Don't trash non-empty directories on Windows

It appears that the Windows backend of send2trash can't guarantee that
files won't actually be deleted. There are some hints that this happens
in particular when you delete a lot of data at once. So this returns to
refusing to delete folders unless they're empty (or contain only ipynb
checkpoints).

Closes gh-3631

notebook/services/contents/filemanager.py

@@ -517,7 +517,21 @@ class FileContentsManager(FileManagerMixin, ContentsManager):
             home_dev = os.stat(os.path.expanduser('~')).st_dev
             return file_dev == home_dev
 
+        def is_non_empty_dir(os_path):
+            if os.path.isdir(os_path):
+                # A directory containing only leftover checkpoints is
+                # considered empty.
+                cp_dir = getattr(self.checkpoints, 'checkpoint_dir', None)
+                if set(os.listdir(os_path)) - {cp_dir}:
+                    return True
+
+            return False
+
         if self.delete_to_trash:
+            if sys.platform == 'win32' and is_non_empty_dir(os_path):
+                # send2trash can really delete files on Windows, so disallow
+                # deleting non-empty files. See Github issue 3631.
+                raise web.HTTPError(400, u'Directory %s not empty' % os_path)
             if _check_trash(os_path):
                 self.log.debug("Sending %s to trash", os_path)
                 # Looking at the code in send2trash, I don't think the errors it
@@ -530,14 +544,9 @@ class FileContentsManager(FileManagerMixin, ContentsManager):
                                  "to home directory", os_path)
 
         if os.path.isdir(os_path):
-            listing = os.listdir(os_path)
             # Don't permanently delete non-empty directories.
-            # A directory containing only leftover checkpoints is
-            # considered empty.
-            cp_dir = getattr(self.checkpoints, 'checkpoint_dir', None)
-            for entry in listing:
-                if entry != cp_dir:
-                    raise web.HTTPError(400, u'Directory %s not empty' % os_path)
+            if is_non_empty_dir(os_path):
+                raise web.HTTPError(400, u'Directory %s not empty' % os_path)
             self.log.debug("Removing directory %s", os_path)
             with self.perm_to_403():
                 shutil.rmtree(os_path)