Random handful of redacted message events visible after joining room with history_visibility==joined

[richvdh]

@​travis:t2l.io joined a room over federation; having done so, he saw two "empty" events in the timeline from before he joined: http://imgur.com/ZC1k3IL.

It is expected that the events be empty - the remote federation server will have served redacted events because nobody on travis's homeserver was a member of the room in these periods. What is not expected is that travis's homeserver served them to the client over /messages.

Inspection of the database showed that the room state at each event was as expected. The problem went away after a synapse restart. This therefore appears to have been caused by a stale cache within synapse.

It is interesting to note that the events in question ($148698101813672eDBdZ:chat.weho.st and $14877908446723tbDHY:chat.weho.st) were the boundary points for the federation backfill requests:

2017-02-23 10:46:26,450 - synapse.http.outbound - 124 - INFO - GET-237401- {GET-O-85787} [chat.weho.st] Sending request: GET matrix://chat.weho.st/_matrix/federation/v1/backfill/!XMHzVqhLqEbFpqBFaf:chat.weho.st/?limit=100&v=%2414877908446723tbDHY%3Achat.weho.st
2017-02-23 10:46:55,305 - synapse.http.outbound - 124 - INFO - GET-237556- {GET-O-85791} [chat.weho.st] Sending request: GET matrix://chat.weho.st/_matrix/federation/v1/backfill/!XMHzVqhLqEbFpqBFaf:chat.weho.st/?limit=100&v=%24148698101813672eDBdZ%3Achat.weho.st

[turt2live]

I've managed to reproduce this (again) on Synapse 0.19.3-rc2 by joining #freenode_#smartos:matrix.org. I'm not too comfortable with dumping logs here, so feel free to message me (@travis:t2l.io) and I'll provide the log package (11mb compressed, 145mb expanded).

[Ralith]

As far as I can recall, this has happened every single time I've joined an IRC portal room on matrix.org. It might be relevant that I'm always joining via my own HS.

[PureTryOut]

I can reproduce this since having my own homeserver. Running Synapse 0.33.9 now, it happens every time I join a new IRC room.

[babolivier]

Reproduced it last evening:

[erikjohnston]

This should be fixed in #4718

[MadLittleMods]

Closing as #4718 looks merged and I don't see any further activity here ⏩