incoming encrypted to-device messages can be lost when the application is restarted

[BillCarsonFr]

If you restart /upgrade (web) or the app is killed in background while processing to_devices coming down the sync, the non decrypted to_devices will be dropped and lost producing UISIs.

This is problem with atomicity of /sync. The sync token is persisted before all to_devices have been processed, so the next time the app launches it will use the saved token, thus ack'ing the previous to_device chunk that was received. In such case the app should start syncing with the previous token to force a redelivery of the initial to_device chunk (or make sure that it permanently persists the to-device messages before storing the /sync token). Olm session internal state should also be reverted if the sync does not complete.

• If you restart (e.g. upgrade) Element while it's waiting to process a m.room_key, it'll drop it and you'll get UISIs element-web#23113
• Similar issue on android
• Similar on iOS?

[richvdh]

@BillCarsonFr:

Olm session internal state should also be reverted if the sync does not complete.

Why is this required?

[pmaier1]

Will be fixed with the introduction of ER.

[BillCarsonFr]

Will be fixed with the introduction of ER.

Looks like not. The sync token is save way to early (on purpose?), before the to_devices have been processed by the rust sdk.
So in case of restart, the next token will be used and thus the to_devices not processed are lost.

https://github.com/matrix-org/matrix-js-sdk/blob/748d03ba1129dd0ab6ce6e44f6202b81e604a4b3/src/sync.ts#L895

            // set the sync token NOW *before* processing the events. We do this so
            // if something barfs on an event we can skip it rather than constantly
            // polling with the same token.
            this.client.store.setSyncToken(data.next_batch);