[sqli-ze] sessions: atomic session creation

[ellemouton]

• Today, a session is created as follows:
  1) Lock Lit's sessRegMu mutex to ensure only one thread is trying to create a session at a time
  2) call the session store's db.GetUnusedIDAndKeyPair method to find an unused alias & private key pair.
  3) call the NewSession function to construct a new Sessions object (note: this does not touch the DB yet) .
  4) Finally, call the store's db.CreateSession method to actually go persist the new store.
  5) unlock sessRegMu.
  6) (For linked sessions: we also use the CheckSessionGroupPredicate to ensure any linked sessions are inactive.)

• How the above flow came about: For Autopilot sessions, we wanted to make sure we could successfully register a session with Autopilot before actually going and persisting it to our DB. But at the time of registration, we already want to know the ID/Local pub key pair we will use for the session. This is why we would do all this in-memory only work before actually persisting the session. This is also why we needed to lock the mutex to ensure only one thread calls db.GetUnusedIDAndKeyPair at a time.

• For a clean interface though, all the above should really happen atomically under a single database read transaction & we should not need to expose things like db.GetUnusedIDAndKeyPair and db.CheckSessionGroupPredicate.

• What we will instead do is:
  - Add a new session.State: SessionReserved, which will be the new first state of a session.
  - Then, we will change NewSession to instead be a method on the Store interface. This will
  - take care of finding a unique ID/Local key pair (so we can remove that from the interface).
  - insert the new session but under the Reserved state.
  - if a group is linked, here is where we will check that all other sessions are revoked. So CheckSessionGroupPredicate can also be removed from the interface.
  - CreateSession will then only move a session from Reserved to Created.
  - On DB startup, we make sure to delete all sessions in the Reserved state.

[ellemouton]

@ViktorTigerstrom & @bitromortac for concept ACK here 🙏

[ViktorTigerstrom]

concept ACK 🚀

[bitromortac]

sounds good, ACK ✅