normalize unicode notebook filenames

used in comparison check for notebook name change.

Unless the filenames are normalized,
unchanged names may result in false positives for a name change
(e.g. OS X uses NFD on the filesystem,
so u'\xfc' roundtripped to the filesystem will be u'u\u0308'),
which can result in the first save of a notebook after open performing the following actions:

1. save the recently opened notebook
2. `old_name != new_name`, so name change detected
3. delete old_name (which is actually new_name), which ultimately deletes the just-saved notebook

In master, this has a symptom of the first checkpoint failing because the first save actually deleted the file, and you can't checkpoint a notebook that doesn't exist.

closes  #3360

IPython/frontend/html/notebook/services/notebooks/filenbmanager.py

@@ -21,6 +21,7 b' import io'
 import os
 import glob
 import shutil
+from unicodedata import normalize
 
 from tornado import web
 
@@ -78,7 +79,7 b' class FileNotebookManager(NotebookManager):'
         """List all notebook names in the notebook dir."""
         names = glob.glob(os.path.join(self.notebook_dir,
                                        '*' + self.filename_ext))
-        names = [os.path.splitext(os.path.basename(name))[0]
+        names = [normalize('NFC', os.path.splitext(os.path.basename(name))[0])
                  for name in names]
         return names
 
@@ -161,7 +162,7 b' class FileNotebookManager(NotebookManager):'
     def write_notebook_object(self, nb, notebook_id=None):
         """Save an existing notebook object by notebook_id."""
         try:
-            new_name = nb.metadata.name
+            new_name = normalize('NFC', nb.metadata.name)
         except AttributeError:
             raise web.HTTPError(400, u'Missing notebook name')
 
@@ -263,7 +264,7 b' class FileNotebookManager(NotebookManager):'
     
     def get_checkpoint_path_by_name(self, name, checkpoint_id):
         """Return a full path to a notebook checkpoint, given its name and checkpoint id."""
-        filename = "{name}-{checkpoint_id}{ext}".format(
+        filename = u"{name}-{checkpoint_id}{ext}".format(
             name=name,
             checkpoint_id=checkpoint_id,
             ext=self.filename_ext,
@@ -294,7 +295,7 b' class FileNotebookManager(NotebookManager):'
         """Create a checkpoint from the current state of a notebook"""
         nb_path = self.get_path(notebook_id)
         # only the one checkpoint ID:
-        checkpoint_id = "checkpoint"
+        checkpoint_id = u"checkpoint"
         cp_path = self.get_checkpoint_path(notebook_id, checkpoint_id)
         self.log.debug("creating checkpoint for notebook %s", notebook_id)
         if not os.path.exists(self.checkpoint_dir):
@@ -309,7 +310,7 b' class FileNotebookManager(NotebookManager):'
         
         This notebook manager currently only supports one checkpoint per notebook.
         """
-        checkpoint_id = "checkpoint"
+        checkpoint_id = u"checkpoint"
         path = self.get_checkpoint_path(notebook_id, checkpoint_id)
         if not os.path.exists(path):
             return []