Preserve edits for multiline editor when length exceeds field size

Previously when using the multiline option for text edit widgets
the entire contents of the field would be discarded if the entered
value exceeded the maximum length for a string field.

Now the entered string is truncated to the maximum field length.

(cherry-picked from 7d8fba8)

Author: nyalldawson

src/gui/editorwidgets/qgstexteditwrapper.cpp

@@ -66,9 +66,19 @@ QVariant QgsTextEditWrapper::value() const
 
   QVariant res( v );
   if ( field().convertCompatible( res ) )
+  {
     return res;
+  }
+  else if ( field().type() == QVariant::String && field().length() > 0 )
+  {
+    // for string fields convertCompatible may return false due to field length limit - in this case just truncate
+    // input rather then discarding it entirely
+    return QVariant( v.left( field().length() ) );
+  }
   else
+  {
     return QVariant( field().type() );
+  }
 }
 
 QWidget* QgsTextEditWrapper::createWidget( QWidget* parent )

tests/src/python/test_qgseditwidgets.py

@@ -14,10 +14,13 @@
 
 import qgis  # NOQA
 
-from qgis.core import QgsMapLayerRegistry, QgsFeature, QgsGeometry, QgsPoint, QgsProject, QgsRelation, QgsVectorLayer, NULL
+from qgis.core import (QgsMapLayerRegistry, QgsFeature, QgsGeometry, QgsPoint, QgsProject, QgsRelation, QgsVectorLayer, NULL,
+                       QgsField)
 from qgis.gui import QgsEditorWidgetRegistry
 
 from qgis.testing import start_app, unittest
+from qgis.PyQt.QtCore import QVariant
+from qgis.PyQt.QtWidgets import QTextEdit
 
 start_app()
 
@@ -63,6 +66,33 @@ def test_SetValue(self):
         self.doAttributeTest(0, ['value', '123', NULL, NULL])
         self.doAttributeTest(1, [NULL, 123, NULL, NULL])
 
+    def testStringWithMaxLen(self):
+        """ tests that text edit wrappers correctly handle string fields with a maximum length """
+        layer = QgsVectorLayer("none?field=fldint:integer", "layer", "memory")
+        assert layer.isValid()
+        layer.dataProvider().addAttributes([QgsField('max', QVariant.String, 'string', 10),
+                                            QgsField('nomax', QVariant.String, 'string', 0)])
+        layer.updateFields()
+        QgsMapLayerRegistry.instance().addMapLayer(layer)
+
+        reg = QgsEditorWidgetRegistry.instance()
+        config = {'IsMultiline': 'True'}
+
+        # first test for field without character limit
+        editor = QTextEdit()
+        editor.setPlainText('this_is_a_long_string')
+        w = reg.create('TextEdit', layer, 2, config, editor, None)
+        self.assertEqual(w.value(), 'this_is_a_long_string')
+
+        # next test for field with character limit
+        editor = QTextEdit()
+        editor.setPlainText('this_is_a_long_string')
+        w = reg.create('TextEdit', layer, 1, config, editor, None)
+
+        self.assertEqual(w.value(), 'this_is_a_')
+
+        QgsMapLayerRegistry.instance().removeAllMapLayers()
+
     def test_ValueRelation_representValue(self):
 
         first_layer = QgsVectorLayer("none?field=foreign_key:integer",