WebRTC sessions

[allenporter]

Motivation and background

WebRTC streams are typically long lived sessions, and often some camera providers (ring, nest) will allow sessions with some timeout as to not just go forever, and they need to be renewed / send a keepalive poll. The home assistant webrtc interface currently has no notion of a session associated when a webrtc stream when creating it:

def async_handle_web_rtc_offer(self, offer: string) -> string

As a result:

• Currently, nest webrtc sessions just timeout after 5 minutes and are not extended.
• Ring webrtc can't really work without a session (a previous attempt doesn't work without the notion of acting on a specific session).

Proposal

To properly implement WebRTC sessions we need:

• A way to create the offer SDP which returns (1) an optional session id (2) an optional expiration time (3) the original sdp offer.
• A way to extend a stream, which accepts the session id and returns an updated expiration time
• Optionally, a way to stop/invalidate a stream that accepts the session id and closes that streams session id. I'm not sure this is needed. (You can implement a keep alive mechanism with the other two items and I think clients already end streams with the device-- but there may be efficiency or security wins here i'm not thinking of.)

Consequences

• The frontend will need to be changed to manage session state, and extending streams based on the expirations.
• Ring could use this API my continually doing it's keep-alive within an arbitrary expiration window while a session is not expired
• A migration path from the new interface wrapping the old interface could give time for custom components to migrate
• There could be future efficiency improvements for other webrtc integrations, but not clear yet.

Alternatives

• A workaround for Nest can be just to reload the stream on failure, then create a new offer. At a minimum it won't be smooth since the session has to start over. This may be useful to improve reliability anyway, but not really a great user experience for the expiration session problem.

[allenporter]

Summarizing a discussion from Discord: The proposal in #1165 will handle this by integration the session with the websocket session, assigning an id, and ensuring close will be called reliably. This addresses the needs for handling stream extensions and keepalives within the integration. The integrations will handle any session id mappings themselves.