[processing] Add metadata setting for file destination parameters

to avoid file overwrite confirmation prompt

Fixes #37011

python/core/auto_generated/processing/qgsprocessingparameters.sip.in

@@ -3142,6 +3142,19 @@ class QgsProcessingParameterFileDestination : QgsProcessingDestinationParameter
 A generic file based destination parameter, for specifying the destination path for a file (non-map layer)
 created by the algorithm.
 
+In some circumstances it is desirable to avoid the usual file overwriting confirmation prompt when
+users select an existing destination file for this parameter type (e.g., for algorithms which
+append to an existing destination file instead of overwriting them.). This can be done by setting
+the widget wrapper metadata "dontconfirmoverwrite" option:
+
+.. code-block:: python
+
+       param = QgsProcessingParameterFileDestination( 'OUTPUT', 'Destination file')
+       # don't show the file overwrite warning when users select a destination file:
+       param.setMetadata( {'widget_wrapper':
+         { 'dontconfirmoverwrite': True }
+       })
+
 .. versionadded:: 3.0
 %End
 

src/core/processing/qgsprocessingparameters.h

@@ -2986,7 +2986,21 @@ class CORE_EXPORT QgsProcessingParameterRasterDestination : public QgsProcessing
  * \ingroup core
  * A generic file based destination parameter, for specifying the destination path for a file (non-map layer)
  * created by the algorithm.
-  * \since QGIS 3.0
+ *
+ * In some circumstances it is desirable to avoid the usual file overwriting confirmation prompt when
+ * users select an existing destination file for this parameter type (e.g., for algorithms which
+ * append to an existing destination file instead of overwriting them.). This can be done by setting
+ * the widget wrapper metadata "dontconfirmoverwrite" option:
+ *
+ * * \code{.py}
+ *   param = QgsProcessingParameterFileDestination( 'OUTPUT', 'Destination file')
+ *   # don't show the file overwrite warning when users select a destination file:
+ *   param.setMetadata( {'widget_wrapper':
+ *     { 'dontconfirmoverwrite': True }
+ *   })
+ * \endcode
+ *
+ * \since QGIS 3.0
  */
 class CORE_EXPORT QgsProcessingParameterFileDestination : public QgsProcessingDestinationParameter
 {

src/gui/processing/qgsprocessingoutputdestinationwidget.cpp

@@ -404,7 +404,9 @@ void QgsProcessingLayerOutputDestinationWidget::selectFile()
   else
     path = settings.value( QStringLiteral( "/Processing/Configuration/OUTPUTS_FOLDER" ) ).toString();
 
-  QString filename = QFileDialog::getSaveFileName( this, tr( "Save file" ), path, fileFilter, &lastFilter );
+  const bool dontConfirmOverwrite = mParameter->metadata().value( QStringLiteral( "widget_wrapper" ) ).toMap().value( QStringLiteral( "dontconfirmoverwrite" ), false ).toBool();
+
+  QString filename = QFileDialog::getSaveFileName( this, tr( "Save file" ), path, fileFilter, &lastFilter, dontConfirmOverwrite ? QFileDialog::Options( QFileDialog::DontConfirmOverwrite ) : nullptr );
   if ( !filename.isEmpty() )
   {
     mUseTemporary = false;