Enable Python in background threads. Fixed drawing of plugin layers.

Author: wonder-sk

python/core/qgspluginlayer.sip

@@ -17,4 +17,15 @@ class QgsPluginLayer : QgsMapLayer
     //! (defult implementation returns nothing)
     //! @note Added in v2.1
     virtual QgsLegendSymbologyList legendSymbologyItems( const QSize& iconSize );
+
+    /** Return new instance of QgsMapLayerRenderer that will be used for rendering of given context
+     *
+     * The default implementation returns map layer renderer which just calls draw().
+     * This may work, but it is unsafe for multi-threaded rendering because of the run
+     * conditions that may happen (e.g. something is changed in the layer while it is
+     * being rendered).
+     *
+     * @note added in 2.1
+     */
+    virtual QgsMapLayerRenderer* createMapRenderer( QgsRenderContext& rendererContext ) /Factory/;
 };

src/core/qgspluginlayer.cpp

@@ -14,6 +14,8 @@
  ***************************************************************************/
 #include "qgspluginlayer.h"
 
+#include "qgsmaplayerrenderer.h"
+
 QgsPluginLayer::QgsPluginLayer( QString layerType, QString layerName )
     : QgsMapLayer( PluginLayer, layerName ), mPluginLayerType( layerType )
 {
@@ -34,3 +36,28 @@ QgsLegendSymbologyList QgsPluginLayer::legendSymbologyItems( const QSize& iconSi
   Q_UNUSED( iconSize );
   return QgsLegendSymbologyList();
 }
+
+/** Fallback layer renderer implementation for layer that do not support map renderer yet. */
+class QgsPluginLayerRenderer : public QgsMapLayerRenderer
+{
+public:
+  QgsPluginLayerRenderer( QgsPluginLayer* layer, QgsRenderContext& rendererContext )
+    : QgsMapLayerRenderer( layer->id() )
+    , mLayer( layer )
+    , mRendererContext( rendererContext )
+  {}
+
+  virtual bool render()
+  {
+    return mLayer->draw( mRendererContext );
+  }
+
+protected:
+  QgsPluginLayer* mLayer;
+  QgsRenderContext& mRendererContext;
+};
+
+QgsMapLayerRenderer* QgsPluginLayer::createMapRenderer( QgsRenderContext& rendererContext )
+{
+  return new QgsPluginLayerRenderer( this, rendererContext );
+}

src/core/qgspluginlayer.h

@@ -46,6 +46,17 @@ class CORE_EXPORT QgsPluginLayer : public QgsMapLayer
     //! @note Added in v2.1
     virtual QgsLegendSymbologyList legendSymbologyItems( const QSize& iconSize );
 
+    /** Return new instance of QgsMapLayerRenderer that will be used for rendering of given context
+     *
+     * The default implementation returns map layer renderer which just calls draw().
+     * This may work, but it is unsafe for multi-threaded rendering because of the run
+     * conditions that may happen (e.g. something is changed in the layer while it is
+     * being rendered).
+     *
+     * @note added in 2.1
+     */
+    virtual QgsMapLayerRenderer* createMapRenderer( QgsRenderContext& rendererContext );
+
   protected:
     QString mPluginLayerType;
 };

src/python/qgspythonutilsimpl.cpp

@@ -33,6 +33,7 @@
 #include <QStringList>
 #include <QDir>
 
+PyThreadState* _mainState;
 
 QgsPythonUtilsImpl::QgsPythonUtilsImpl()
 {
@@ -53,6 +54,9 @@ void QgsPythonUtilsImpl::initPython( QgisInterface* interface )
   // initialize python
   Py_Initialize();
 
+  // initialize threading AND acquire GIL
+  PyEval_InitThreads();
+
   mPythonEnabled = true;
 
   mMainModule = PyImport_AddModule( "__main__" ); // borrowed reference
@@ -167,6 +171,13 @@ void QgsPythonUtilsImpl::initPython( QgisInterface* interface )
 
   QString startuppath = homePythonPath() + " + \"/startup.py\"";
   runString( "if os.path.exists(" + startuppath + "): from startup import *\n" );
+
+  // release GIL!
+  // Later on, we acquire GIL just before doing some Python calls and
+  // release GIL again when the work with Python API is done.
+  // (i.e. there must be PyGILState_Ensure + PyGILState_Release pair
+  // around any calls to Python API, otherwise we may segfault!)
+  _mainState = PyEval_SaveThread();
 }
 
 void QgsPythonUtilsImpl::exitPython()