Merge pull request #7792 from m-kuhn/threadsafefeaturesource

Add threadsafe method to get featuresource from layer

src/core/qgsvectorlayerutils.cpp

@@ -498,6 +498,37 @@ QgsFeature QgsVectorLayerUtils::duplicateFeature( QgsVectorLayer *layer, const Q
   return newFeature;
 }
 
+std::unique_ptr<QgsVectorLayerFeatureSource> QgsVectorLayerUtils::getFeatureSource( QPointer<QgsVectorLayer> layer )
+{
+  std::unique_ptr<QgsVectorLayerFeatureSource> featureSource;
+
+  auto getFeatureSource = [ layer, &featureSource ]
+  {
+#if QT_VERSION >= QT_VERSION_CHECK( 5, 10, 0 )
+    Q_ASSERT( QThread::currentThread() == qApp->thread() );
+#endif
+    QgsVectorLayer *lyr = layer.data();
+
+    if ( lyr )
+    {
+      featureSource.reset( new QgsVectorLayerFeatureSource( lyr ) );
+    }
+  };
+
+#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;
+}
+
 QList<QgsVectorLayer *> QgsVectorLayerUtils::QgsDuplicateFeatureContext::layers() const
 {
   QList<QgsVectorLayer *> layers;

src/core/qgsvectorlayerutils.h

@@ -19,6 +19,7 @@
 #include "qgis_core.h"
 #include "qgsvectorlayer.h"
 #include "qgsgeometry.h"
+#include "qgsvectorlayerfeatureiterator.h"
 
 /**
  * \ingroup core
@@ -155,6 +156,18 @@ class CORE_EXPORT QgsVectorLayerUtils
      */
     static QgsFeature duplicateFeature( QgsVectorLayer *layer, const QgsFeature &feature, QgsProject *project, int depth, QgsDuplicateFeatureContext &duplicateFeatureContext SIP_OUT );
 
+    /**
+     * Gets the feature source from a QgsVectorLayer pointer.
+     * This method is thread-safe but will block the main thread for execution. Executing it from the main
+     * thread is safe too.
+     * This should be used in scenarios, where a ``QWeakPointer<QgsVectorLayer>`` is kept in a thread
+     * and features should be fetched from this layer. Using the layer directly is not safe to do.
+     * The result will be ``nullptr`` if the layer has been deleted.
+     *
+     * \note Requires Qt >= 5.10 to make use of the thread-safe implementation
+     * \since QGIS 3.4
+     */
+    static std::unique_ptr<QgsVectorLayerFeatureSource> getFeatureSource( QPointer<QgsVectorLayer> layer ) SIP_SKIP;
 };
 
 

tests/src/core/CMakeLists.txt

@@ -192,6 +192,7 @@ SET(TESTS
  testqgsvectorlayercache.cpp
  testqgsvectorlayerjoinbuffer.cpp
  testqgsvectorlayer.cpp
+ testqgsvectorlayerutils.cpp
  testziplayer.cpp
  testqgsmeshlayer.cpp
  testqgsmeshlayerrenderer.cpp

tests/src/core/testqgsvectorlayerutils.cpp

@@ -0,0 +1,129 @@
+/***************************************************************************
+     test_template.cpp
+     --------------------------------------
+    Date                 : Sun Sep 16 12:22:23 AKDT 2007
+    Copyright            : (C) 2007 by Gary E. Sherman
+    Email                : sherman at mrcc dot com
+ ***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+#include "qgstest.h"
+
+#include "qgsvectorlayerutils.h"
+
+/**
+ * \ingroup UnitTests
+ * This is a unit test for the vector layer class.
+ */
+class TestQgsVectorLayerUtils : public QObject
+{
+    Q_OBJECT
+  public:
+    TestQgsVectorLayerUtils() = default;
+
+  private slots:
+
+    void initTestCase(); // will be called before the first testfunction is executed.
+    void cleanupTestCase(); // will be called after the last testfunction was executed.
+    void init() {} // will be called before each testfunction is executed.
+    void cleanup() {} // will be called after every testfunction.
+
+    void testGetFeatureSource();
+};
+
+void TestQgsVectorLayerUtils::initTestCase()
+{
+  QgsApplication::init();
+  QgsApplication::initQgis();
+  QgsApplication::showSettings();
+
+}
+
+void TestQgsVectorLayerUtils::cleanupTestCase()
+{
+  QgsApplication::exitQgis();
+}
+
+class FeatureFetcher : public QThread
+{
+    Q_OBJECT
+
+  public:
+    FeatureFetcher( QPointer<QgsVectorLayer> layer )
+      : mLayer( layer )
+    {
+    }
+
+    void run() override
+    {
+      QgsFeature feat;
+      auto fs = QgsVectorLayerUtils::getFeatureSource( mLayer );
+      if ( fs )
+        fs->getFeatures().nextFeature( feat );
+      emit resultReady( feat.attribute( QStringLiteral( "col1" ) ) );
+    }
+
+  signals:
+    void resultReady( const QVariant &attribute );
+
+  private:
+    QPointer<QgsVectorLayer> mLayer;
+};
+
+
+void TestQgsVectorLayerUtils::testGetFeatureSource()
+{
+  std::unique_ptr<QgsVectorLayer> vl = qgis::make_unique<QgsVectorLayer>( QStringLiteral( "Point?field=col1:integer" ), QStringLiteral( "vl" ), QStringLiteral( "memory" ) );
+  vl->startEditing();
+  QgsFeature f1( vl->fields(), 1 );
+  f1.setAttribute( QStringLiteral( "col1" ), 10 );
+  vl->addFeature( f1 );
+
+  QPointer<QgsVectorLayer> vlPtr( vl.get() );
+
+  QgsFeature feat;
+  QgsVectorLayerUtils::getFeatureSource( vlPtr )->getFeatures().nextFeature( feat );
+  QCOMPARE( feat.attribute( QStringLiteral( "col1" ) ).toInt(), 10 );
+
+  FeatureFetcher *thread = new FeatureFetcher( vlPtr );
+
+  bool finished = false;
+  QVariant result;
+
+  auto onResultReady = [&finished, &result]( const QVariant & res )
+  {
+    finished = true;
+    result = res;
+  };
+
+  connect( thread, &FeatureFetcher::resultReady, this, onResultReady );
+  connect( thread, &QThread::finished, thread, &QThread::deleteLater );
+
+  thread->start();
+  while ( !finished )
+    QCoreApplication::processEvents();
+  QCOMPARE( result.toInt(), 10 );
+  thread->quit();
+
+  FeatureFetcher *thread2 = new FeatureFetcher( vlPtr );
+
+  finished = false;
+  result = QVariant();
+  connect( thread2, &FeatureFetcher::resultReady, this, onResultReady );
+  connect( thread2, &QThread::finished, thread, &QThread::deleteLater );
+
+  vl.reset();
+  thread2->start();
+  while ( !finished )
+    QCoreApplication::processEvents();
+  QVERIFY( result.isNull() );
+  thread2->quit();
+}
+
+QGSTEST_MAIN( TestQgsVectorLayerUtils )
+#include "testqgsvectorlayerutils.moc"