Element R | After a backup is deleted the sdk keeps trying to download keys from it

[BillCarsonFr]

Step to reproduce

• You must have several backup version
• Create a new version n , and ensure that the backup private key is cached locally

=> Delete the backup (maybe from another client). Now the current backup is n-1

Actual

The queryKeyBackup() api (called on fail to decrypt) keeps querying to version n

=> This is because the backup key in cache is still (version:n, privKey: xxx)
https://github.com/matrix-org/matrix-js-sdk/blob/5931a5119c9f792a34ec5c1461da2c939b6326d3/src/rust-crypto/rust-crypto.ts#L202

Legacy is a bit different as the version of the backup is not saved in cache, so it has to check what's the current backup version.

Expected

This is a case of desynchronised cache / 4S, the silver bullet fix should ensure that deleting a backup keeps the "recovery" in a valid state.

• If you delete a backup the 4S should be updated (remove/clear the m.megolm_backup.v1 account data), update the cache
• If a client detect that a version is deleted => should clear cache and re-request missing secrets + eventually ask user to enter passphrase to sync with 4S (hoping it has the correct secret)
• What should we do with mulitple backup version? Is there any reason to maintain several versions? given that on change the private key is probably lost anyhow)

[richvdh]

Apparently, once this happens:

• BackupManager.activeBackupVersion is set to the previous version (n-1). I think this is what is shown in the UI as "Active backup version".
• IndexedDB backup_keys.recovery_key_v1 holds the decryption key of the deleted version.
• IndexedDB backup_keys.backup_key_v1, which (I think) stores the version for the decryption key, contrary to the implication of the name, is set to the deleted version (n).