Synapse does not tell the clients when it runs out of OTKs

[matrixbot]

This issue has been migrated from #10484.

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Following discussion in #10479 (and especially matrix-org/synapse#10479 (comment)), I've checked the sync response when Synapse does not have OTK (one time key) anymore. So when the number of OTK is changing from N to 0.

Instead of receiving

"device_one_time_keys_count":{"signed_curve25519":0}

The client receives

"device_one_time_keys_count":{}

and so the client cannot distinguish "no change" and "no OTK left", and do not upload new OTK, which at the end breaks decryption of some messages.

Server version: 1.38.0 commit 12c92ee

[kegsay]

Empirically this does work and is tested in sliding sync as of matrix-org/sliding-sync#390

[richvdh]

It's unclear what remains to be done here. Seems like the implementation is compliant with the spec.