Failures uploading one-time keys

[richvdh]

Rageshake log says:

05-06 15:54:56.118 1 D/MXCrypto: ## encryptEventContent() : wait after e2e init

[richvdh]

see https://github.com/matrix-org/riot-android-rageshakes/issues/121

[ylecollen]

05-06 13:40:32.571 597 D/RestAdapterCallback: Trigger the event [uploadKeys]
05-06 13:40:32.640 1 D/RestAdapterCallback: ## failure(): [uploadKeys] with error 400 Bad Request
05-06 13:40:32.641 1 E/MXCrypto: ## maybeUploadOneTimeKeys() : failed One time key with key_id 'AAAA2Q' already exists
05-06 13:40:32.643 1 E/MXCrypto: ## start failed : One time key with key_id 'AAAA2Q' already exists

[richvdh]

05-06 13:40:32.643 1 E/MXCrypto: ## start failed : One time key with key_id 'AAAA2Q' already exists

This error will only happen if you are trying to change the one-time key, which suggests that, in the past, one-time-keys have been generated but not persisted to the permanent store before uploading - and then, when the app has been restarted, another key with the same key_id has been generated :/.

This might explain #1209

[richvdh]

05-06 13:40:32.643 1 E/MXCrypto: ## start failed : One time key with key_id 'AAAA2Q' already exists

This error will only happen if you are trying to change the one-time key,

This turns out to be not quite right: synapse also returns the error if the key is the same but the signature is different. That could happen if the client had previously tried to upload the key, but not got the response. It would then later retry the upload, having re-signed the key.

This could also affect riot-web.

I think the most pragmatic fix to this is to fudge the server so that it does not raise an error if the key is the same and the signature has changed.