Avoid race condition crash on directory deletion

Given the directory structure /a/b/c.txt there is a race condition
in DirectorySnapshot if it reads the list of entries in /a, then
/a/b is deleted, and then it tries to read /a/b. This happens
often in practice when changing between very different branches in
git.

The correct behaviour would be to report /a/b as not existing in this
case, but we cannot do this without either changing the order of the
walk (from parent-first to parent-last) or significantly increasing
memory usage and copies, so instead we report /a/b as existing but
empty, which is not ideal, but is better than the current behavior
of silently crashing.

src/watchdog/utils/dirsnapshot.py

@@ -208,7 +208,16 @@ def __init__(self, path, recursive=True,
         self._inode_to_path[(st.st_ino, st.st_dev)] = path
 
         def walk(root):
-            paths = [os.path.join(root, name) for name in listdir(root)]
+            try:
+                paths = [os.path.join(root, name) for name in listdir(root)]
+            except OSError as e:
+                # Directory may have been deleted between finding it in the directory
+                # list of its parent and trying to delete its contents. If this
+                # happens we treat it as empty, which is wrong but better than crashing
+                if e.errno == 2:
+                    return
+                else:
+                    raise
             entries = []
             for p in paths:
                 try: