Fix part of the problem with notebook losing trust.

See jupyter/nbformat#236 and
https://github.com/jupyter/nbformat/issue/235.

This mostly affects frontends that do not always add ids to cells,
like classic notebook, and that issue is not limited to cells ids, but
also to empty cells, that may not have/need a trusted field.

But that is another issue.

notebook/services/contents/filemanager.py

@@ -39,6 +39,29 @@
 
 _script_exporter = None
 
+# workaround nbformat bug:
+# nbformat save logic will mutate the notebook if it does not
+# have cells ids, so if we sign the notebook,
+# the signature of the saved notebook is changed.
+# we thus need to add cell ids to the notebook before signing.
+try:
+    from nbformat.validator import normalize
+except ImportError:
+    from nbformat.corpus.words import generate_corpus_id
+    from copy import deepcopy
+
+    def normalize(nbdict, version, version_minor):
+        nbdict = depcopy(nbdict)
+
+        if version >= 4 and version_minor >= 5:
+            # if we support cell ids ensure default ids are provided
+            for cell in nbdict["cells"]:
+                if "id" not in cell:
+                    changes += 1
+                    # Generate cell ids if any are missing
+                    cell["id"] = generate_corpus_id()
+        return nbdict
+
 
 def _post_save_script(model, os_path, contents_manager, **kwargs):
     """convert notebooks to Python script after save with nbconvert
@@ -473,6 +496,8 @@ def save(self, model, path=''):
         try:
             if model['type'] == 'notebook':
                 nb = nbformat.from_dict(model['content'])
+
+                nb = normalize(nb, nb.get("nbformat"), nb.get("nbformat_minor"))
                 self.check_and_sign(nb, path)
                 self._save_notebook(os_path, nb)
                 # One checkpoint should always exist for notebooks.