sync v2 implicit todevice acks considered incredibly harmful.

[ara4n]

I think we’ve tracked the main final reliability problem on matrix-video-chat to the fact that people leave instances lying around in bg tabs, which sit there running a sync loop which races to consume msc matrix-org/matrix-spec-proposals#3401 todevice messages.

I think the race is that if one sync loop calls /sync before the other tab’s /sync returns, it can “steal” the todevice messages out from the other loop by acknowledging and thus deleting them. As a result, half your todevice messages go missing (hence all my accusations in matrix-org/synapse#9533 about unreliable todevice msgs).

@kegsay - any suggestions on a backwards compatible change to v2 sync to fix this so we’re not blocked waiting for sliding sync to land?

See also matrix-org/synapse#6450 as another instance of this wreaking havoc.

[kegsay]

Move the sync loop to a SharedWorker maybe?

[turt2live]

Why are there multiple tabs in play? Pantalaimon already works around this problem and has multiple sync loops, which puts into question the application's architecture.

[kegsay]

people leave instances lying around in bg tabs

This is likely why there are multiple tabs in play.

[turt2live]

I'm a little bit surprised that happens for a voip stack though, and that the client isn't built to handle it without the need for protocol changes. Element Web (for example) doesn't choke when it happens.