QgsGmlStreamingParser: fix quadratic performance pattern when parsing…

… huge JSON content

Addresses issue with #55847 (comment)
and parsing of "https://www.wfs.nrw.de/geobasis/wfs_nw_inspire-gewaesser-netzwerk_atkis-basis-dlm?SERVICE=WFS&REQUEST=GetFeature&VERSION=2.0.0&TYPENAMES=net:Network&COUNT=1&SRSNAME=urn:ogc:def:crs:EPSG::25832&NAMESPACES=xmlns(net,http://inspire.ec.europa.eu/schemas/net/4.0)&NAMESPACE=xmlns(net,http://inspire.ec.europa.eu/schemas/net/4.0"
which is a 44 MB single WFS feature...

src/core/qgsgml.cpp

@@ -1109,18 +1109,14 @@ void QgsGmlStreamingParser::endElement( const XML_Char *el )
 
     if ( mAttributeValIsNested )
     {
-      //find index with attribute name
-      const QMap<QString, QPair<int, QgsField> >::const_iterator att_it = mThematicAttributes.constFind( mAttributeName );
-      Q_ASSERT( mCurrentFeature );
-      const int attrIndex = att_it.value().first;
-      auto attrVal = mCurrentFeature->attribute( attrIndex );
-      if ( attrVal.isNull() )
+      auto iter = mMapFieldNameToJSONContent.find( mAttributeName );
+      if ( iter == mMapFieldNameToJSONContent.end() )
       {
-        mCurrentFeature->setAttribute( attrIndex, QString::fromStdString( mAttributeJson.dump() ) );
+        mMapFieldNameToJSONContent[mAttributeName] = QString::fromStdString( mAttributeJson.dump() );
       }
       else
       {
-        QString str = attrVal.toString();
+        QString &str = iter.value();
         if ( str[0] == '[' && str.back() == ']' )
         {
           str.back() = ',';
@@ -1132,7 +1128,6 @@ void QgsGmlStreamingParser::endElement( const XML_Char *el )
         }
         str.append( QString::fromStdString( mAttributeJson.dump() ) );
         str.append( ']' );
-        mCurrentFeature->setAttribute( attrIndex, str );
       }
     }
     else
@@ -1242,6 +1237,14 @@ void QgsGmlStreamingParser::endElement( const XML_Char *el )
     }
     mCurrentFeature->setValid( true );
 
+    for ( auto iter = mMapFieldNameToJSONContent.constBegin(); iter != mMapFieldNameToJSONContent.constEnd(); ++iter )
+    {
+      const QMap<QString, QPair<int, QgsField> >::const_iterator att_it = mThematicAttributes.constFind( iter.key() );
+      const int attrIndex = att_it.value().first;
+      mCurrentFeature->setAttribute( attrIndex, iter.value() );
+    }
+    mMapFieldNameToJSONContent.clear();
+
     mFeatureList.push_back( QgsGmlFeaturePtrGmlIdPair( mCurrentFeature, mCurrentFeatureId ) );
 
     mCurrentFeature = nullptr;

src/core/qgsgml.h

@@ -335,6 +335,8 @@ class CORE_EXPORT QgsGmlStreamingParser
     QString mAttributeName;
     int mAttributeDepth = -1;
     bool mAttributeValIsNested = false;
+    //! Map from field name to JSON content.
+    QMap< QString, QString > mMapFieldNameToJSONContent;
     nlohmann::json mAttributeJson;
     QStack<nlohmann::json *> mAttributeJsonCurrentStack;
     char mEndian;