[Concurrency] Actors & actor isolation

[Douglas_Gregor]

@nonsensery

Regarding this (code) comment, from the Actor Isolation section (near the end):

// Safe: this operation is the only one that has access to the actor's local
// state right now, and there have not been any suspension points between
// the place where we checked for sufficient funds and here.

Does this mean that, if one actor method call suspends (because it calls an async function), then other method calls on that same actor could run while the original is suspended? That is, could separate method calls on an actor be interleaved?

Yes.

@nonsensery

From the rest of the proposal, I would have expected that a given actor method call would run completely before any others were allowed to run.

That's not correct. Each async call is potentially a suspension point where other code could be interleaved on the actor. This prevents deadlocks. It's also why we consider it important to mark these in the code with await.

(I think we need to call this out specifically in the proposal)

Doug