Potential race when knocking to join a call over federation

[Half-Shot]

Steps to reproduce

(N.B. I suspect this is more of a Synapse problem, but reporting at application level.

There are two users here. Alice is on Server A, and Bob is on Server B.

1. Alice creates a new call, and sends the link along to Bob
2. Bob requests to join the call (knocks)
3. Alice accepts and invites Bo,b
4. Bob sees an error about not being able to join
5. (workaround) Bob is able to refresh and immediately join the call.

Outcome

What did you expect?

Upon being inviited, I should be joined to the call.

What happened instead?

I suspect this is invites racing;

Operating system

Arch Linux

Browser information

Firefox

URL for webapp

No response

Will you send logs?

No

[Half-Shot]

I strongly suspect this is element-hq/synapse#15012

[Half-Shot]

#2637 reportedly makes this less of a problem, but this causes some delay in joining a room so it would be good to fix upstream.