Formalise use of Agent-cluster

[gterzian]

This comment provides a very good overview of the "agent/agent-cluster" model emerging in the HTML standard.

This sums it up nicely:

A user agent:

• Holds a set of browsing context groups.
• Holds a shared worker map of origin to agent clusters.
• Holds a service worker map of origin to agent clusters.

and "A browsing context group: Holds a map of similar-origin window keys to agent clusters. (...)"

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The good news is that Servo's current architecture pretty much already follows that model, with the exception, in my opinion, of Service Worker.

We currently already have

servo/components/constellation/constellation.rs

Line 194 in a38f28f

struct BrowsingContextGroup {

which contains a

servo/components/constellation/constellation.rs

Line 205 in a38f28f

event_loops: HashMap<Host, Weak<EventLoop>>,

It's not really accurate to call our EventLoop an "event-loop", since it actually contains, potentially, several event-loops, or should we say "agents", in the form of a single window agent, and potentially multiple dedicated worker agents.

An EventLoop can also potentially contain several worklet agents.

So, its really not an "event-loop", and in fact is fits the definition of agent-cluster very well.

So basically, we're lucky, because we pretty much already have "agent-cluster" right, and we could just rename EventLoop -> AgentCluster.

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Now, note that service-worker-agent
and shared-worker-agent, on the other hand, are never part of the same "agent-cluster" as a window/worklet/dedicated-worker agent.

We don't have shared-worker yet, and service-workers currently run inside an EventLoop.

So, we could move service worker to it's own "agent-cluster", kept in a map on the constellation. It's discussed at #19302

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On a related note, I think the "agent-cluster" concept could influence other architectural choices. A good example is media, since an audio worklet would have to run in the agent-cluster used by a window/dedicated worker agents, yet the audio backend probably needs to run in a different process.

So there it's not so much about where "media" runs, it's rather about splitting up media so that some part(the "audio rendering thread") runs in a agent-cluster(the same one as the window agent using media), and another part runs in another process(perhaps the "main" process, or a separate one). This is a change that is discussed as part of #23807

[gterzian]

Also just seeing the spec now also defines the allocation and re-use of agent-cluster for similiar-origin window agents, like obtain-agent-cluster-key, and this matches our operation at the constellation such as

servo/components/constellation/constellation.rs

Line 827 in 119cb89

fn get_event_loop(

So here some renaming and linking to the spec would be useful.