We need the configuration instructions necessary for using RESTBagOStuff and Kask as our session store, and leaving the main stash where it is.
I think this means:
- an addition to $wgObjectCaches to define the store (I suggest 'kask-session' as the key)
- an addition to $wgObjectCaches to define the transitional store per T222742 using MultiwriteBagOStuff (I suggest 'kask-transition')
- setting $wgSessionCacheType to 'kask-transition'
- setting $wgObjectCacheSessionExpiry to the same value as is configured for kask (9 * 3600?)
- A comment above/near the $wgObjectCacheSessionExpiry reminding the reader that setting the session expiry to something different than what is configured in Kask will give unexpected results, so don't do that.
At some point we'd need to switch $wgSessionCacheType to 'kask-session'. Since our configuration code is PHP, there's a temptation to do something like check the date, and use the different stores based on how much time has passed since the switchover date, but... that's probably being too clever.