Re: openat(..., AT_UNLINKED) was Re: New copyfile system call - discussbefore LSF?

[Ric Wheeler]

On 03/31/2013 07:18 PM, Pavel Machek wrote:
> Hi!
>
> > > > > > Take a look at how many actively used filesystems out there that have
> > > > > > some variant of sillyrename(), and explain what you want to do in those
> > > > > > cases.
> > > > > Well. Yes, there are non-unix filesystems around. You have to deal
> > > > > with silly files on them, and this will not be different.
> > > > So this would be a local POSIX filesystem only solution to a problem
> > > > that has yet to be formulated?
> > > Problem is "clasical create temp file then delete it" is racy. See the
> > > archives. That is useful & common operation.
> > Which race are you concerned with exactly?
> >
> > User wants to test for a file with name "foo.txt"
> >
> > * create "foo.txt~" (or whatever)
> > * write contents into "foo.txt~"
> > * rename "foo.txt~" to "foo.txt"
> >
> > Until rename is done, the file does not exists and is not complete.
> > You will potentially have a garbage file to clean up if the program
> > (or system) crashes, but that is not racy in a classic sense, right?
> Well. If people rsync from you, they will start fetching incomplete
> foo.txt~. Plus the garbage issue.

That is not racy, just garbage (not trying to be pedantic, just trying to 
understand). I can see that the "~" file is annoying, but we have dealt with it 
for a *long* time :)

Until it has the right name (on either the source or target system for rsync), 
it is not the file you are looking for.
> > This is more of a garbage clean up issue?
> Also. Plus sometimes you want temporary "file" that is
> deleted. Terminals use it for history, etc...

There you would have a race, you can create a file and unlink it of course and 
still write to it, but you would have a potential empty file issue?

Ric


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