Discards user work when multiple users are editing

[aarchiba]

I have experienced a number of instances where the Numbas editor has lost work that I did: changes I have made to question statements and to grading settings have disappeared overnight. I think that the problem is that my teaching assistant was also working on the same question, or at least had the question open in a tab.

My guess as to the process:

• Person A opens the question/part, loading the content.
• Person B opens the question/part, loading the content.
• Person A makes some change, and it is automatically saved.
• Person B does not see the change, and makes some other unrelated change. The auto-save reverts person A's change.

There is an alternative way to lose changes described in #731 that can happen if you leave pages too soon; this happens if you leave pages open too long.

[christianp]

Yes, this is a known problem, sorry. I've been putting off dealing with this because ideally I'd like to allow real-time collaboration, which would require a clever merging strategy. But really we should implement something like an editing lock in the meantime.