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gh-81793: always call linkat() from os.link(), if available #24997

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gh-81793: always call linkat() from os.link(), if available #24997

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21b0ab7
Add failing test for os.link() with symlink
takluyver Mar 23, 2021
772b3e9
bpo-37612: always call linkat() from os.link(), if available
jo-he Mar 23, 2021
0200894
📜🤖 Added by blurb_it.
blurb-it[bot] Jul 19, 2019
beada2b
Merge branch 'main' into bpo-37612
serhiy-storchaka Apr 12, 2025
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bpo-37612: always call linkat() from os.link(), if available 
The issue with link() is that POSIX does not define its behavior
regarding symbolic links:

"If path1 names a symbolic link, it is implementation-defined whether
link() follows the symbolic link, or creates a new link to the symbolic
link itself."

And it is indeed implemented differently on Linux and NetBSD.
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@jo-he @takluyver
jo-he authored and takluyver committed Mar 23, 2021
commit 772b3e936a390a4f662c0e3e46a90a434c024c98
50 changes: 30 additions & 20 deletions Modules/posixmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3930,31 +3930,41 @@ os_link_impl(PyObject *module, path_t *src, path_t *dst, int src_dir_fd,
#else
Py_BEGIN_ALLOW_THREADS
#ifdef HAVE_LINKAT
if ((src_dir_fd != DEFAULT_DIR_FD) ||
(dst_dir_fd != DEFAULT_DIR_FD) ||
(!follow_symlinks)) {

if (HAVE_LINKAT_RUNTIME) {

result = linkat(src_dir_fd, src->narrow,
dst_dir_fd, dst->narrow,
follow_symlinks ? AT_SYMLINK_FOLLOW : 0);

}
if (HAVE_LINKAT_RUNTIME) {
result = linkat(src_dir_fd, src->narrow,
dst_dir_fd, dst->narrow,
follow_symlinks ? AT_SYMLINK_FOLLOW : 0);
}
#ifdef __APPLE__
else {
if (src_dir_fd == DEFAULT_DIR_FD && dst_dir_fd == DEFAULT_DIR_FD) {
/* See issue 41355: This matches the behaviour of !HAVE_LINKAT */
result = link(src->narrow, dst->narrow);
} else {
linkat_unavailable = 1;
}
else {
if (src_dir_fd == DEFAULT_DIR_FD && dst_dir_fd == DEFAULT_DIR_FD && follow_symlinks) {
/* See issue 41355: This matches the behaviour of !HAVE_LINKAT */
result = link(src->narrow, dst->narrow);
} else {
linkat_unavailable = 1;
}
}
#endif

#else /* linkat not available */
/* See issue 41355: link() on Linux works like linkat without AT_SYMLINK_FOLLOW,
but on Mac it works like linkat *with* AT_SYMLINK_FOLLOW. */
#ifdef __APPLE__
if (!follow_symlinks) {
PyErr_SetString(PyExc_NotImplementedError,
"link: follow_symlinks=False unavailable on this platform");
return NULL;
} else {
#else
if (follow_symlinks) {
PyErr_SetString(PyExc_NotImplementedError,
"link: follow_symlinks=True unavailable on this platform");
Comment on lines +4412 to +4414
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@serhiy-storchaka serhiy-storchaka Apr 12, 2025

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follow_symlinks is true by default, so os.link() will always fail if linkat is not available.

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@takluyver takluyver Apr 14, 2025

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I think that's desirable? follow_symlinks is documented as true by default, so if that's unlikely to work, it seems better to error out than to ignore it.

linkat has been in Linux since 2006 and FreeBSD since 2009, so hopefully this isn't a case that many people will run into.

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@serhiy-storchaka serhiy-storchaka Apr 14, 2025

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No, it is not desirable. Otherwise we would just not implement os.link() without linkat.

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@takluyver takluyver Apr 14, 2025

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Well, you could still pass follow_symlinks=False if you're happy with that behaviour, so it's more useful than the function not existing at all. What alternative are you suggesting?

  • Should os.link() implement a symlink-following fallback with 2 system calls? Is there a race condition there?
  • Should the follow_symlinks default be documented as platform dependent? That would be accurate but surprising, since most functions with that parameter default to True, and it makes it easy to write code that does something slightly different on Mac & Linux.

But is there any relevant platform or configuration where linkat() is not available? It looks like this has been ubiquitous for well over a decade, so maybe the fallback is kind of academic anyway.

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@serhiy-storchaka serhiy-storchaka Apr 14, 2025

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I am working on this issue now.

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@serhiy-storchaka serhiy-storchaka Apr 14, 2025

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See #81793.

return NULL;
} else {
#endif /* __APPLE__ */
result = link(src->narrow, dst->narrow);
}
else
#endif /* HAVE_LINKAT */
result = link(src->narrow, dst->narrow);
Py_END_ALLOW_THREADS

#ifdef HAVE_LINKAT
Expand Down