Issue 14773: fwalk breaks on dangling symlinks (original) (raw)
Created on 2012-05-10 19:28 by hynek, last changed 2022-04-11 14:57 by admin. This issue is now closed.
Messages (7)
Author: Hynek Schlawack (hynek) * 
Date: 2012-05-10 19:28
I'm implementing a safe rmtree using fwalk. Everything works perfectly except for one thing: if the directory contains dangling symlinks, fwalk goes belly-up:
$ ls -l test/ total 0 lrwxrwxrwx 1 vagrant vagrant 4 May 10 16:36 doesntwork -> this
$ ./python Python 3.3.0a3+ (default:b32baa5b7626+, May 10 2012, 14:56:20) [GCC 4.6.3] on linux Type "help", "copyright", "credits" or "license" for more information.
import os [71253 refs] list(os.fwalk('test')) Traceback (most recent call last): File "", line 1, in File "/home/vagrant/p/Lib/os.py", line 342, in fwalk for x in _fwalk(topfd, top, topdown, onerror, followlinks): File "/home/vagrant/p/Lib/os.py", line 361, in _walk if st.S_ISDIR(fstatat(topfd, name).st_mode): FileNotFoundError: [Errno 2] No such file or directory
Unfortunately this makes it impossible to implement rmtree. The reason is the following code:
for name in names: # Here, we don't use AT_SYMLINK_NOFOLLOW to be consistent with # walk() which reports symlinks to directories as directories. We do # however check for symlinks before recursing into a subdirectory. if st.S_ISDIR(fstatat(topfd, name).st_mode): dirs.append(name) else: nondirs.append(name)
The "unsafe" walk tree uses os.path.isdir() instead of os.fstatat() and handles this case gracefully.
A simple try-except protection with a symlink check fixes it and the tests pass. This is a blocker for #4489. I have expanded the test of the WalkerTests suite.
Tested on Linux (= works) and OS X (= skipped).
Author: Charles-François Natali (neologix) *
Date: 2012-05-12 15:57
I'm not sure we really need to check for a dangling symlink in case of FileNotFoundError: whether it's a dangling symlink, or the file disappeared in-between-, skipping it is OK.
Author: Hynek Schlawack (hynek) *
Date: 2012-05-12 18:13
But if it is a dangling symlink, you want to add it to nondirs while missing files could be skipped, no? You can't skip dangling symlinks if you want to implement rmtree. The normal walk() doesn't too.
Author: Hynek Schlawack (hynek) *
Date: 2012-05-12 22:37
So I've changed the patch to ignore everything missing except for dangling links (which throw unfortunately the same exception).
Just to stress it once more: a fwalk that ignores dangling symlinks is worthless for rmtree. And wasn't rmtree the initial reason to implement fwalk in the first place? ;)
Author: Charles-François Natali (neologix) *
Date: 2012-05-13 10:52
Just to stress it once more: a fwalk that ignores dangling symlinks is worthless for rmtree. And wasn't rmtree the initial reason to implement fwalk in the first place? ;)
Indeed, my bad :-)
So I've changed the patch to ignore everything missing except for dangling links (which throw unfortunately the same exception).
Looks good to me.
Author: Hynek Schlawack (hynek) *
Date: 2012-05-15 13:29
I just realized it doesn't really make sense because if a file disappears for real, we'll get another FileNotFoundException when checking whether it's a symlink and the continue is never reached.
So behold v3. :)
This time, I have tested it by injecting a
if name == 'tmp4': import os os.unlinkat(topfd, name)
right before the S_ISDIR in fwalk.
Some tests failed because said tmp4 was obviously missing – the old code threw FileNotFoundExceptions. After restoration the whole test suite passes in regression mode.
Author: Roundup Robot (python-dev) 
Date: 2012-05-15 14:34
New changeset cbe7560d4443 by Hynek Schlawack in branch 'default': #14773: Fix os.fwalk() failing on dangling symlinks http://hg.python.org/cpython/rev/cbe7560d4443
History
Date
User
Action
Args
2022-04-11 14:57:30
admin
set
github: 58978
2012-05-16 09:39:11
hynek
set
resolution: fixed
2012-05-16 09:36:17
hynek
set
status: open -> closed
stage: commit review -> resolved
2012-05-15 14:34:48
python-dev
set
nosy: + python-dev
messages: +
2012-05-15 14:09:50
pitrou
set
stage: patch review -> commit review
2012-05-15 13:29:22
hynek
set
files: + fwalk.diff
messages: +
stage: commit review -> patch review
2012-05-13 10:52:36
neologix
set
messages: +
stage: patch review -> commit review
2012-05-12 22:37:09
hynek
set
files: + fwalk-ignore-missing-files.diff
messages: +
2012-05-12 18:13:49
hynek
set
messages: +
2012-05-12 15:57:23
neologix
set
messages: +
2012-05-10 19:31:59
hynek
link
2012-05-10 19:28:39
hynek
create