Description

So I was trying to figure out why the /auth/xxx/fallback/web was working on a local homeserver instance but not on matrix.org.

In this flow, there's a sessions dict which governs the current state of your login/registration progress. When you complete a auth stage, your session in this dict is modified. This sessions dict is available to registration and login methods.

Turns out that in the recommended worker setup, you can end up with two workers, each with their own instance of sessions. One tracks your login progress, and another that gets modified when you successfully complete web fallback. However, because they are separate, you don't actually see any progress when you query /register after successfully completing web fallback registration stages.

Essentially /auth/xxx/fallback/web needs to get routed to the same worker as /register.

Synapse v1.8.0