https://github.com/matrix-org/synapse/blob/de655e66/synapse/storage/devices.py#L100 is the code which looks for device list updates to send out.

In brief, the logic is:

• get up to 20 rows from the update table for the current destination
• if we hit the limit, note the maximum stream_id so that we can start from there next time
• for each device, get the e2e keys, and add it to the list.

On the next pass, we will return only updates which have a stream_id greater than the maximum of the previous pass.

There are two problems with this, both of which mean that we will drop update messages:

• The first is trivially solved: the query lacks an ORDER BY clause, so there is no guarantee that we will get the oldest updates. Suppose there are 50 updates waiting to go to a particular destination, with stream IDs N to N+49. We will get an arbitrary 20 rows - for example N+25 to N+44. That would mean that the updates with stream IDs N to N+24 will be lost.

• The second is maybe a bit harder to solve, but relates to the fact that there may be several updates (for different devices) with the same stream_id. Suppose we've solved the previous problem, and we now have, in the table:

  • 15 updates, for one device each, with stream ids N to N+14
  • At stream id N+15, a single update for 6 devices.
    So then our query will return the first 15 updates, and 5 of the 6 updates at N+15. Again, the final update gets lost.