When marking newly-backed up sessions, only fetch ones that are not backed up

[andybalaam]

Fixes element-hq/element-web#26488

(No public API changes.)

Explanation

When we receive a response saying that a key backup is complete, we must mark the sessions whose keys are not backed up as such, so we don't back them up again.

In the current code. we call store.get_inbound_group_sessions(), then filter the results to find those sessions that are associated with this backup. This is very slow as there can be a very large number of sessions.

This PR is to call store.inbound_group_sessions_for_backup() instead. This method only fetches sessions that are not already backed up, which seems, if I understand it correctly, to be the exact set of sessions that we might want to mark as backed up at this time.

What limit to use?

The big question is what maximum limit to pass in (get_inbound_group_sessions has no limit, which is part of the cause of this problem). The point is that there may be new sessions that are not part of this backup, so we can't just use Self::BACKUP_BATCH_SIZE, which is the maximum number of sessions in this backup. I chose a large number that should mean that we usually get all the relevant sessions.

If we ever don't mark sessions as backed up even though they are, I am not sure of the consequences - I think maybe we will back them up again in the next backup, which would hopefully not be a problem.

If we ever had so many new sessions that the limit meant we never found any sessions to mark as backed up, that would be a problem, because we would never stop trying to back up the same sessions over and over again. I added an error log so we can at least identify the problem if it happens.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (e652069) 82.77% compared to head (b0a8904) 82.75%.

Files	Patch %	Lines
crates/matrix-sdk-crypto/src/backups/mod.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2922      +/-   ##
==========================================
- Coverage   82.77%   82.75%   -0.03%     
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  Files         220      220              
  Lines       22462    22464       +2     
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- Hits        18593    18589       -4     
- Misses       3869     3875       +6     

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[BillCarsonFr]

I'd recommand to create a new store method mark_session_as_backed_up(roomId, sessionId, sender_key).

Then in mark_request_as_sent you take the sessions in PendingBackup.session and iterate to mark them all as marked.

As for now we might need to decrypt to check any mismatch on sender_key

[andybalaam]

OK, after discussion, we prefer to attempt to add a mark_sessions_as_backed_up method to the crypto store. Hopefully, this won't require us to decrypt any sessions at all.

[andybalaam]

Closed in favour of #2934