Prevent users from removing layers or closing projects when

running tasks depend on those layers

It's not foolproof because there's many ways to remove layers
not through the QGIS gui and tasks should be written to nicely handle
layers being removed mid-way through execution.

But it's still a good warning for users if they accidently try to
load a new project and don't realise some task is still running
in the background.

Author: nyalldawson

src/app/qgisapp.cpp

@@ -4485,6 +4485,9 @@ void QgisApp::fileNewBlank()
 //as file new but accepts flags to indicate whether we should prompt to save
 void QgisApp::fileNew( bool thePromptToSaveFlag, bool forceBlank )
 {
+  if ( checkTasksDependOnProject() )
+    return;
+
   if ( thePromptToSaveFlag )
   {
     if ( !saveDirty() )
@@ -4594,6 +4597,9 @@ void QgisApp::fileNew( bool thePromptToSaveFlag, bool forceBlank )
 
 bool QgisApp::fileNewFromTemplate( const QString& fileName )
 {
+  if ( checkTasksDependOnProject() )
+    return false;
+
   if ( !saveDirty() )
   {
     return false; //cancel pressed
@@ -4883,6 +4889,9 @@ void QgisApp::showAlignRasterTool()
 
 void QgisApp::fileOpen()
 {
+  if ( checkTasksDependOnProject() )
+    return;
+
   // possibly save any pending work before opening a new project
   if ( saveDirty() )
   {
@@ -5289,6 +5298,9 @@ void QgisApp::openProject( QAction *action )
   // possibly save any pending work before opening a different project
   Q_ASSERT( action );
 
+  if ( checkTasksDependOnProject() )
+    return;
+
   QString debugme = action->data().toString();
   if ( saveDirty() )
     addProject( debugme );
@@ -5317,6 +5329,9 @@ void QgisApp::runScript( const QString &filePath )
   */
 void QgisApp::openProject( const QString & fileName )
 {
+  if ( checkTasksDependOnProject() )
+    return;
+
   // possibly save any pending work before opening a different project
   if ( saveDirty() )
   {
@@ -8388,6 +8403,26 @@ void QgisApp::removeLayer()
       return;
   }
 
+  QStringList activeTaskDescriptions;
+  Q_FOREACH ( QgsMapLayer * layer, mLayerTreeView->selectedLayers() )
+  {
+    QList< QgsTask* > tasks = QgsApplication::taskManager()->tasksDependentOnLayer( layer->id() );
+    if ( !tasks.isEmpty() )
+    {
+      Q_FOREACH ( QgsTask* task, tasks )
+      {
+        activeTaskDescriptions << tr( " • %1" ).arg( task->description() );
+      }
+    }
+  }
+
+  if ( !activeTaskDescriptions.isEmpty() )
+  {
+    QMessageBox::warning( this, tr( "Active tasks" ),
+                          tr( "The following tasks are currently running which depend on this layer:\n\n%1\n\nPlease cancel these tasks and retry." ).arg( activeTaskDescriptions.join( "\n" ) ) );
+    return;
+  }
+
   QList<QgsLayerTreeNode*> selectedNodes = mLayerTreeView->selectedNodes( true );
 
   //validate selection
@@ -9542,7 +9577,34 @@ bool QgisApp::saveDirty()
   mMapCanvas->freeze( false );
 
   return answer != QMessageBox::Cancel;
-} // QgisApp::saveDirty()
+}
+
+bool QgisApp::checkTasksDependOnProject()
+{
+  QSet< QString > activeTaskDescriptions;
+  QMap<QString, QgsMapLayer*> layers = QgsMapLayerRegistry::instance()->mapLayers();
+  QMap<QString, QgsMapLayer*>::const_iterator layerIt = layers.constBegin();
+
+  for ( ; layerIt != layers.constEnd(); ++layerIt )
+  {
+    QList< QgsTask* > tasks = QgsApplication::taskManager()->tasksDependentOnLayer( layerIt.key() );
+    if ( !tasks.isEmpty() )
+    {
+      Q_FOREACH ( QgsTask* task, tasks )
+      {
+        activeTaskDescriptions.insert( tr( " • %1" ).arg( task->description() ) );
+      }
+    }
+  }
+
+  if ( !activeTaskDescriptions.isEmpty() )
+  {
+    QMessageBox::warning( this, tr( "Active tasks" ),
+                          tr( "The following tasks are currently running which depend on layers in this project:\n\n%1\n\nPlease cancel these tasks and retry." ).arg( activeTaskDescriptions.toList().join( "\n" ) ) );
+    return true;
+  }
+  return false;
+}
 
 void QgisApp::closeProject()
 {

src/app/qgisapp.h

@@ -1473,6 +1473,8 @@ class APP_EXPORT QgisApp : public QMainWindow, private Ui::MainWindow
     // void pasteTransformations();
     //! check to see if file is dirty and if so, prompt the user th save it
     bool saveDirty();
+    //! Checks for running tasks dependent on the open project
+    bool checkTasksDependOnProject();
 
     /** Helper function to union several geometries together (used in function mergeSelectedFeatures)
       @return empty geometry in case of error or if canceled */