Minimal pubsub from `@endo/stream`

[kriskowal]

The Endo Dæmon packages/daemon needs a minimal pubsub mechanism which can be based on the @endo/stream async iterator design using three possible variations on an async singly-linked-list queue:

• A topic that is a time series of a value or snapshots of a complex object would use makeLatestTopic such that any published value may be dropped and replaced and only observed if subscribers are polling faster than updates. New subscribers see the most recently published value immediately then each subsequent request for an iteration waits for a fresher value, so the sequence they see may be a subsequence of the published values in order and without duplicates.
• A topic that is a time series of changes after capturing a value or snapshot would use makeChangeTopic. Subscribers would see every delta published after getting a subscription. This is useful for differential updates but can’t be lossy.
• Existing Agoric notifiers use the equivalent of makeUpdateTopic. Subscribers receive the most recently published value and also see every intermediate value. We would probably want to encourage users of makeUpdateTopic to instead use makeLatestTopic or makeChangeTopic depending on whether they’re publishing a time series or time series deltas.

I’ve created this ticket to track work ongoing in #1432.

[kriskowal]

We decided this was the the form of pubsub that we wanted to expose in the Pet Dæmon. It’s currently living as an exported library of the daemon until it can be replaced with the notifiers from Agoric.

I’m inclined at this point to bring this idea back into @endo/streams and find a forward-compatibility story that allows things that currently can be iterated to eventually have proper pub-sub on the wire by dint of an alternate light-client. attn @michaelfig