thousands of to-device messages to cross-signing keys

[richvdh]

My server has accumulated hundreds of thousands of to-device messages to three particular device IDs:

synapse=# select count(*),device_id from device_inbox group by 2 order by 1 desc limit 3;
 count  |                  device_id                  
--------+---------------------------------------------
 263318 | 93fB6y9SJ3183h6zpYjImjRE8om8X01O7Xjl7UkcHeo
 263318 | 4CDFPgvgQryGrOAGLlscaauGQPLTLmlYKP/8VgLHzEo
 263318 | fTBqO/U1BB6rTBo+PEa3pExSx5bWVipFa8F1OhRyKrk

these turn out to be the public halves of master, self-signing and user-signing cross-signing keys, and they are mostly key-share requests (and subsequent cancellations). They are coming from both element-web and element-android.

Clearly nothing is ever going to read these messages. It's a shame that synapse accepts/keeps them (cf matrix-org/synapse#3656), but it also seems wasteful that element sends the requests.

[uhoreg]

Key share requests are usually sent to "*" in element, so I guess it's synapse expanding "*" to include the cross-signing "devices".

[anoadragon453]

Indeed we are: https://github.com/matrix-org/synapse/blob/c7b823525e9598ee4de76099d9ca5ea1ce3977f4/synapse/storage/databases/main/deviceinbox.py#L486-L498

Adding a clause for hidden should prevent to_device messages from being added to devices which should not be visible.

So this isn't any client's fault after all, as * is intended to have messages sent only to visible devices. Thus closing this issue in favour of one for Synapse: matrix-org/synapse#9348