsFeature f;
   QgsMultiPolyline mpl;
 
@@ -499,23 +503,37 @@ bool QgsTracer::initGraph()
 
   t2.start();
 
+  int timeNodingCall = 0;
+
 #if 0
   // without noding - if data are known to be noded beforehand
-  int timeNodingCall = 0;
 #else
   QgsGeometry* allGeom = QgsGeometry::fromMultiPolyline( mpl );
 
-  t2a.start();
-  GEOSGeometry* allNoded = GEOSNode_r( QgsGeometry::getGEOSHandler(), allGeom->asGeos() );
-  int timeNodingCall = t2a.elapsed();
+  try
+  {
+    t2a.start();
+    // GEOSNode_r may throw an exception
+    GEOSGeometry* allNoded = GEOSNode_r( QgsGeometry::getGEOSHandler(), allGeom->asGeos() );
+    timeNodingCall = t2a.elapsed();
+
+    QgsGeometry* noded = new QgsGeometry;
+    noded->fromGeos( allNoded );
+    delete allGeom;
 
-  QgsGeometry* noded = new QgsGeometry;
-  noded->fromGeos( allNoded );
-  delete allGeom;
+    mpl = noded->asMultiPolyline();
 
-  mpl = noded->asMultiPolyline();
+    delete noded;
+  }
+  catch ( GEOSException &e )
+  {
+    // no big deal... we will just not have nicely noded linework, potentially
+    // missing some intersections
 
-  delete noded;
+    mHasTopologyProblem = true;
+
+    QgsDebugMsg( "Tracer Noding Exception: " + e.what() );
+  }
 #endif
 
   int timeNoding = t2.elapsed();

src/core/qgstracer.h

@@ -76,6 +76,11 @@ class CORE_EXPORT QgsTracer : public QObject
     //! Whether the internal data structures have been initialized
     bool isInitialized() const { return mGraph != nullptr; }
 
+    //! Whether there was an error during graph creation due to noding exception,
+    //! indicating some input data topology problems
+    //! @note added in QGIS 2.14.2
+    bool hasTopologyProblem() const { return mHasTopologyProblem; }
+
     //! Possible errors that may happen when calling findShortestPath()
     enum PathError
     {
@@ -127,6 +132,9 @@ class CORE_EXPORT QgsTracer : public QObject
     //! Limit of how many features can be in the graph (0 means no limit).
     //! This is to avoid possibly long graph preparation for complicated layers
     int mMaxFeatureCount;
+    //! A flag indicating that there was an error during graph creation
+    //! due to noding exception, indicating some input data topology problems
+    bool mHasTopologyProblem;
 };
 
 

src/gui/qgsmapcanvastracer.cpp

@@ -65,6 +65,11 @@ void QgsMapCanvasTracer::reportError( QgsTracer::PathError err, bool addingVerte
       break;
   }
 
+  if ( message.isEmpty() && hasTopologyProblem() )
+  {
+    message = tr( "Tracing may not work correctly. Please check topology of the input layers." );
+  }
+
   if ( message.isEmpty() )
     return;