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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.


Well, you could do subprocess.check_output(['ln', '-sf', ...]) (yuck), but it
does essentially the same thing anyway.

I don't think a race here would cause anything.  I would probably remove the
file and then create the symlink and if there is something/someone else
installing the same file than it is good that we error our because there are
bigger issues than that.  The whole installation process should not be
interfered with and you cannot make it atomic anyway.

Thoughts?

Signed-off-by: Erik Skultety <eskultet redhat com>
---
scripts/meson-install-symlink.py | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/scripts/meson-install-symlink.py b/scripts/meson-install-symlink.py
index e38507072d..630dfc079f 100644
--- a/scripts/meson-install-symlink.py
+++ b/scripts/meson-install-symlink.py
@@ -2,6 +2,7 @@

import os
import sys
+import tempfile

destdir = os.environ.get('DESTDIR', os.sep)
dirname = sys.argv[1]
@@ -12,4 +13,7 @@ workdir = os.path.join(destdir, dirname.strip(os.sep))

os.makedirs(workdir, exist_ok=True)
os.chdir(workdir)
-os.symlink(target, link)
+
+templink = tempfile.mktemp(dir=workdir)
+os.symlink(target, templink)
+os.replace(templink, link)
--
2.26.2

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