if_not_exists for batch.create_* doesn't work

[Natim]

The if_not_exists behavior doesn't ignore the 412 if they happen in a batch.

[Natim]

A work-around is to use safe=False for now but it is not exactly the same behavior.

[glasserc]

This is going to be pretty tricky. The current way if_not_exists is implemented is:

• Make the request with if-none-match
• If the request succeeded, return the response.
• If you got a 412, then it existed. The real value was returned, but not its permissions. Fetch its permissions.

For this to work in a batch, the request would have to enqueue some kind of post-request processing, and that processing might need to issue another batch of requests. Tying all of that together seems complicated and error-prone.

I'm trying to think if there's a larger solution here and I'm not really seeing one. Even if we had something like Futures to tie the results of a batch to the requests, we'd still need some kind of after-response-with-potentially-new-batch mechanism, which is ugly. There's also the fact that what we have right now isn't really re-entrant. It's possible that the first request gets a 412, but the conflicting record gets deleted before the second request, which gets a 404. Right now we don't handle this at all, but we could imagine the second request re-triggering the first request, which could retrigger the second request, on and on forever.

I feel like the right solution here is to change the server to return the permissions as well as the data on a 412. If we did that, then we would never need a follow-up request and we could just add some kind of post-processing that transformed 412 responses. But I'm not sure this is extensible. Is it possible for Kinto plugins to add other fields to record responses, but not to 412 responses?