Create backup file when writing new xml file?

[David Faure]

On Thursday 20 February 2014 00:57:36 Thiago Macieira wrote:
> Em qui 20 fev 2014, às 09:07:46, David Faure escreveu:
> > > That's not an atomic operation. There's a brief moment when a file with
> > > the
> > > original name does not exist anymore.
> > 
> > I know. But again: how do you use a backup file? You have to open a file
> > manager and rename it to the main file name, or you have to use the open
> > dialog in the application, with a permissive filter, and open it.
> > This is no different whether the main file exists or has been deleted. In
> > both cases manual user action is needed to use the backup file.
> > 
> > If you say such manual action should never be required, then either the
> > backup file is useless, or the app needs to provide a GUI for it.
> > 
> > My conclusion: for a document, user action is required to use the backup
> > file anyway, and losing the main file on a crash *is* one of the few cases
> > where the backup file actually turns out to be useful. And for an internal
> > file (e.g. config file), a backup file is not useful (since the user
> > doesn't know about it), so plain QSaveFile is sufficient.
> 
> I don't care much about the backup file. It's a good idea to try and be as
> safe as possible, in case there's a problem with saving of the new file and
> the user wants to restore from backup. As you said, restoring the backup
> requires the use of a file manager and direct user intervention.
> 
> My problem is only that the a file with the name of the original file name
> ceases to exist for a brief period. What happens if there's a filesystem
> notifier running on that system, watching the directory to reload the
> contents? It might get woken up by the rename of the original to backup and
> then it might decide that the data is gone.
> 
> It might mean the difference between a "File has changed, Reload?" dialog
> and a "File was deleted, Close?" one.

OK, that's a good point.

Now for a stupid-but-simple suggestion, then:
* copy (not rename) main file to backup
* save to temp file
* atomically rename-overwrite temp file to main file

This way, the main file always exists.

-- 
David Faure, faure at kde.org, http://www.davidfaure.fr
Working on KDE, in particular KDE Frameworks 5