[processing] preserve field length and precision for temporary outputs (#5654)

* add test case to cover length and precision through uri

Author: nirvn

src/core/providers/memory/qgsmemoryproviderutils.cpp

@@ -63,7 +63,16 @@ QgsVectorLayer *QgsMemoryProviderUtils::createMemoryLayer( const QString &name,
   }
   for ( const auto &field : fields )
   {
-    parts << QStringLiteral( "field=%1:%2" ).arg( field.name(), memoryLayerFieldType( field.type() ) );
+    QString lengthPrecision;
+    if ( field.length() > 0 && field.precision() > 0 )
+    {
+      lengthPrecision = QStringLiteral( "(%1,%2)" ).arg( field.length() ).arg( field.precision() );
+    }
+    else if ( field.length() > 0 )
+    {
+      lengthPrecision = QStringLiteral( "(%1)" ).arg( field.length() );
+    }
+    parts << QStringLiteral( "field=%1:%2%3" ).arg( field.name(), memoryLayerFieldType( field.type() ), lengthPrecision );
   }
 
   QString uri = geomType + '?' + parts.join( '&' );

tests/src/python/test_provider_memory.py

@@ -263,6 +263,16 @@ def testFromUri(self):
         myProvider = myMemoryLayer.dataProvider()
         assert myProvider is not None
 
+    def testLengthPrecisionFromUri(self):
+        """Test we can assign length and precision from a uri"""
+        myMemoryLayer = QgsVectorLayer(
+            ('Point?crs=epsg:4326&field=size:double(12,9)&index=yes'),
+            'test',
+            'memory')
+
+        self.assertEqual(myMemoryLayer.fields().field('size').length(), 12)
+        self.assertEqual(myMemoryLayer.fields().field('size').precision(), 9)
+
     def testSaveFields(self):
         # Create a new memory layer with no fields
         myMemoryLayer = QgsVectorLayer(