Synapse does not attempt to send events in the device outbox at startup

[matrixbot]

This issue has been migrated from #16680.

---

Description

If you call /sendToDevice to a remote user, and that user's HS is temporarily down (not offline long enough to be blacklisted), it will not be sent. It will be stored in the device outbox federation table. If you then restart Synapse, it does not try to empty that table at startup. I waited 30s for it to do something.

This happens on sqlite, postgres and worker mode.

If the user sends another to-device event, the outbox is then cleared.

Steps to reproduce

Run this complement test and remove the "kick" event which happens after 10s in a goroutine: matrix-org/complement#694

Logs for HS1 (the one which gets restarted) running Postgres (no worker mode) attached.
postgres-hs1.log

Homeserver

Local

Synapse Version

v1.95

Installation Method

Docker (matrixdotorg/synapse)

Database

PostgreSQL

Workers

Single process

Platform

complement

Configuration

No response

Relevant log output

Attached.

Anything else that would be useful to know?

No response

[richvdh]

If the user sends another to-device event, the outbox is then cleared.

... indeed, if any outgoing federation traffic happens (room events, typing notifications, device list updates) that will be enough to prompt us to process the outbox. Also, in general, an incoming federation transaction on a server previously marked as "down" will cause us to restart the federation transmission loop and process the outbox.