In some cases, restoring client state from backups may cause Olm sessions
to become corrupted, resulting in a backward ratchet. As a consequence,
the receiving side is unable to decrypt messages. To address this issue,
the failed side initiates a new Olm session and sends a dummy encrypted
message.
However, this dummy message also creates a new Olm session on the
sender's side. To ensure that both sides use the same new session, we
must sort sessions by their creation timestamp before encrypting new
messages.
Although the session list was sorted correctly previously, we selected
the wrong side of the list, resulting in an outdated session being
selected instead of the newest one. This patch resolves the issue by
selecting the newest Olm session and adds a test to the session getter
logic to prevent reintroduction of this bug.
Damir Jelić
ccedfb41f3