Avoid race condition crash when a directory is swapped for a file

This commit builds off the race condition checking introduced in
2e83199, but also correctly handles the case where a directory is
replaced with a file before it can be walked.

For example:
1. [external] create /a/b/c.txt
2. [watchdog] walk /a (contains directory /a/b/ at this point)
3. [external] rm -rf /a/b/
4. [external] touch /a/b
5. [watchdog] walk /a/b/ (raises NotADirectoryError/ENOTDIR)

Also adds a test case that runs through the above example to ensure that
an exception is no longer raised.

src/watchdog/utils/dirsnapshot.py

@@ -218,8 +218,9 @@ def walk(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.
-                if e.errno == errno.ENOENT:
+                # happens we treat it as empty. Likewise if the directory was replaced
+                # with a file of the same name (less likely, but possible).
+                if e.errno == errno.ENOENT or e.errno == errno.ENOTDIR:
                     return
                 else:
                     raise

tests/test_snapshot_diff.py

@@ -14,8 +14,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import os
 import time
-from .shell import mkdir, touch, mv
+from .shell import mkdir, touch, mv, rm
 from watchdog.utils.dirsnapshot import DirectorySnapshot
 from watchdog.utils.dirsnapshot import DirectorySnapshotDiff
 from watchdog.utils import platform
@@ -105,3 +106,19 @@ def test_detect_modify_for_moved_files(p):
     diff = DirectorySnapshotDiff(ref, DirectorySnapshot(p('')))
     assert diff.files_moved == [(p('a'), p('b'))]
     assert diff.files_modified == [p('a')]
+
+def test_replace_dir_with_file(p):
+    # Replace a dir with a file of the same name just before the normal listdir
+    # call and ensure it doesn't cause an exception
+
+    def listdir_fcn(path):
+        if path == p("root", "dir"):
+            rm(path, recursive=True)
+            touch(path)
+        return os.listdir(path)
+
+    mkdir(p('root'))
+    mkdir(p('root', 'dir'))
+
+    # Should NOT raise an OSError (ENOTDIR)
+    DirectorySnapshot(p('root'), listdir=listdir_fcn)