[FEATURE] Give access to geometry errors

When methods are called that use GEOS to create new geometries, the
result geometries now contain information about what has gone wrong in
case of an error.

In practice, this means it's possible to give more detailed information
in place (and not only in the message log) when things like processing
algorithms fail.

python/core/geometry/qgsgeometry.sip

@@ -716,6 +716,12 @@ Returns a simplified version of this geometry using a specified tolerance value
     QgsGeometry centroid() const;
 %Docstring
  Returns the center of mass of a geometry.
+
+ If the input is a NULL geometry, the output will also be a NULL geometry.
+
+ If an error was encountered while creating the result, more information can be retrieved
+ by calling `error()` on the returned geometry.
+
 .. note::
 
    for line based geometries, the center point of the line is returned,
@@ -730,6 +736,12 @@ Returns a simplified version of this geometry using a specified tolerance value
  Returns a point guaranteed to lie on the surface of a geometry. While the centroid()
  of a geometry may be located outside of the geometry itself (e.g., for concave shapes),
  the point on surface will always be inside the geometry.
+
+ If the input is a NULL geometry, the output will also be a NULL geometry.
+
+ If an error was encountered while creating the result, more information can be retrieved
+ by calling `error()` on the returned geometry.
+
 .. seealso:: centroid()
 .. seealso:: poleOfInaccessibility()
  :rtype: QgsGeometry
@@ -752,7 +764,12 @@ Returns a simplified version of this geometry using a specified tolerance value
 
     QgsGeometry convexHull() const;
 %Docstring
-Returns the smallest convex polygon that contains all the points in the geometry.
+ Returns the smallest convex polygon that contains all the points in the geometry.
+
+ If the input is a NULL geometry, the output will also be a NULL geometry.
+
+ If an error was encountered while creating the result, more information can be retrieved
+ by calling `error()` on the returned geometry.
  :rtype: QgsGeometry
 %End
 
@@ -799,14 +816,25 @@ Returns the smallest convex polygon that contains all the points in the geometry
 
  Curved geometries will be segmentized before subdivision.
 
+ If the input is a NULL geometry, the output will also be a NULL geometry.
+
+ If an error was encountered while creating the result, more information can be retrieved
+ by calling `error()` on the returned geometry.
+
 .. versionadded:: 3.0
  :rtype: QgsGeometry
 %End
 
     QgsGeometry interpolate( double distance ) const;
 %Docstring
- Return interpolated point on line at distance
-.. versionadded:: 1.9
+ Return interpolated point on line at distance.
+
+ If the input is a NULL geometry, the output will also be a NULL geometry.
+
+ If an error was encountered while creating the result, more information can be retrieved
+ by calling `error()` on the returned geometry.
+
+.. versionadded:: 2.0
 .. seealso:: lineLocatePoint()
  :rtype: QgsGeometry
 %End
@@ -841,7 +869,12 @@ Returns the smallest convex polygon that contains all the points in the geometry
 
     QgsGeometry intersection( const QgsGeometry &geometry ) const;
 %Docstring
-Returns a geometry representing the points shared by this geometry and other.
+ Returns a geometry representing the points shared by this geometry and other.
+
+ If the input is a NULL geometry, the output will also be a NULL geometry.
+
+ If an error was encountered while creating the result, more information can be retrieved
+ by calling `error()` on the returned geometry.
  :rtype: QgsGeometry
 %End
 
@@ -859,6 +892,12 @@ Returns a geometry representing the points shared by this geometry and other.
 %Docstring
  Returns a geometry representing all the points in this geometry and other (a
  union geometry operation).
+
+ If the input is a NULL geometry, the output will also be a NULL geometry.
+
+ If an error was encountered while creating the result, more information can be retrieved
+ by calling `error()` on the returned geometry.
+
 .. note::
 
    this operation is not called union since its a reserved word in C++.
@@ -878,13 +917,23 @@ Returns a geometry representing the points shared by this geometry and other.
 
     QgsGeometry difference( const QgsGeometry &geometry ) const;
 %Docstring
-Returns a geometry representing the points making up this geometry that do not make up other.
+ Returns a geometry representing the points making up this geometry that do not make up other.
+
+ If the input is a NULL geometry, the output will also be a NULL geometry.
+
+ If an error was encountered while creating the result, more information can be retrieved
+ by calling `error()` on the returned geometry.
  :rtype: QgsGeometry
 %End
 
     QgsGeometry symDifference( const QgsGeometry &geometry ) const;
 %Docstring
-Returns a geometry representing the points making up this geometry that do not make up other.
+ Returns a geometry representing the points making up this geometry that do not make up other.
+
+ If the input is a NULL geometry, the output will also be a NULL geometry.
+
+ If an error was encountered while creating the result, more information can be retrieved
+ by calling `error()` on the returned geometry.
  :rtype: QgsGeometry
 %End
 
@@ -1184,6 +1233,15 @@ Ring 0 is outer ring and can't be deleted.
  :rtype: int
 %End
 
+    QString error() const;
+%Docstring
+ Returns an error string referring to an error that was produced
+ when this geometry was created.
+
+.. versionadded:: 3.0
+ :rtype: str
+%End
+
 
     static QgsGeometry fromQPointF( QPointF point );
 %Docstring

src/core/geometry/qgsgeometry.cpp

@@ -51,6 +51,7 @@ struct QgsGeometryPrivate
   ~QgsGeometryPrivate() { delete geometry; }
   QAtomicInt ref;
   QgsAbstractGeometry *geometry = nullptr;
+  QString error;
 };
 
 QgsGeometry::QgsGeometry(): d( new QgsGeometryPrivate() )
@@ -1586,10 +1587,13 @@ QgsGeometry QgsGeometry::centroid() const
 
   QgsGeos geos( d->geometry );
   QgsPoint centroid;
-  bool ok = geos.centroid( centroid );
+  QString error;
+  bool ok = geos.centroid( centroid, &error );
   if ( !ok )
   {
-    return QgsGeometry();
+    QgsGeometry geom;
+    geom.d->error = error;
+    return geom;
   }
   return QgsGeometry( centroid.clone() );
 }
@@ -1602,13 +1606,17 @@ QgsGeometry QgsGeometry::pointOnSurface() const
   }
 
   QgsGeos geos( d->geometry );
-  QgsPoint pt;
-  bool ok = geos.pointOnSurface( pt );
+  std::unique_ptr<QgsPoint>pt( new QgsPoint() );
+
+  QString error;
+  bool ok = geos.pointOnSurface( *pt.get(), &error );
   if ( !ok )
   {
-    return QgsGeometry();
+    QgsGeometry geom;
+    geom.d->error = error;
+    return geom;
   }
-  return QgsGeometry( pt.clone() );
+  return QgsGeometry( pt.release() );
 }
 
 QgsGeometry QgsGeometry::poleOfInaccessibility( double precision, double *distanceToBoundary ) const
@@ -1625,10 +1633,13 @@ QgsGeometry QgsGeometry::convexHull() const
     return QgsGeometry();
   }
   QgsGeos geos( d->geometry );
-  QgsAbstractGeometry *cHull = geos.convexHull();
+  QString error;
+  QgsAbstractGeometry *cHull = geos.convexHull( &error );
   if ( !cHull )
   {
-    return QgsGeometry();
+    QgsGeometry geom;
+    geom.d->error = error;
+    return geom;
   }
   return QgsGeometry( cHull );
 }
@@ -1670,11 +1681,14 @@ QgsGeometry QgsGeometry::subdivide( int maxNodes ) const
     geom = segmentizedCopy.get();
   }
 
+  QString error;
   QgsGeos geos( geom );
-  QgsAbstractGeometry *result = geos.subdivide( maxNodes );
+  QgsAbstractGeometry *result = geos.subdivide( maxNodes, &error );
   if ( !result )
   {
-    return QgsGeometry();
+    QgsGeometry geom;
+    geom.d->error = error;
+    return geom;
   }
   return QgsGeometry( result );
 }
@@ -1691,10 +1705,13 @@ QgsGeometry QgsGeometry::interpolate( double distance ) const
     line = QgsGeometry( d->geometry->boundary() );
 
   QgsGeos geos( line.geometry() );
-  QgsAbstractGeometry *result = geos.interpolate( distance );
+  QString error;
+  QgsAbstractGeometry *result = geos.interpolate( distance, &error );
   if ( !result )
   {
-    return QgsGeometry();
+    QgsGeometry geom;
+    geom.d->error = error;
+    return geom;
   }
   return QgsGeometry( result );
 }
@@ -1780,7 +1797,16 @@ QgsGeometry QgsGeometry::intersection( const QgsGeometry &geometry ) const
 
   QgsGeos geos( d->geometry );
 
-  QgsAbstractGeometry *resultGeom = geos.intersection( *( geometry.d->geometry ) );
+  QString error;
+  QgsAbstractGeometry *resultGeom = geos.intersection( *( geometry.d->geometry ), &error );
+
+  if ( !resultGeom )
+  {
+    QgsGeometry geom;
+    geom.d->error = error;
+    return geom;
+  }
+
   return QgsGeometry( resultGeom );
 }
 
@@ -1792,11 +1818,14 @@ QgsGeometry QgsGeometry::combine( const QgsGeometry &geometry ) const
   }
 
   QgsGeos geos( d->geometry );
+  QString error;
 
-  QgsAbstractGeometry *resultGeom = geos.combine( *( geometry.d->geometry ) );
+  QgsAbstractGeometry *resultGeom = geos.combine( *( geometry.d->geometry ), &error );
   if ( !resultGeom )
   {
-    return QgsGeometry();
+    QgsGeometry geom;
+    geom.d->error = error;
+    return geom;
   }
   return QgsGeometry( resultGeom );
 }
@@ -1827,10 +1856,13 @@ QgsGeometry QgsGeometry::difference( const QgsGeometry &geometry ) const
 
   QgsGeos geos( d->geometry );
 
-  QgsAbstractGeometry *resultGeom = geos.difference( *( geometry.d->geometry ) );
+  QString error;
+  QgsAbstractGeometry *resultGeom = geos.difference( *( geometry.d->geometry ), &error );
   if ( !resultGeom )
   {
-    return QgsGeometry();
+    QgsGeometry geom;
+    geom.d->error = error;
+    return geom;
   }
   return QgsGeometry( resultGeom );
 }
@@ -1844,10 +1876,14 @@ QgsGeometry QgsGeometry::symDifference( const QgsGeometry &geometry ) const
 
   QgsGeos geos( d->geometry );
 
-  QgsAbstractGeometry *resultGeom = geos.symDifference( *( geometry.d->geometry ) );
+  QString error;
+
+  QgsAbstractGeometry *resultGeom = geos.symDifference( *( geometry.d->geometry ), &error );
   if ( !resultGeom )
   {
-    return QgsGeometry();
+    QgsGeometry geom;
+    geom.d->error = error;
+    return geom;
   }
   return QgsGeometry( resultGeom );
 }
@@ -2275,6 +2311,11 @@ int QgsGeometry::vertexNrFromVertexId( QgsVertexId id ) const
   return -1;
 }
 
+QString QgsGeometry::error() const
+{
+  return d->error;
+}
+
 void QgsGeometry::convertPointList( const QList<QgsPointXY> &input, QgsPointSequence &output )
 {
   output.clear();