Fix memory layers with spatial index crash when rendering

Fixes #17705

src/core/providers/memory/qgsmemoryfeatureiterator.cpp

@@ -231,13 +231,17 @@ bool QgsMemoryFeatureIterator::close()
 QgsMemoryFeatureSource::QgsMemoryFeatureSource( const QgsMemoryProvider *p )
   : mFields( p->mFields )
   , mFeatures( p->mFeatures )
-  , mSpatialIndex( p->mSpatialIndex ? new QgsSpatialIndex( *p->mSpatialIndex ) : nullptr )  // just shallow copy
+  , mSpatialIndex( p->mSpatialIndex ? qgis::make_unique< QgsSpatialIndex >( *p->mSpatialIndex ) : nullptr ) // this is just a shallow copy, but see below...
   , mSubsetString( p->mSubsetString )
   , mCrs( p->mCrs )
 {
   mExpressionContext << QgsExpressionContextUtils::globalScope()
                      << QgsExpressionContextUtils::projectScope( QgsProject::instance() );
   mExpressionContext.setFields( mFields );
+
+  // QgsSpatialIndex is not thread safe - so make spatial index safe to use across threads by forcing a full deep copy
+  if ( mSpatialIndex )
+    mSpatialIndex->detach();
 }
 
 QgsFeatureIterator QgsMemoryFeatureSource::getFeatures( const QgsFeatureRequest &request )