[feature] Feature request: Allow connect proxy to set SSE response headers

[eduardbareev]

Please provide a mechanism for Connect Proxy to set and unset SSE response headers.

If a header name provided by Connect Proxy is among those normally managed by Centrifugo (e.g., CORS-related), the Connect Proxy's setting should take priority.

For example, {access-control-allow-origin: None} or {access-control-allow-origin: ""} would prevent access-control-allow-origin from appearing in the SSE response, even if Centrifugo would otherwise include it.

[FZambia]

This is not very simple for Centrifugo given how proxy works and current design. Transport layer doesn’t have a mechanism to modify headers based on proxy response. Moreover, in bidirectional case, it happens after headers are written and connection may be hijacked already. So sometimes it's too late to be able to modify.

Also, Centrifugo generally tries to keep all transports to work in similar way where possible without providing specific features for specific types of transports until really there is no alternative solution.

My current thoughts is that in Centrifugo case CORS should be handled outside of connect proxy. If you need dynamic CORS (from what I understood it's your primary goal here) – then probably you can consider using wildcard origins and do extra checks on the backend connect handler side, disconnecting connections from unknown origins.

Dynamic CORS outside the advice above may be handled somehow differently using sort of webhook from Centrifugo to the backend with a configurable cache time for successful responses.