Properly discard edit commands when calling QgsVectorLayer::destroyEditCommand()

Since it's not directly possible to pop the last command off
an QUndoStack (the command which is destroyed/discard by calling
this method), we add a dummy obsolete command to force this to occur.
Pushing the new command deletes the destroyed one, and since the new
command is obsolete it's automatically deleted by the undo stack.

Author: nyalldawson

src/core/qgsvectorlayer.cpp

@@ -32,6 +32,7 @@
 #include <QDomNode>
 #include <QVector>
 #include <QStringBuilder>
+#include <QUndoCommand>
 
 #include "qgssettings.h"
 #include "qgsvectorlayer.h"
@@ -3005,6 +3006,15 @@ void QgsVectorLayer::destroyEditCommand()
   }
   undoStack()->endMacro();
   undoStack()->undo();
+
+  // it's not directly possible to pop the last command off the stack (the destroyed one)
+  // and delete, so we add a dummy obsolete command to force this to occur.
+  // Pushing the new command deletes the destroyed one, and since the new
+  // command is obsolete it's automatically deleted by the undo stack.
+  std::unique_ptr< QUndoCommand > command = qgis::make_unique< QUndoCommand >();
+  command->setObsolete( true );
+  undoStack()->push( command.release() );
+
   mEditCommandActive = false;
   mDeletedFids.clear();
   emit editCommandDestroyed();

tests/src/python/test_qgsvectorlayer.py

@@ -238,6 +238,60 @@ def test_FeatureCount(self):
         myCount = myLayer.featureCount()
         self.assertEqual(myCount, 6)
 
+    # undo stack
+    def testUndoStack(self):
+        layer = createLayerWithOnePoint()
+        layer.startEditing()
+
+        self.assertEqual(layer.undoStack().count(), 0)
+        self.assertEqual(layer.undoStack().index(), 0)
+        f = QgsFeature()
+        f.setAttributes(["test", 123])
+        f.setGeometry(QgsGeometry.fromPointXY(QgsPointXY(100, 200)))
+        self.assertTrue(layer.addFeatures([f]))
+        self.assertEqual(layer.undoStack().count(), 1)
+        self.assertEqual(layer.undoStack().index(), 1)
+        self.assertEqual(layer.featureCount(), 2)
+
+        layer.undoStack().undo()
+        self.assertEqual(layer.undoStack().count(), 1)
+        self.assertEqual(layer.undoStack().index(), 0)
+        self.assertEqual(layer.featureCount(), 1)
+
+        layer.undoStack().redo()
+        self.assertEqual(layer.undoStack().count(), 1)
+        self.assertEqual(layer.undoStack().index(), 1)
+        self.assertEqual(layer.featureCount(), 2)
+
+        # macro commands
+        layer.beginEditCommand("Test command 1")
+        self.assertTrue(layer.addFeatures([f]))
+        self.assertTrue(layer.addFeatures([f]))
+        layer.endEditCommand()
+        self.assertEqual(layer.undoStack().count(), 2)
+        self.assertEqual(layer.undoStack().index(), 2)
+        self.assertEqual(layer.featureCount(), 4)
+
+        layer.undoStack().undo()
+        self.assertEqual(layer.undoStack().count(), 2)
+        self.assertEqual(layer.undoStack().index(), 1)
+        self.assertEqual(layer.featureCount(), 2)
+
+        layer.undoStack().redo()
+        self.assertEqual(layer.undoStack().count(), 2)
+        self.assertEqual(layer.undoStack().index(), 2)
+        self.assertEqual(layer.featureCount(), 4)
+
+        # throw away a macro command
+        layer.beginEditCommand("Test command 1")
+        self.assertTrue(layer.addFeatures([f]))
+        self.assertTrue(layer.addFeatures([f]))
+        self.assertEqual(layer.featureCount(), 6)
+        layer.destroyEditCommand()
+        self.assertEqual(layer.undoStack().count(), 2)
+        self.assertEqual(layer.undoStack().index(), 2)
+        self.assertEqual(layer.featureCount(), 4)
+
     # ADD FEATURE
 
     def test_AddFeature(self):