Fix for recursive symlink issue #4669

[scanon]

This is a potential fix to issue #4669. The fix simply catches the
recursive symlink error and moves on. It is possible that the
try/except belongs in the utils but I wanted to limit the scope.

[takluyver]

Spelling of 'recursive'.

Also, we should use the assertIn and assertNotIn methods rather than assertTrue and assertFalse. The more specific methods give more useful messages when things fail.

[takluyver]

I fixed the spelling, but this test is now failing on Windows:

======================================================================
FAIL: test_recursive_symlink (notebook.services.contents.tests.test_manager.TestFileContentsManager)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Miniconda36-x64\lib\site-packages\ipython_genutils\testing\decorators.py", line 186, in skipper_func
    return f(*args, **kwargs)
  File "C:\projects\notebook\notebook\services\contents\tests\test_manager.py", line 150, in test_recursive_symlink
    self.assertNotIn('recursive', contents)
AssertionError: 'recursive' unexpectedly found in {'recursive': {'name': 'recursive', 'path': 'test recursive symlink/recursive', 'last_modified': datetime.datetime(2019, 6, 25, 8, 35, 46, 553212, tzinfo=<notebook._tz.tzUTC object at 0x000000B0AB495D68>), 'created': datetime.datetime(2019, 6, 25, 8, 35, 46, 553212, tzinfo=<notebook._tz.tzUTC object at 0x000000B0AB495D68>), 'content': None, 'format': None, 'mimetype': None, 'size': 0, 'writable': True, 'type': 'file'}, 'untitled.txt': {'name': 'untitled.txt', 'path': 'test recursive symlink/untitled.txt', 'last_modified': datetime.datetime(2019, 6, 25, 8, 35, 46, 553212, tzinfo=<notebook._tz.tzUTC object at 0x000000B0AB495D68>), 'created': datetime.datetime(2019, 6, 25, 8, 35, 46, 553212, tzinfo=<notebook._tz.tzUTC object at 0x000000B0AB495D68>), 'content': None, 'format': None, 'mimetype': 'text/plain', 'size': 0, 'writable': True, 'type': 'file'}}

I guess Windows doesn't handle this in the same way Unix does. It's probably fine to just skip the test on Windows completely. I think it's unusual to use symlinks on Windows anyway.

[takluyver]

I've got the tests passing.

I was informed of a related issue someone at EuXFEL encountered, where there was a permissions error stat-ing a symlink. So I've broadened this to catch all OSError, logging the exception details for anything besides the recursive symlink case.

[blink1073]

LGTM, thanks!