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Fix map layer memory leak with bad layer handling

Long story: if the store contains a layer with the same ID
it silently fails to add it to the store.

This is the cause of leak in case client code calls
store->addLayers() within the (iterative) process of fixing
bad layers by patching the XML and re-reading it.

The proposed solution is to check in QgsProject::addLayer()
- which creates the layer object - if the newly created layer
was already in the store and delete it at the end of the
layer reading process if it was.

Deleting the layer from within the store->addLayers is
extremely dangerous, because the caller could keep a reference
to the layer and use it.
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elpaso committed Mar 26, 2019
1 parent e75308e commit bd9b3736d6f854fd6d9c75d25520105d59b801e5
Showing with 22 additions and 8 deletions.
  1. +22 −8 src/core/qgsproject.cpp
@@ -944,30 +944,30 @@ bool QgsProject::addLayer( const QDomElement &layerElem, QList<QDomNode> &broken
{
QString type = layerElem.attribute( QStringLiteral( "type" ) );
QgsDebugMsgLevel( "Layer type is " + type, 4 );
QgsMapLayer *mapLayer = nullptr;
std::unique_ptr<QgsMapLayer> mapLayer;

if ( type == QLatin1String( "vector" ) )
{
mapLayer = new QgsVectorLayer;
mapLayer = qgis::make_unique<QgsVectorLayer>();

// apply specific settings to vector layer
if ( QgsVectorLayer *vl = qobject_cast<QgsVectorLayer *>( mapLayer ) )
if ( QgsVectorLayer *vl = qobject_cast<QgsVectorLayer *>( mapLayer.get() ) )
{
vl->setReadExtentFromXml( mTrustLayerMetadata );
}
}
else if ( type == QLatin1String( "raster" ) )
{
mapLayer = new QgsRasterLayer;
mapLayer = qgis::make_unique<QgsRasterLayer>();
}
else if ( type == QLatin1String( "mesh" ) )
{
mapLayer = new QgsMeshLayer;
mapLayer = qgis::make_unique<QgsMeshLayer>();
}
else if ( type == QLatin1String( "plugin" ) )
{
QString typeName = layerElem.attribute( QStringLiteral( "name" ) );
mapLayer = QgsApplication::pluginLayerRegistry()->createLayer( typeName );
mapLayer.reset( QgsApplication::pluginLayerRegistry()->createLayer( typeName ) );
}

if ( !mapLayer )
@@ -978,13 +978,19 @@ bool QgsProject::addLayer( const QDomElement &layerElem, QList<QDomNode> &broken

Q_CHECK_PTR( mapLayer ); // NOLINT

// This is tricky: to avoid a leak we need to check if the layer was already in the store
// because if it was, the newly created layer will not be added to the store and it would leak.
const QString layerId { layerElem.namedItem( QStringLiteral( "id" ) ).toElement().text() };
Q_ASSERT( ! layerId.isEmpty() );
const bool layerWasStored { layerStore()->mapLayer( layerId ) };

// have the layer restore state that is stored in Dom node
bool layerIsValid = mapLayer->readLayerXml( layerElem, context ) && mapLayer->isValid();
QList<QgsMapLayer *> newLayers;
newLayers << mapLayer;
newLayers << mapLayer.get();
if ( layerIsValid )
{
emit readMapLayer( mapLayer, layerElem );
emit readMapLayer( mapLayer.get(), layerElem );
addMapLayers( newLayers );
}
else
@@ -995,6 +1001,14 @@ bool QgsProject::addLayer( const QDomElement &layerElem, QList<QDomNode> &broken
QgsDebugMsg( "Unable to load " + type + " layer" );
brokenNodes.push_back( layerElem );
}

// It should be safe to delete the layer now if layer was stored, because all the store
// had to to was to reset the data source in case the validity changed.
if ( ! layerWasStored )
{
mapLayer.release();
}

return layerIsValid;
}

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