Element-R: lock out multiple tabs to avoid races

[richvdh]

Using the Rust crypto SDK from two tabs at once will cause data corruption, due to the fact that the rust-sdk caches information in memory.

Examples of the sorts of problems that could arise:

• If you send messages from both tabs to the same room, we could use the same megolm ratchet index (this is exactly the problem in Wrong message index when sending an image via Share Extension, message not decryptable in Element-Web, can break rooms when session-ID is affected! element-ios#7499)

• We might create two Olm one-time-keys with the same ID, causing problems like Lost OTK, leading to "OneTime key already exists" error and later UTDs matrix-org/matrix-rust-sdk#1415.

• Most disasterously of all, suppose:

  • You share two encrypted rooms with your colleague Bob
  • In one tab, you send a message in one room; in the other tab, you send a message in the other room.

  In this case, each message will necessitate a megolm keyshare, which will cause the ratchet for the Olm session shared between you and Bob to be advanced. However, this will cause unrecoverable corruption to the Olm session.
  In other words, you and Bob will never be able to communicate again.

[richvdh]

A (somewhat hacky) option might be to lock out multiple tabs, by sending messages over a BroadcastChannel

[richvdh]

for now at least, we will lock out one of the tabs.

Element Call have done something like this and we may be able to benefit from their implementation

[richvdh]

I could probably do with some design input on this. Currently I have thrown together a page that looks like this:

[nadonomy]

@richvdh can we add some kind of interaction to choose which tab to use Element in? For example, WhatsApp web has this:

Separately, can we use Element UI components? Or does this page need to be some kind of barebones HTML/styling?

If the latter, I'll whip something together copying the mobile install guide.

[richvdh]

the answer is (I think) "yes" to both, though we're deliberately trying to keep this feature lightweight as it's meant to be a quick alternative to "proper" multitab support.

I might need help from someone like @germain-gg in using UI components in EW properly, but it's just react components like the rest of EW so should be fine.

[richvdh]

@nadonomy I made a loading screen that looks like this: