Reload the data source when a dependency layer is changed (#37475)

 Refreshing the data provider right away causes problems on a layer
 dependent on itself. In that case two `dataChanged` signals are
 expected to be emitted when deleting or creating a feature, but as the
 layer is reloaded, the second `dataChanged` is not emitted.

python/core/auto_generated/qgsvectorlayer.sip.in

@@ -2672,6 +2672,13 @@ Emitted before changes are committed to the data provider.
     void beforeRollBack();
 %Docstring
 Emitted before changes are rolled back.
+%End
+
+    void afterCommitChanges();
+%Docstring
+Emitted after changes are commited to the data provider.
+
+.. versionadded:: 3.16
 %End
 
     void afterRollBack();

src/core/qgsvectorlayer.cpp

@@ -3367,6 +3367,7 @@ bool QgsVectorLayer::commitChanges()
     delete mEditBuffer;
     mEditBuffer = nullptr;
     undoStack()->clear();
+    emit afterCommitChanges();
     emit editingStopped();
   }
   else
@@ -5335,6 +5336,12 @@ void QgsVectorLayer::emitDataChanged()
   mDataChangedFired = false;
 }
 
+void QgsVectorLayer::onAfterCommitChangesDependency()
+{
+  mDataChangedFired = true;
+  reload();
+}
+
 bool QgsVectorLayer::setDependencies( const QSet<QgsMapLayerDependency> &oDeps )
 {
   QSet<QgsMapLayerDependency> deps;
@@ -5358,6 +5365,7 @@ bool QgsVectorLayer::setDependencies( const QSet<QgsMapLayerDependency> &oDeps )
     disconnect( lyr, &QgsVectorLayer::geometryChanged, this, &QgsVectorLayer::emitDataChanged );
     disconnect( lyr, &QgsVectorLayer::dataChanged, this, &QgsVectorLayer::emitDataChanged );
     disconnect( lyr, &QgsVectorLayer::repaintRequested, this, &QgsVectorLayer::triggerRepaint );
+    disconnect( lyr, &QgsVectorLayer::afterCommitChanges, this, &QgsVectorLayer::onAfterCommitChangesDependency );
   }
 
   // assign new dependencies
@@ -5378,6 +5386,7 @@ bool QgsVectorLayer::setDependencies( const QSet<QgsMapLayerDependency> &oDeps )
     connect( lyr, &QgsVectorLayer::geometryChanged, this, &QgsVectorLayer::emitDataChanged );
     connect( lyr, &QgsVectorLayer::dataChanged, this, &QgsVectorLayer::emitDataChanged );
     connect( lyr, &QgsVectorLayer::repaintRequested, this, &QgsVectorLayer::triggerRepaint );
+    connect( lyr, &QgsVectorLayer::afterCommitChanges, this, &QgsVectorLayer::onAfterCommitChangesDependency );
   }
 
   // if new layers are present, emit a data change

src/core/qgsvectorlayer.h

@@ -2482,6 +2482,12 @@ class CORE_EXPORT QgsVectorLayer : public QgsMapLayer, public QgsExpressionConte
     //! Emitted before changes are rolled back.
     void beforeRollBack();
 
+    /**
+     * Emitted after changes are commited to the data provider.
+     * \since QGIS 3.16
+     */
+    void afterCommitChanges();
+
     /**
      * Emitted after changes are rolled back.
      * \since QGIS 3.4
@@ -2709,6 +2715,7 @@ class CORE_EXPORT QgsVectorLayer : public QgsMapLayer, public QgsExpressionConte
     void onSymbolsCounted();
     void onDirtyTransaction( const QString &sql, const QString &name );
     void emitDataChanged();
+    void onAfterCommitChangesDependency();
 
   private:
     void updateDefaultValues( QgsFeatureId fid, QgsFeature feature = QgsFeature() );

src/gui/qgsrelationeditorwidget.cpp

@@ -872,6 +872,8 @@ void QgsRelationEditorWidget::toggleEditing( bool state )
     if ( mNmRelation.isValid() )
       mEditorContext.vectorLayerTools()->stopEditing( mNmRelation.referencedLayer() );
   }
+
+  updateButtons();
 }
 
 void QgsRelationEditorWidget::saveEdits()