Snapshot: don't walk directories without read permissions

[BoboTiG]

Original patch by Joshua Skelton (@joshuaskelly) on issue #408.

[BoboTiG]

@rrzaripov, do you mind reviewing 🙏 ?

[rrzaripov]

@BoboTiG I was tested this patch under Windows.
For example, next directory structure exists c:\tmp\A\B\C. All directories for simplifying empty. While testing we will disabling access to B.

Python 2.7

>>> from watchdog.utils import dirsnapshot
>>> dirsnapshot.DirectorySnapshot("c:\\tmp\\A")
{'c:\\tmp\\A': StatResult(st_dev=2195292832L, st_ino=6473924464939757L, st_mode=16895, st_mtime=1560413735L),
'c:\\tmp\\A\\B': StatResult(st_dev=2195292832L, st_ino=15199648742969916L, st_mode=16895, st_mtime=1560413742L), 
'c:\\tmp\\A\\B\\C': StatResult(st_dev=2195292832L, st_ino=5348024558097312L, st_mode=16895, st_mtime=1560413739L),}
>>> # Disable access to C:\tmp\A\B
>>> dirsnapshot.DirectorySnapshot("c:\\tmp\\A")
{'c:\\tmp\\A': StatResult(st_dev=2195292832L, st_ino=6473924464939757L, st_mode=16895, st_mtime=1560413735L)}

Python 3.6

>>> from watchdog.utils import dirsnapshot
>>> dirsnapshot.DirectorySnapshot("c:\\tmp\\A")
{'c:\\tmp\\A': os.stat_result(st_mode=16895, st_ino=6473924464939757, st_dev=2195292832, st_nlink=1, st_uid=0, st_gid=0, st_size=0, st_atime=1560413735, st_mtime=1560413735, st_ctime=1560411735),
'c:\\tmp\\A\\B': os.stat_result(st_mode=16895, st_ino=15199648742969916, st_dev=2195292832, st_nlink=1, st_uid=0, st_gid=0, st_size=0, st_atime=1560413742, st_mtime=1560413742, st_ctime=1560413306), 
'c:\\tmp\\A\\B\\C': os.stat_result(st_mode=16895, st_ino=5348024558097312, st_dev=2195292832, st_nlink=1, st_uid=0, st_gid=0, st_size=0, st_atime=1560413739, st_mtime=1560413739, st_ctime=1560413739)}
>>> # Disable access to C:\tmp\A\B
>>> dirsnapshot.DirectorySnapshot("c:\\tmp\\A")
{'c:\\tmp\\A': os.stat_result(st_mode=16895, st_ino=6473924464939757, st_dev=2195292832, st_nlink=1, st_uid=0, st_gid=0, st_size=0, st_atime=1560413735, st_mtime=1560413735, st_ctime=1560411735),
'c:\\tmp\\A\\B': os.stat_result(st_mode=16895, st_ino=0, st_dev=0, st_nlink=0, st_uid=0, st_gid=0, st_size=0, st_atime=1560413742, st_mtime=1560413742, st_ctime=1560413306)}

In both cases we not get content of B directory, without exceptions, this is great and I think #408 may be closed after merging this patch. But on Python 3.6 snapshot of directory A contains directory B. I think this is just Python 2 specificity, and just need to keep in mind.

[BoboTiG]

This is great, thank you very much for the review and test 💪

[BoboTiG]

I should add a test, this is somewhat crucial :)

[BoboTiG]

Test added + some refactoring.

[BoboTiG]

I will merge if you are still OK @rrzaripov.

[rrzaripov]

@BoboTiG I'm checkout latest version of branch, test passed, all looks good for merging!

[joshuaskelly]

@BoboTiG Thanks for driving this and getting it merged back in! 🎉🏆

[BoboTiG]

Thansk to you for the original patch ;)