UNIX: QSaveFile fails to write after changing the file name using setFileName() when hitting on a read-only file first

Consider an editor application implementing its save function such that it instantiates a QSaveFile and gives the user a way to retry with a different name should saving fail (implement a loop using QSaveFile::setFileName() ).

Steps to reproduce:

• Create a file and chmod ugo-w
• Open the editor
• Save, enter new file name

Expected: Saving works
Actual: QSaveFile::write() returns -1, commit fails

This was observed for https://codereview.qt-project.org/c/qt/qttools/+/264690 and a test would look like https://codereview.qt-project.org/c/qt/qtbase/+/267669 .