Ensure device list updates are robust to race conditions and network failures #432
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As we are selecting... for updates. Without this, we can drop updates to the floor incorrectly. See #430
Otherwise we can cause lost device list updates if the client resets the connection (as the device list update won't be in any in-memory cache).
They were A) confusing and B) testing the wrong thing as they got refactored after the bad refactor of the source code..
devonh
approved these changes
May 9, 2024
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Fixes #430
This does two things:
.. FOR UPDATE)This adds a regression test and refactors the unit tests to be testing the correct semantics.
History on this is:
resultpreviously had theNewitems put intoSent(the original intent) but now we were copyingresulttowriteBackand then swapping_NewtoSent. As the caller only looks inSent, it won't see any device list updates. This would fix itself on the next request because we then returnSentconfusingly. However, the semantics are like this to guard against network failures, and since this refactor specific network failures would drop the update.I found this out when trying to fix this test in complement crypto, which is sensitive to how quickly the device list update will be delivered to the EX client. Hence me finding the bug and fixing it.
This is effectively a UTD cause for the purposes of element-hq/element-meta#245 because it meant EX clients may miss a device list update, causing EX clients to fail to encrypt for newly logged in devices (just like that failing complement crypto test).