Write notebook.json file atomically

[mdboom]

The motivation for this was a bug in the jupyter-notebook-gist extension whereby a LazyConfigValue was being added to the JSON config tree in some cases. That's definitely a bug on the part of the extension that needs to be fixed over there.

However, this caused the notebook.json writer to crash mid-writing, leaving a corrupted notebook.json on disk, eg.:

{
  "load_extensions": {
    "jupyter-notebook-gist/extension": true,
    "jupyter-js-widgets/extension": true
  },
  "oauth_client_id":

So on subsequent startups of the Jupyter notebook, parsing fails unless the user manually fixes the JSON file. Potentially there could have been data loss in the config file if there were keys after oauth_client_id that failed to write.

I would argue that notebook shouldn't write the corrupted JSON to disk, but leave it in its last state if this happens. Manual testing shows that if a rogue extension runs, subsequent extensions are still run and can successfully add their own values to the JSON. In other words, only the data in single call to set or update is thrown out by this change.

I considered using the "write to a tempfile and replace" method for this. That's complicated to get correct in a cross-platform way (requires os.replace on Windows, which is only in Python 3.3 and later, so would require a backfill for Python 2.7 etc.) If streaming directly to disk is important, I'm happy to redo this that way, but I thought for config files that are generally quite small, doing this in-memory should be fine.

[takluyver]

Thanks, I think this makes sense. The error on Appveyor is some npm problem, not related to this change.