The bug is os.fwalk() crashes with unhandled exception when there is an error
accessing symbolic link targets.

To reproduce the bug, create a symbolic link that targets a file that you do not
have permission to access:

    $ touch handsoff
    $ sudo chown root:root handsoff
    $ sudo chmod 700 handsoff
    $ ln -s handsoff blah

Now, os.fwalk() fails:

    >>> for root, dirs, files, fd in os.fwalk('.'):
    ...     print(root, dirs, files)
    ...
    Traceback (most recent call last):
      File "test_fwalk_permission_error.py", line 3, in <module>
        for root, dirs, files, fd in os.fwalk('.'):
      File "/usr/lib64/python3.5/os.py", line 520, in fwalk
        yield from _fwalk(topfd, top, topdown, onerror, follow_symlinks)
      File "/usr/lib64/python3.5/os.py", line 537, in _fwalk
        if st.S_ISDIR(stat(name, dir_fd=topfd).st_mode):
    PermissionError: [Errno 13] Permission denied: 'blah'


The cause of the problem is in this part of os.py:

for name in names:
    try:
        # 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(stat(name, dir_fd=topfd).st_mode):
            dirs.append(name)
        else:
            nondirs.append(name)
    except FileNotFoundError:
        try:
            # Add dangling symlinks, ignore disappeared files
            if st.S_ISLNK(stat(name, dir_fd=topfd, follow_symlinks=False)
                        .st_mode):
                nondirs.append(name)
        except FileNotFoundError:
            continue

To fix it, simply replace FileNotFoundError with more general OSError.

Cheers

Similar to http://bugs.python.org/issue25860

I can't reproduce the issue.

$ mkdir testdir
$ touch testdir/handsoff
$ sudo chown root:root testdir/handsoff
$ sudo chmod 700 testdir/handsoff
$ ln -s handsoff testdir/blah
$ python3.5
>>> import os
>>> list(os.walk('testdir'))
[('testdir', [], ['handsoff', 'blah'])]
>>> list(os.fwalk('testdir'))
[('testdir', [], ['handsoff', 'blah'], 3)]

Ubuntu 16.04, Linux 4.4, tested on ext4 and tmpfs filesystems.

Sorry, the target file needs to be in a directory you don't have permission to access:

    $ mkdir handsoff
    $ sudo chown root:root handsoff
    $ sudo chmod 700 handsoff
    $ ln -s handsoff/anything blah

Yes, there is a difference between os.walk() and os.fwalk() in some cases. I don't know whether it was introduced deliberately or by accident. os.fwalk() was added in issue13734, FileNotFoundError is used since issue14773.

This is definitely a bug because there is no way an unhandled exception like this is acceptable behaviour. The behaviour is that the fwalk iteration will stop and there is no way to recover / continue. Also fwalk takes an onerror callback function that won't be called with this error.

Here is a patch with tests. Needed to test it on Windows.

New changeset fe1fea4ded04 by Serhiy Storchaka in branch '3.5':
Issue #28353: os.fwalk() no longer fails on broken links.
https://hg.python.org/cpython/rev/fe1fea4ded04

New changeset e99ec3c77a63 by Serhiy Storchaka in branch '3.6':
Issue #28353: os.fwalk() no longer fails on broken links.
https://hg.python.org/cpython/rev/e99ec3c77a63

New changeset 33a1a3dd0051 by Serhiy Storchaka in branch 'default':
Issue #28353: os.fwalk() no longer fails on broken links.
https://hg.python.org/cpython/rev/33a1a3dd0051

New changeset ec12e16ea6a1 by Serhiy Storchaka in branch '3.6':
Issue #28353: Try to fix tests.
https://hg.python.org/cpython/rev/ec12e16ea6a1

New changeset a0913dbadea6 by Serhiy Storchaka in branch 'default':
Issue #28353: Try to fix tests.
https://hg.python.org/cpython/rev/a0913dbadea6

New changeset 470224ec16b6 by Serhiy Storchaka in branch '3.5':
Issue #28353: Fixed tests of os.fwalk() with broken links.
https://hg.python.org/cpython/rev/470224ec16b6

Serhiy, after your commits, test_os requires root privileges or it'll fail. This is not the case before.

[cpython]$ ./python -m test test_os
Run tests sequentially
0:00:00 [1/1] test_os
test test_os crashed -- Traceback (most recent call last):
  File "/home/angwer/cpython/Lib/test/libregrtest/runtest.py", line 164, in runtest_inner
    test_runner()
  File "/home/angwer/cpython/Lib/test/libregrtest/runtest.py", line 163, in test_runner
    support.run_unittest(tests)
  File "/home/angwer/cpython/Lib/test/support/__init__.py", line 1849, in run_unittest
    _run_suite(suite)
  File "/home/angwer/cpython/Lib/test/support/__init__.py", line 1824, in _run_suite
    raise TestFailed(err)
test.support.TestFailed: multiple errors occurred; run in verbose mode for details

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/angwer/cpython/Lib/test/libregrtest/runtest.py", line 167, in runtest_inner
    test_time = time.time() - start_time
  File "/home/angwer/cpython/Lib/test/libregrtest/save_env.py", line 278, in __exit__
    restore(original)
  File "/home/angwer/cpython/Lib/test/libregrtest/save_env.py", line 235, in restore_files
    support.rmtree(fn)
  File "/home/angwer/cpython/Lib/test/support/__init__.py", line 377, in rmtree
    _rmtree(path)
  File "/home/angwer/cpython/Lib/shutil.py", line 474, in rmtree
    _rmtree_safe_fd(fd, path, onerror)
  File "/home/angwer/cpython/Lib/shutil.py", line 412, in _rmtree_safe_fd
    _rmtree_safe_fd(dirfd, fullname, onerror)
  File "/home/angwer/cpython/Lib/shutil.py", line 412, in _rmtree_safe_fd
    _rmtree_safe_fd(dirfd, fullname, onerror)
  File "/home/angwer/cpython/Lib/shutil.py", line 408, in _rmtree_safe_fd
    onerror(os.open, fullname, sys.exc_info())
  File "/home/angwer/cpython/Lib/shutil.py", line 406, in _rmtree_safe_fd
    dirfd = os.open(name, os.O_RDONLY, dir_fd=topfd)
PermissionError: [Errno 13] Permission denied: 'SUB21'

'test_os' left behind directory '@test_23400_tmp' and it couldn't be removed: [Errno 13] Permission denied: 'SUB21'
test_os failed

1 test failed:
    test_os

Total duration: 292 ms
Tests result: FAILURE
Traceback (most recent call last):
  File "/home/angwer/cpython/Lib/test/support/__init__.py", line 914, in temp_dir
    yield path
  File "/home/angwer/cpython/Lib/test/support/__init__.py", line 963, in temp_cwd
    yield cwd_dir
  File "/home/angwer/cpython/Lib/test/libregrtest/main.py", line 468, in main
    self._main(tests, kwargs)
  File "/home/angwer/cpython/Lib/test/libregrtest/main.py", line 497, in _main
    sys.exit(len(self.bad) > 0 or self.interrupted)
SystemExit: True

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/angwer/cpython/Lib/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/home/angwer/cpython/Lib/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/angwer/cpython/Lib/test/__main__.py", line 2, in <module>
    main()
  File "/home/angwer/cpython/Lib/test/libregrtest/main.py", line 532, in main
    Regrtest().main(tests=tests, **kwargs)
  File "/home/angwer/cpython/Lib/test/libregrtest/main.py", line 468, in main
    self._main(tests, kwargs)
  File "/home/angwer/cpython/Lib/contextlib.py", line 100, in __exit__
    self.gen.throw(type, value, traceback)
  File "/home/angwer/cpython/Lib/test/support/__init__.py", line 963, in temp_cwd
    yield cwd_dir
  File "/home/angwer/cpython/Lib/contextlib.py", line 100, in __exit__
    self.gen.throw(type, value, traceback)
  File "/home/angwer/cpython/Lib/test/support/__init__.py", line 917, in temp_dir
    rmtree(path)
  File "/home/angwer/cpython/Lib/test/support/__init__.py", line 377, in rmtree
    _rmtree(path)
  File "/home/angwer/cpython/Lib/shutil.py", line 474, in rmtree
    _rmtree_safe_fd(fd, path, onerror)
  File "/home/angwer/cpython/Lib/shutil.py", line 412, in _rmtree_safe_fd
    _rmtree_safe_fd(dirfd, fullname, onerror)
  File "/home/angwer/cpython/Lib/shutil.py", line 412, in _rmtree_safe_fd
    _rmtree_safe_fd(dirfd, fullname, onerror)
  File "/home/angwer/cpython/Lib/shutil.py", line 412, in _rmtree_safe_fd
    _rmtree_safe_fd(dirfd, fullname, onerror)
  File "/home/angwer/cpython/Lib/shutil.py", line 408, in _rmtree_safe_fd
    onerror(os.open, fullname, sys.exc_info())
  File "/home/angwer/cpython/Lib/shutil.py", line 406, in _rmtree_safe_fd
    dirfd = os.open(name, os.O_RDONLY, dir_fd=topfd)
PermissionError: [Errno 13] Permission denied: 'SUB21'

Hmm, tests were passed when I ran them before committing. Seems this is heisenbug.

Seems like the dir you add breaks other cases. I change addCleanUp to os.chmod and get:

======================================================================
FAIL: test_file_like_path (test.test_os.BytesWalkTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/angwer/cpython/Lib/test/test_os.py", line 950, in test_file_like_path
    self.test_walk_prune(_PathLike(self.walk_path))
  File "/home/angwer/cpython/Lib/test/test_os.py", line 941, in test_walk_prune
    self.assertEqual(len(all), 2)
AssertionError: 3 != 2

======================================================================
FAIL: test_walk_bottom_up (test.test_os.BytesWalkTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/angwer/cpython/Lib/test/test_os.py", line 956, in test_walk_bottom_up
    self.assertEqual(len(all), 4, all)
AssertionError: 5 != 4 : [('@test_24702_tmp/TEST1/SUB2/SUB21', [], ['tmp3']), ('@test_24702_tmp/TEST1/SUB2', ['link', 'SUB21'], ['tmp3', 'broken_link3', 'broken_link', 'broken_link2']), ('@test_24702_tmp/TEST1/SUB1/SUB11', [], []), ('@test_24702_tmp/TEST1/SUB1', ['SUB11'], ['tmp2']), ('@test_24702_tmp/TEST1', ['SUB2', 'SUB1'], ['tmp1'])]

======================================================================
FAIL: test_walk_prune (test.test_os.BytesWalkTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/angwer/cpython/Lib/test/test_os.py", line 941, in test_walk_prune
    self.assertEqual(len(all), 2)
AssertionError: 3 != 2

======================================================================
FAIL: test_walk_topdown (test.test_os.BytesWalkTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/angwer/cpython/Lib/test/test_os.py", line 916, in test_walk_topdown
    self.assertEqual(len(all), 4)
AssertionError: 5 != 4

======================================================================
FAIL: test_file_like_path (test.test_os.FwalkTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/angwer/cpython/Lib/test/test_os.py", line 950, in test_file_like_path
    self.test_walk_prune(_PathLike(self.walk_path))
  File "/home/angwer/cpython/Lib/test/test_os.py", line 941, in test_walk_prune
    self.assertEqual(len(all), 2)
AssertionError: 3 != 2

======================================================================
FAIL: test_walk_bottom_up (test.test_os.FwalkTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/angwer/cpython/Lib/test/test_os.py", line 956, in test_walk_bottom_up
    self.assertEqual(len(all), 4, all)
AssertionError: 5 != 4 : [('@test_24702_tmp/TEST1/SUB2/SUB21', [], ['tmp3']), ('@test_24702_tmp/TEST1/SUB2', ['link', 'SUB21'], ['tmp3', 'broken_link3', 'broken_link', 'broken_link2']), ('@test_24702_tmp/TEST1/SUB1/SUB11', [], []), ('@test_24702_tmp/TEST1/SUB1', ['SUB11'], ['tmp2']), ('@test_24702_tmp/TEST1', ['SUB2', 'SUB1'], ['tmp1'])]

======================================================================
FAIL: test_walk_prune (test.test_os.FwalkTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/angwer/cpython/Lib/test/test_os.py", line 941, in test_walk_prune
    self.assertEqual(len(all), 2)
AssertionError: 3 != 2

======================================================================
FAIL: test_walk_topdown (test.test_os.FwalkTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/angwer/cpython/Lib/test/test_os.py", line 916, in test_walk_topdown
    self.assertEqual(len(all), 4)
AssertionError: 5 != 4

======================================================================
FAIL: test_file_like_path (test.test_os.WalkTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/angwer/cpython/Lib/test/test_os.py", line 950, in test_file_like_path
    self.test_walk_prune(_PathLike(self.walk_path))
  File "/home/angwer/cpython/Lib/test/test_os.py", line 941, in test_walk_prune
    self.assertEqual(len(all), 2)
AssertionError: 3 != 2

======================================================================
FAIL: test_walk_bottom_up (test.test_os.WalkTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/angwer/cpython/Lib/test/test_os.py", line 956, in test_walk_bottom_up
    self.assertEqual(len(all), 4, all)
AssertionError: 5 != 4 : [('@test_24702_tmp/TEST1/SUB2/SUB21', [], ['tmp3']), ('@test_24702_tmp/TEST1/SUB2', ['link', 'SUB21'], ['tmp3', 'broken_link3', 'broken_link', 'broken_link2']), ('@test_24702_tmp/TEST1/SUB1/SUB11', [], []), ('@test_24702_tmp/TEST1/SUB1', ['SUB11'], ['tmp2']), ('@test_24702_tmp/TEST1', ['SUB2', 'SUB1'], ['tmp1'])]

======================================================================
FAIL: test_walk_prune (test.test_os.WalkTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/angwer/cpython/Lib/test/test_os.py", line 941, in test_walk_prune
    self.assertEqual(len(all), 2)
AssertionError: 3 != 2

======================================================================
FAIL: test_walk_topdown (test.test_os.WalkTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/angwer/cpython/Lib/test/test_os.py", line 916, in test_walk_topdown
    self.assertEqual(len(all), 4)
AssertionError: 5 != 4

New changeset 655510dd46fd by Serhiy Storchaka in branch '3.5':
Issue #28353: Make test_os.WalkTests.test_walk_bad_dir stable.
https://hg.python.org/cpython/rev/655510dd46fd

New changeset df28e536f19d by Serhiy Storchaka in branch '3.6':
Issue #28353: Make test_os.WalkTests.test_walk_bad_dir stable.
https://hg.python.org/cpython/rev/df28e536f19d

New changeset aa891d13e1cf by Serhiy Storchaka in branch 'default':
Issue #28353: Make test_os.WalkTests.test_walk_bad_dir stable.
https://hg.python.org/cpython/rev/aa891d13e1cf

test_walk_bad_dir renamed one of subdirs and tearDown failed to clean up changed tree. An error happened depending on the order of listed subdirectories: ['SUB1', 'SUB2'] -- passed, ['SUB2', 'SUB1'] -- failed.

It works. Close. :-)