[libvirt PATCH] scripts: Fix meson-install-symlink.py overwriting existing links

[Erik Skultety]

On Wed, Aug 05, 2020 at 09:57:17AM +0100, Daniel P. Berrangé wrote:
> On Tue, Aug 04, 2020 at 06:27:08PM +0200, Erik Skultety wrote:
> > By default, symlink re-creation fails if the link already exists, more
> > specifically in case of meson-install-symlink.py:
> > 
> > Traceback (most recent call last):
> >   File "/<path_to_libvirt_repo>/scripts/meson-install-symlink.py",
> >     line 15, in <module>
> >         os.symlink(target, link)
> > FileExistsError: File exists: '../default.xml' -> 'default.xml'
> > 
> > Unfortunately, Python can't mimic "ln -sf", so we have to fix this
> > differently - create a temporary name which is then going to be used
> > for the temporary link followed by a rename with the original link's
> > name.
> > Note that this solution is racy as mktemp() doesn't guarantee
> > atomicity in link creation, so theoretically another process could come
> > and create a file with the same name as the temporary link name, but
> > a proper solution would be longer and not as elegant.
> 
> You've just described exactly what python does for "ln -sf foo bar".
> try stracing it and you'll see it do:
> 
>   symlinkat("foo", AT_FDCWD, "CubmxbFT")  = 0
>   renameat(AT_FDCWD, "CubmxbFT", AT_FDCWD, "bar") = 0

Oh, I didn't know that, in which case v1 is actually the correct fix here. I
sent v2 in the meantime where I remove the pre-existing link which is even more
compact, so I don't care which one we go for.

Erik