Thread safety for get_feature

Author: m-kuhn

python/core/auto_generated/qgsvectorlayerfeatureiterator.sip.in

@@ -52,6 +52,13 @@ this source.
 Returns the coordinate reference system for features retrieved from this source.
 
 .. versionadded:: 3.0
+%End
+
+    QString id() const;
+%Docstring
+Returns the layer id of the source layer.
+
+.. versionadded:: 3.4
 %End
 
   protected:
@@ -63,6 +70,7 @@ Returns the coordinate reference system for features retrieved from this source.
 
 
 
+
 };
 
 

src/core/expression/qgsexpressionfunction.cpp

@@ -43,6 +43,7 @@
 #include "qgscolorramp.h"
 #include "qgsfieldformatterregistry.h"
 #include "qgsfieldformatter.h"
+#include "qgsvectorlayerfeatureiterator.h"
 
 const QString QgsExpressionFunction::helpText() const
 {
@@ -3582,24 +3583,25 @@ static QVariant fcnGetFeatureById( const QVariantList &values, const QgsExpressi
 static QVariant fcnGetFeature( const QVariantList &values, const QgsExpressionContext *context, QgsExpression *parent, const QgsExpressionNodeFunction * )
 {
   //arguments: 1. layer id / name, 2. key attribute, 3. eq value
-  QgsVectorLayer *vl = QgsExpressionUtils::getVectorLayer( values.at( 0 ), parent );
+
+  std::unique_ptr<QgsVectorLayerFeatureSource> featureSource = QgsExpressionUtils::getFeatureSource( values.at( 0 ), parent );
 
   //no layer found
-  if ( !vl )
+  if ( !featureSource )
   {
     return QVariant();
   }
 
   QString attribute = QgsExpressionUtils::getStringValue( values.at( 1 ), parent );
-  int attributeId = vl->fields().lookupField( attribute );
+  int attributeId = featureSource->fields().lookupField( attribute );
   if ( attributeId == -1 )
   {
     return QVariant();
   }
 
   const QVariant &attVal = values.at( 2 );
 
-  const QString cacheValueKey = QStringLiteral( "getfeature:%1:%2:%3" ).arg( vl->id(), QString::number( attributeId ), attVal.toString() );
+  const QString cacheValueKey = QStringLiteral( "getfeature:%1:%2:%3" ).arg( featureSource->id(), QString::number( attributeId ), attVal.toString() );
   if ( context && context->hasCachedValue( cacheValueKey ) )
   {
     return context->cachedValue( cacheValueKey );
@@ -3613,7 +3615,7 @@ static QVariant fcnGetFeature( const QVariantList &values, const QgsExpressionCo
   {
     req.setFlags( QgsFeatureRequest::NoGeometry );
   }
-  QgsFeatureIterator fIt = vl->getFeatures( req );
+  QgsFeatureIterator fIt = featureSource->getFeatures( req );
 
   QgsFeature fet;
   QVariant res;

src/core/expression/qgsexpressionutils.h

@@ -23,6 +23,7 @@
 #include "qgsexpression.h"
 #include "qgscolorramp.h"
 #include "qgsvectorlayer.h"
+#include "qgsvectorlayerfeatureiterator.h"
 #include "qgsrasterlayer.h"
 #include "qgsproject.h"
 #include "qgsrelationmanager.h"
@@ -351,6 +352,34 @@ class QgsExpressionUtils
       return ml;
     }
 
+    static std::unique_ptr<QgsVectorLayerFeatureSource> getFeatureSource( const QVariant &value, QgsExpression *e )
+    {
+      std::unique_ptr<QgsVectorLayerFeatureSource> featureSource;
+
+      auto getFeatureSource = [ &value, e, &featureSource ]
+      {
+        QgsVectorLayer *layer = getVectorLayer( value, e );
+
+        if ( layer )
+        {
+          featureSource.reset( new QgsVectorLayerFeatureSource( layer ) );
+        }
+      };
+
+#if QT_VERSION >= QT_VERSION_CHECK( 5, 10, 0 )
+      // Make sure we only deal with the vector layer on the main thread where it lives.
+      // Anything else risks a crash.
+      if ( QThread::currentThread() == qApp->thread() )
+        getFeatureSource();
+      else
+        QMetaObject::invokeMethod( qApp, getFeatureSource, Qt::BlockingQueuedConnection );
+#else
+      getFeatureSource();
+#endif
+
+      return featureSource;
+    }
+
     static QgsVectorLayer *getVectorLayer( const QVariant &value, QgsExpression *e )
     {
       return qobject_cast<QgsVectorLayer *>( getMapLayer( value, e ) );

src/core/qgsvectorlayerfeatureiterator.cpp

@@ -32,6 +32,7 @@ QgsVectorLayerFeatureSource::QgsVectorLayerFeatureSource( const QgsVectorLayer *
   QMutexLocker locker( &layer->mFeatureSourceConstructorMutex );
   mProviderFeatureSource = layer->dataProvider()->featureSource();
   mFields = layer->fields();
+  mId = layer->id();
 
   // update layer's join caches if necessary
   if ( layer->mJoinBuffer->containsJoins() )
@@ -107,6 +108,11 @@ QgsCoordinateReferenceSystem QgsVectorLayerFeatureSource::crs() const
   return mCrs;
 }
 
+QString QgsVectorLayerFeatureSource::id() const
+{
+  return mId;
+}
+
 
 QgsVectorLayerFeatureIterator::QgsVectorLayerFeatureIterator( QgsVectorLayerFeatureSource *source, bool ownSource, const QgsFeatureRequest &request )
   : QgsAbstractFeatureIteratorFromSource<QgsVectorLayerFeatureSource>( source, ownSource, request )

src/core/qgsvectorlayerfeatureiterator.h

@@ -78,6 +78,13 @@ class CORE_EXPORT QgsVectorLayerFeatureSource : public QgsAbstractFeatureSource
      */
     QgsCoordinateReferenceSystem crs() const;
 
+    /**
+     * Returns the layer id of the source layer.
+     *
+     * \since QGIS 3.4
+     */
+    QString id() const;
+
   protected:
 
     QgsAbstractFeatureSource *mProviderFeatureSource = nullptr;
@@ -88,6 +95,8 @@ class CORE_EXPORT QgsVectorLayerFeatureSource : public QgsAbstractFeatureSource
 
     QgsFields mFields;
 
+    QString mId;
+
     QgsExpressionContextScope mLayerScope;
 
     bool mHasEditBuffer;