feat(sdk): Introduce the new LatestEvents API, part 3: Computing from the Send Queue

[Hywan]

This patch adds a new feature to LatestEvents: it now listens to the SendQueue!

Most of the addition are related to tests. I've tried to keep the commits are small and atomic as possible. We follow and update the code gradually.

LatestEventValuesForLocalEvents

A new type is introduced to manage a buffer of LatestEventValue for local events: LatestEventValuesForLocalEvents. A bit of explanations.

The system does only receive RoomSendQueueUpdates. It's not designed to iterate over local events in the send queue when a local event is changed (cancelled, or updated for example). That's why we keep our own buffer here. Imagine the system receives 4 RoomSendQueueUpdate:

1. RoomSendQueueUpdate::NewLocalEvent: new local event,
2. RoomSendQueueUpdate::NewLocalEvent: new local event,
3. RoomSendQueueUpdate::ReplacedLocalEvent: replaced the first local event,
4. RoomSendQueueUpdate::CancelledLocalEvent: cancelled the second local event.

NewLocalEvents will trigger the computation of new LatestEventValues, but CancelledLocalEvent for example doesn't hold any information to compute a new LatestEventValue, so we need to remember the previous values, until the local events are sent and removed from this buffer.

Another reason why we need a buffer is to handle wedged local event. Imagine the system receives 3 RoomSendQueueUpdate:

1. RoomSendQueueUpdate::NewLocalEvent: new local event,
2. RoomSendQueueUpdate::NewLocalEvent: new local event,
3. RoomSendQueueUpdate::SendError: the first local event has failed to be sent.

Because a SendError is received (targeting the first NewLocalEvent), the send queue is stopped. However, the LatestEventValue targets the second NewLocalEvent. The system must consider that when a local event is wedged, all the following local events must also be marked as wedged. And vice versa, when the send queue is able to send an event again, all the following local events must be marked as unwedged.

This type isolates a couple of methods designed to manage these specific behaviours.

---

• Address [meta] Latest Event API, the missing one #4112
• Address https://github.com/element-hq/element-internal/issues/689
• Sequel of feat(sdk): Introduce the new LatestEvents API, part 2: Computing from the Event Cache #5340

[codecov]

Codecov Report

❌ Patch coverage is 92.31398% with 94 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.62%. Comparing base (13c30f6) to head (63ae70c).
⚠️ Report is 10 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...rates/matrix-sdk/src/latest_events/latest_event.rs	93.27%	26 Missing and 44 partials ⚠️
crates/matrix-sdk/src/latest_events/mod.rs	85.79%	23 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5482      +/-   ##
==========================================
+ Coverage   88.57%   88.62%   +0.04%     
==========================================
  Files         340      340              
  Lines       93743    94891    +1148     
  Branches    93743    94891    +1148     
==========================================
+ Hits        83036    84097    +1061     
- Misses       6575     6617      +42     
- Partials     4132     4177      +45     

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[codspeed-hq]

CodSpeed Performance Report

Merging #5482 will not alter performance

_{Comparing Hywan:feat-sdk-latest-events-take-3 (63ae70c) with main (4f0415d)}

Summary

✅ 31 untouched benchmarks