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



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



Regards,
Daniel
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