Processing: fix crash in alg runner task with bad scripts

Fixes #21270

With test

src/core/processing/qgsprocessingalgorithm.cpp

@@ -40,6 +40,8 @@ QgsProcessingAlgorithm::~QgsProcessingAlgorithm()
 QgsProcessingAlgorithm *QgsProcessingAlgorithm::create( const QVariantMap &configuration ) const
 {
   std::unique_ptr< QgsProcessingAlgorithm > creation( createInstance() );
+  if ( ! creation )
+    throw QgsProcessingException( QObject::tr( "Error creating algorithm from createInstance()" ) );
   creation->setProvider( provider() );
   creation->initAlgorithm( configuration );
   return creation.release();

src/core/processing/qgsprocessingalgrunnertask.cpp

@@ -34,7 +34,7 @@ QgsProcessingAlgRunnerTask::QgsProcessingAlgRunnerTask( const QgsProcessingAlgor
     mOwnedFeedback.reset( new QgsProcessingFeedback() );
     mFeedback = mOwnedFeedback.get();
   }
-  if ( !mAlgorithm->prepare( mParameters, context, mFeedback ) )
+  if ( !( mAlgorithm && mAlgorithm->prepare( mParameters, context, mFeedback ) ) )
     cancel();
 }
 

tests/src/python/CMakeLists.txt

@@ -154,6 +154,7 @@ ADD_PYTHON_TEST(PyQgsPointDisplacementRenderer test_qgspointdisplacementrenderer
 ADD_PYTHON_TEST(PyQgsPostgresDomain test_qgspostgresdomain.py)
 ADD_PYTHON_TEST(PyQgsProcessingRecentAlgorithmLog test_qgsprocessingrecentalgorithmslog.py)
 ADD_PYTHON_TEST(PyQgsProcessingInPlace test_qgsprocessinginplace.py)
+ADD_PYTHON_TEST(PyQgsProcessingAlgRunner test_qgsprocessingalgrunner.py)
 ADD_PYTHON_TEST(PyQgsProcessingAlgDecorator test_processing_alg_decorator.py)
 ADD_PYTHON_TEST(PyQgsProjectionSelectionWidgets test_qgsprojectionselectionwidgets.py)
 ADD_PYTHON_TEST(PyQgsProjectMetadata test_qgsprojectmetadata.py)

tests/src/python/test_qgsprocessingalgrunner.py

@@ -0,0 +1,99 @@
+# -*- coding: utf-8 -*-
+"""QGIS Unit tests for Processing algorithm runner(s).
+
+.. note:: 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.
+"""
+__author__ = 'Alessandro Pasotti'
+__date__ = '2019-02'
+__copyright__ = 'Copyright 2019, The QGIS Project'
+# This will get replaced with a git SHA1 when you do a git archive
+__revision__ = '$Format:%H$'
+
+import re
+from qgis.PyQt.QtCore import QCoreApplication
+from qgis.testing import start_app, unittest
+from qgis.core import QgsProcessingAlgRunnerTask
+
+from processing.core.Processing import Processing
+from processing.core.ProcessingConfig import ProcessingConfig
+from qgis.testing import start_app, unittest
+from qgis.analysis import QgsNativeAlgorithms
+from qgis.core import (
+    QgsApplication,
+    QgsSettings,
+    QgsProcessingContext,
+    QgsProcessingAlgRunnerTask,
+    QgsProcessingAlgorithm,
+    QgsProject,
+)
+
+start_app()
+
+
+class CrashingProcessingAlgorithm(QgsProcessingAlgorithm):
+    """
+    Wrong class in factory createInstance()
+    """
+
+    INPUT = 'INPUT'
+    OUTPUT = 'OUTPUT'
+
+    def tr(self, string):
+        return QCoreApplication.translate('Processing', string)
+
+    def createInstance(self):
+        """Wrong!"""
+        return ExampleProcessingAlgorithm()
+
+    def name(self):
+        return 'mycrashingscript'
+
+    def displayName(self):
+        return self.tr('My Crashing Script')
+
+    def group(self):
+        return self.tr('Example scripts')
+
+    def groupId(self):
+        return 'examplescripts'
+
+    def shortHelpString(self):
+        return self.tr("Example algorithm short description")
+
+    def initAlgorithm(self, config=None):
+        pass
+
+    def processAlgorithm(self, parameters, context, feedback):
+        return {self.OUTPUT: 'an_id'}
+
+
+class TestQgsProcessingAlgRunner(unittest.TestCase):
+
+    @classmethod
+    def setUpClass(cls):
+        """Run before all tests"""
+        QCoreApplication.setOrganizationName("QGIS_Test")
+        QCoreApplication.setOrganizationDomain(
+            "QGIS_TestPyQgsProcessingInPlace.com")
+        QCoreApplication.setApplicationName("QGIS_TestPyQgsProcessingInPlace")
+        QgsSettings().clear()
+        Processing.initialize()
+        QgsApplication.processingRegistry().addProvider(QgsNativeAlgorithms())
+        cls.registry = QgsApplication.instance().processingRegistry()
+
+    def test_bad_script_dont_crash(self):
+        """Test regression #21270 (segfault)"""
+
+        context = QgsProcessingContext()
+        context.setProject(QgsProject.instance())
+        with self.assertRaises(Exception) as e:
+            QgsProcessingAlgRunnerTask(CrashingProcessingAlgorithm(), {}, context=context)
+
+        self.assertEqual(str(e.exception), 'unknown')
+
+
+if __name__ == '__main__':
+    unittest.main()