Don't block when canceling canvas render jobs

In some cases canceling render jobs can take a long time. Eg when
using database layers over a sloooooow connection, canceling a job
can be blocked by minutes while waiting for the first batch of feature
fetching to finish. (Since eg postgres features are fetched in batches
of 2000 with no opportunity to abort mid-way through this).

This meant that while the first render allows the GUI to remain
responsive, any subsequent render operations which occured before
the first render completes locks up the whole ui until the first
render can finish cancellation.

With this change, the render cancelation happens with blocking.
It means that you can pan and zoom around a map over of slow
connection without any ui locks.

(cherry-picked from 3b56b79)

python/core/qgsmaprenderercustompainterjob.sip

@@ -19,6 +19,7 @@ class QgsMapRendererCustomPainterJob : QgsMapRendererJob
 
     virtual void start();
     virtual void cancel();
+    virtual void cancelWithoutBlocking();
     virtual void waitForFinished();
     virtual bool isActive() const;
     virtual QgsLabelingResults* takeLabelingResults() /Transfer/;

python/core/qgsmaprendererjob.sip

@@ -56,6 +56,8 @@ class QgsMapRendererJob : QObject
     //! Does nothing if the rendering is not active.
     virtual void cancel() = 0;
 
+    virtual void cancelWithoutBlocking() = 0;
+
     //! Block until the job has finished.
     virtual void waitForFinished() = 0;
 

python/core/qgsmaprendererparalleljob.sip

@@ -18,6 +18,7 @@ class QgsMapRendererParallelJob : QgsMapRendererQImageJob
 
     virtual void start();
     virtual void cancel();
+    virtual void cancelWithoutBlocking();
     virtual void waitForFinished();
     virtual bool isActive() const;
 

python/core/qgsmaprenderersequentialjob.sip

@@ -19,6 +19,7 @@ class QgsMapRendererSequentialJob : QgsMapRendererQImageJob
 
     virtual void start();
     virtual void cancel();
+    virtual void cancelWithoutBlocking();
     virtual void waitForFinished();
     virtual bool isActive() const;
 

src/core/qgsmaprenderercustompainterjob.cpp

@@ -126,14 +126,7 @@ void QgsMapRendererCustomPainterJob::cancel()
 
   QgsDebugMsg( "QPAINTER cancelling" );
   disconnect( &mFutureWatcher, SIGNAL( finished() ), this, SLOT( futureFinished() ) );
-
-  mLabelingRenderContext.setRenderingStopped( true );
-  for ( LayerRenderJobs::iterator it = mLayerJobs.begin(); it != mLayerJobs.end(); ++it )
-  {
-    it->context.setRenderingStopped( true );
-    if ( it->renderer && it->renderer->feedback() )
-      it->renderer->feedback()->cancel();
-  }
+  cancelWithoutBlocking();
 
   QTime t;
   t.start();
@@ -144,7 +137,24 @@ void QgsMapRendererCustomPainterJob::cancel()
 
   futureFinished();
 
-  QgsDebugMsg( "QPAINTER cancelled" );
+  QgsDebugMsg( "QPAINTER canceled" );
+}
+
+void QgsMapRendererCustomPainterJob::cancelWithoutBlocking()
+{
+  if ( !isActive() )
+  {
+    QgsDebugMsg( "QPAINTER not running!" );
+    return;
+  }
+
+  mLabelingRenderContext.setRenderingStopped( true );
+  for ( LayerRenderJobs::iterator it = mLayerJobs.begin(); it != mLayerJobs.end(); ++it )
+  {
+    it->context.setRenderingStopped( true );
+    if ( it->renderer && it->renderer->feedback() )
+      it->renderer->feedback()->cancel();
+  }
 }
 
 void QgsMapRendererCustomPainterJob::waitForFinished()

src/core/qgsmaprenderercustompainterjob.h

@@ -38,6 +38,7 @@ class CORE_EXPORT QgsMapRendererCustomPainterJob : public QgsMapRendererJob
 
     virtual void start() override;
     virtual void cancel() override;
+    virtual void cancelWithoutBlocking() override;
     virtual void waitForFinished() override;
     virtual bool isActive() const override;
     virtual QgsLabelingResults* takeLabelingResults() override;

src/core/qgsmaprendererjob.h

@@ -96,6 +96,13 @@ class CORE_EXPORT QgsMapRendererJob : public QObject
     //! Does nothing if the rendering is not active.
     virtual void cancel() = 0;
 
+    /**
+     * Triggers cancelation of the rendering job without blocking. The render job will continue
+     * to operate until it is able to cancel, at which stage the finished() signal will be emitted.
+     * Does nothing if the rendering is not active.
+     */
+    virtual void cancelWithoutBlocking() = 0;
+
     //! Block until the job has finished.
     virtual void waitForFinished() = 0;
 
@@ -206,6 +213,7 @@ class CORE_EXPORT QgsMapRendererQImageJob : public QgsMapRendererJob
 
     //! Get a preview/resulting image
     virtual QImage renderedImage() = 0;
+
 };
 
 

src/core/qgsmaprendererparalleljob.cpp

@@ -123,6 +123,28 @@ void QgsMapRendererParallelJob::cancel()
   Q_ASSERT( mStatus == Idle );
 }
 
+void QgsMapRendererParallelJob::cancelWithoutBlocking()
+{
+  if ( !isActive() )
+    return;
+
+  QgsDebugMsg( QString( "PARALLEL cancel at status %1" ).arg( mStatus ) );
+
+  mLabelingRenderContext.setRenderingStopped( true );
+  for ( LayerRenderJobs::iterator it = mLayerJobs.begin(); it != mLayerJobs.end(); ++it )
+  {
+    it->context.setRenderingStopped( true );
+    if ( it->renderer && it->renderer->feedback() )
+      it->renderer->feedback()->cancel();
+  }
+
+  if ( mStatus == RenderingLayers )
+  {
+    disconnect( &mFutureWatcher, SIGNAL( finished() ), this, SLOT( renderLayersFinished() ) );
+    connect( &mFutureWatcher, SIGNAL( finished() ), this, SLOT( renderingFinished() ) );
+  }
+}
+
 void QgsMapRendererParallelJob::waitForFinished()
 {
   if ( !isActive() )

src/core/qgsmaprendererparalleljob.h

@@ -35,6 +35,7 @@ class CORE_EXPORT QgsMapRendererParallelJob : public QgsMapRendererQImageJob
 
     virtual void start() override;
     virtual void cancel() override;
+    virtual void cancelWithoutBlocking() override;
     virtual void waitForFinished() override;
     virtual bool isActive() const override;
 

src/core/qgsmaprenderersequentialjob.cpp

@@ -86,6 +86,15 @@ void QgsMapRendererSequentialJob::cancel()
   Q_ASSERT( !mInternalJob && !mPainter );
 }
 
+void QgsMapRendererSequentialJob::cancelWithoutBlocking()
+{
+  if ( !isActive() )
+    return;
+
+  QgsDebugMsg( "sequential - cancel internal" );
+  mInternalJob->cancelWithoutBlocking();
+}
+
 void QgsMapRendererSequentialJob::waitForFinished()
 {
   if ( !isActive() )

src/core/qgsmaprenderersequentialjob.h

@@ -37,6 +37,7 @@ class CORE_EXPORT QgsMapRendererSequentialJob : public QgsMapRendererQImageJob
 
     virtual void start() override;
     virtual void cancel() override;
+    virtual void cancelWithoutBlocking() override;
     virtual void waitForFinished() override;
     virtual bool isActive() const override;
 

src/gui/qgsmapcanvas.cpp

@@ -835,8 +835,10 @@ void QgsMapCanvas::stopRendering()
   {
     QgsDebugMsg( "CANVAS stop rendering!" );
     mJobCancelled = true;
-    mJob->cancel();
-    Q_ASSERT( !mJob ); // no need to delete here: already deleted in finished()
+    disconnect( mJob, SIGNAL( finished() ), this, SLOT( rendererJobFinished() ) );
+    connect( mJob, SIGNAL( finished() ), mJob, SLOT( deleteLater() ) );
+    mJob->cancelWithoutBlocking();
+    mJob = nullptr;
   }
 }