After reloading, my own failed-to-send message is UTD

[andybalaam]

See rageshake https://github.com/matrix-org/element-web-rageshakes/issues/25454 (cc @bnjbvr )

It is possible to get a UTD (Unable To Decrypt) for your own message by following these steps.

• Send an encrypted message that fails to send
• Refresh Element Web
• Observe that the message in your timeline is UTD, even if it then successfully sends

Here is what we think is happening:

• JS wants to send an event
• JS asks Rust to encrypt the event, which works and the megolm session is saved by Rust
• JS tries to PUT the event to the homeserver but fails
• User refreshes
• JS has the event pending to send but with no ID because it hasn't sent. It uses a "fake" ID based on the transaction ID it chose for the PUT request
• JS asks Rust to decrypt the event so it can display it
• Rust refuses because the event ID is incorrect.

The Rust returns an error like:

DecryptionError[msg: leading sigil is incorrect or missing at line 1 column 63

Because the JS is providing event JSON containing an event ID that does not begin with $.

Possible fixes:

1. Change the "fake"/temporary ID the JS uses for events that failed to send. We don't know whether this will have impact on other code - maybe some code somewhere uses the lack of a $ to infer that this event failed to send? Or
2. Check the event ID just before we send the event JSON to rust and add some fixed prefix e.g. $TRANS- to make the ID acceptable to the Rust code.
3. Change the Rust to accept non-$-prefixed IDs, but this doesn't seem a good idea, since it is correct to expect event IDs to start with $.

[NetSysFire]

Could reproduce just that yesterday with 1.11.86.