Sync recovery loop failed to handle successful /sync request and contiunued trying to request the same ?since value

I noticed that my client was failing to sync (which is fairly typical after a homeserver restart) but looking at the network log something struck me as unusual:


It's my understanding that once Element has got a successful sync back, it will persist the data and immediately retry with the new since provided in the response. The network log suggests element tried for a bit with the same sync token, which meant Synapse had to generate a completely fresh response several times.

I looked at the response format of the first successful response, and the since token in the response is `s44114914_293425298_135_27463679_540759_1696_968250_19575942_52`, which means that Synapse IS providing the correct since token.

Start by reproducing the failed sync after a homeserver restart and inspect the network log around the first successful /sync response. Trace how the returned since token is persisted and reused; done means a successful response immediately leads to a request using its new token rather than repeating the previous value.

Written by the indexing model from the issue text.