Implement atomic save

Ping @fperez, this should avoid issues with corrupted/lost notebooks
when the disk is full, though I haven't worked out how to test it just
yet.

Closes gh-6254

IPython/html/services/contents/filemanager.py

@@ -13,6 +13,7 b' from tornado import web'
 
 from .manager import ContentsManager
 from IPython.nbformat import current
+from IPython.utils.io import atomic_writing
 from IPython.utils.path import ensure_dir_exists
 from IPython.utils.traitlets import Unicode, Bool, TraitError
 from IPython.utils.py3compat import getcwd
@@ -295,7 +296,7 b' class FileContentsManager(ContentsManager):'
         if 'name' in nb['metadata']:
             nb['metadata']['name'] = u''
 
-        with io.open(os_path, 'w', encoding='utf-8') as f:
+        with atomic_writing(os_path, encoding='utf-8') as f:
             current.write(nb, f, u'json')
 
     def _save_file(self, os_path, model, name='', path=''):
@@ -312,7 +313,7 b' class FileContentsManager(ContentsManager):'
                 bcontent = base64.decodestring(b64_bytes)
         except Exception as e:
             raise web.HTTPError(400, u'Encoding error saving %s: %s' % (os_path, e))
-        with io.open(os_path, 'wb') as f:
+        with atomic_writing(os_path, 'wb') as f:
             f.write(bcontent)
 
     def _save_directory(self, os_path, model, name='', path=''):

IPython/utils/io.py

@@ -16,6 +16,7 b' from __future__ import absolute_import'
 # Imports
 #-----------------------------------------------------------------------------
 import codecs
+from contextlib import contextmanager
 import os
 import sys
 import tempfile
@@ -217,6 +218,23 b" def temp_pyfile(src, ext='.py'):"
     f.flush()
     return fname, f
 
+@contextmanager
+def atomic_writing(path, mode='w', encoding='utf-8', **kwargs):
+    tmp_file = path + '.tmp-write'
+    if 'b' in mode:
+        encoding = None
+
+    with open(tmp_file, mode, encoding=encoding, **kwargs) as f:
+        yield f
+
+    # Written successfully, now rename it
+
+    if os.name == 'nt' and os.path.exists(path):
+        # Rename over existing file doesn't work on Windows
+        os.remove(path)
+
+    os.rename(tmp_file, path)
+
 
 def raw_print(*args, **kw):
     """Raw print to sys.__stdout__, otherwise identical interface to print()."""