MSC4450: Identity Provider selection for User-Interactive Authentication with Legacy Single Sign-On

[reivilibre]

Tracking issue for MSC4450: Identity Provider selection for User-Interactive Authentication with Legacy Single Sign-On as an experimental feature in Synapse

Context

We have a customer with an exotic multi-Identity Provider setup,
where the client is configured with a specific Identity Provider but users can be associated to multiple providers and Synapse does not know which one to choose.

Presenting a web UI for manual selection is not useful in this customer's situation as it would only confuse the user.

See:

• https://github.com/element-hq/backend-internal/issues/229

Current design in Synapse

We return 400 errors if the IDP ID is not a configured provider or is not in the set of providers that the user is associated with.

Caveats to be aware of

How do we know when to remove this

I imagine one of the two following cases will happen:

1. Legacy authentication gets phased out, or at least no longer used by the customer needing this feature, so we just walk away from the MSC.
2. The MSC gets merged and we can gracefully migrate the customer to the stable parameter.