Olm sessions are vulnerable to wedging on OS crash or power failure, leading to UTDs

[richvdh]

Consider:

• Alice and Bob have an active Olm session.
• Alice is using a client which uses the indexeddb backend of matrix-crypto-sdk. She wants to send a new Olm message, so she generates a new Olm ratchet key and updates the session. She can then encrypt the new message, and stores the updated session in the database
• Alice sends the message.
• Alice's computer crashes.
• Alice reboots
• If the updated session was not correctly persisted before the crash, this Olm session is now wedged: Alice cannot decrypt any replies from Bob, and, because Alice will use the wrong ratchet key in the next sender chain, Bob will be unable to decrypt any further messages from Alice.

Currently, we use the "default" durability mode of IndexedDB, which can lead to data loss. We should instead use "strict", at least for transactions which update Olm sessions.

[kegsay]

Amazingly.. Firefox still doesn't support the durability property. https://developer.mozilla.org/en-US/docs/Web/API/IDBTransaction/durability though pre-release versions do.

EDIT: Ah that's reading the durability from an existing transaction, the option to enable it has been supported much longer https://developer.mozilla.org/en-US/docs/Web/API/IDBDatabase/transaction#options

[kegsay]

Filed https://github.com/rustwasm/wasm-bindgen/issues/3959 which needs to land first before we can specify the durability mode.