Sliding Sync: Notify and sync when one-time keys or fallback keys are claimed/uploaded

[MadLittleMods]

Notify and sync when one-time keys or fallback keys are claimed/uploaded

Applies to Sync v2 and Sliding Sync although Sync v2 is untested in this area.

Fix #17474

Dev notes

Devices will generate one-time keys and upload them to the server; these will later be claimed by other users. Servers must ensure that each one-time key is only claimed once: a homeserver should discard the one time key once it has been given to another user.

Devices will be informed, via /sync, about the number of one-time keys remaining that can be claimed, as well as whether the fallback keys have been used. The device can thus ensure that, while it is online, there is a sufficient supply of one-time keys available, and that the fallback keys get replaced if they have been used.

https://spec.matrix.org/v1.11/client-server-api/#one-time-and-fallback-keys

Claiming one-time keys

POST /_matrix/client/v3/keys/claim

e2e_keys_handler.claim_one_time_keys
e2e_keys_handler.claim_local_one_time_keys

store.claim_e2e_one_time_keys
store.claim_e2e_fallback_keys

Uploading one-time keys

POST /_matrix/client/v3/keys/upload

e2e_keys_handler.upload_keys_for_user
e2e_keys_handler._upload_one_time_keys_for_user
e2e_keys_handler.set_e2e_fallback_keys

Todo

• Handle workers/replication (keys are uploaded/claimed on one worker and we are notifier.wait_for_events() on a another worker like a syncotron)

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct
  (run the linters)

[MadLittleMods]

I suppose the way to actually fix this is to introduce a new stream like StreamKeyType.E2EE_KEYS/E2eeKeysStream for changes to one-time or fallback keys changing?

Just want to sanity check before I go that route.

[MadLittleMods]

cc @erikjohnston