Turns out I didn't read my own internals docs/comments and despite it
not being used previously, this adds the real use case: a root,
per-actor, scope which ensures parent comms are the last conc-thing to
be cancelled.
Also, the impl changes here make the test from 6410e45 (or wtv
it's rebased to) pass, i.e. we can support crash handling in the root
actor despite the root-tn having been (self) cancelled.
Superficial adjustments,
- rename `Actor._service_n` -> `._service_tn` everywhere.
- add asserts to `._runtime.async_main()` which ensure that the any
`.trionics.maybe_open_nursery()` calls against optionally passed
`._[root/service]_tn` are allocated-if-not-provided (the
`._service_tn`-case being an i-guess-prep-for-the-future-anti-pattern
Bp).
- obvi adjust all internal usage to match new naming.
Serious/real-use-case changes,
- add (back) a `Actor._root_tn` which sits a scope "above" the
service-tn and is either,
+ assigned in `._runtime.async_main()` for sub-actors OR,
+ assigned in `._root.open_root_actor()` for the root actor.
**THE primary reason** to keep this "upper" tn is that during
a full-`Actor`-cancellation condition (more details below) we want to
ensure that the IPC connection with a sub-actor's parent is **the last
thing to be cancelled**; this is most simply implemented by ensuring
that the `Actor._parent_chan: .ipc.Channel` is handled in an upper
scope in `_rpc.process_messages()`-subtask-terms.
- for the root actor this `root_tn` is allocated in `.open_root_actor()`
body and assigned as such.
- extend `Actor.cancel_soon()` to be cohesive with this entire teardown
"policy" by scheduling a task in the `._root_tn` which,
* waits for the `._service_tn` to complete and then,
* cancels the `._root_tn.cancel_scope`,
* includes "sclangy" console logging throughout.
a9f06df3fb