Prevent double-submit of forms about addons

[andymckay]

It's rare to see a double form submit prevention scheme that isn't crappy. Doing this requires a bit of work and something I hesitant to recommend we work on. Especially because we plan on re-doing the entire front end of AMO next year to have a different structure, this is one of those things that wouldn't last the transition.

[andymckay]

Closing based on last comment.

[Rob--W]

@andymckay
I agree that a general form submission prevention scheme takes a lot of work and rarely results in the desired result, that's why I scoped this bug report to the use case. At the review dashboard on AMO, I frequently see double (and triple!!!) submissions (not just by others, I also commit multiple submissions due to the lag on AMO). The URL is at https://addons.mozilla.org/en-US/editors/review/[addon-id]`

The back-end logic would be something along the lines of:

1. At form creation time: Include an identifier of the last message (all messages are printed, so this comes at no cost).
2. At form submission: Query the last row of the message sequence.
3. If 2 differs from 1, show the form again and ask the user to confirm the submission.

If implemented at the front-end, then I would do:

1. At form.onsubmit, use event.preventDefault() and request the page using AJAX.
2. If the last item in the form is equal to the current last element, use an unchecked form.submit().
3. Otherwise show a warning and request a confirmation.

In both implementations I assume that the data is up-to-date (and not some old cached value).

The logic is not that difficult. If I knew a bit more about the stack that you're using for the backend, then I would have submitted a patch. Given that you're already familiar with the code, is it possible to fix it?