Fix race condition when updating competition tabs order

[jonatanklosko]

Apparently there was competition tab with NULL display_order in our database, which shouldn't be possible. Took me quite some clicking to reproduce this on the staging (as we run multiple workers there), but eventually I managed to get to that state. After analyzing SQL UPDATE queries log I found out this race condition:

tab	display order
A	1 (action: down)
B	2
C	3 (action: up)

If these actions happen simultaneously then tabs' desired state is A | 2 and B | 2, so one of the updates must fail. The thing is that to preserve display order uniqueness an update is done in three steps, e.g.:

1. A = NULL
2. B = 1
3. A = 2

So if the step 3. fails, we end up with A being NULL.
Wrapping these 3 steps in a transaction should solve the issue.

[jfly]

LGTM! Nice catch :)

Do we enforce uniqueness of tab order via a mysql constraint?

[jonatanklosko]

We do, that's one of the updates must've failed and left NULL value.

worldcubeassociation.org/WcaOnRails/db/structure.sql

Line 723 in e5a9bcb

UNIQUE KEY `index_competition_tabs_on_display_order_and_competition_id` (`display_order`,`competition_id`)