Go back to a `global` single-ton nursery per actor

Turns out the lifetime mgmt of separate nurseries per delegate manager
is tricky; a new nursery can't be naively allocated on cache-misses since
it may get closed by some early terminating task instead of by the "last
using" consumer task. In theory if we allocate using the same logic as
that used for the last-task-triggers-exit then this should work?

For now just go back to a single global nursery per `_Cache` which still
avoids use of the internal actor service nursery.

tractor/trionics/_mngrs.py

@@ -35,6 +35,7 @@ from typing import (
 import trio
 from trio_typing import TaskStatus
 
+from .._state import current_actor
 from ..log import get_logger
 
 
@@ -140,6 +141,7 @@ class _Cache:
     a kept-alive-while-in-use async resource.
 
     '''
+    service_n: Optional[trio.Nursery] = None
     locks: dict[Hashable, trio.Lock] = {}
     users: int = 0
     values: dict[Any,  Any] = {}
@@ -147,7 +149,7 @@ class _Cache:
         Hashable,
         tuple[trio.Nursery, trio.Event]
     ] = {}
-    nurseries: dict[int, trio.Nursery] = {}
+    # nurseries: dict[int, trio.Nursery] = {}
     no_more_users: Optional[trio.Event] = None
 
     @classmethod
@@ -205,6 +207,18 @@ async def maybe_open_context(
     lock = _Cache.locks.setdefault(fid, trio.Lock())
     await lock.acquire()
 
+    # XXX: one singleton nursery per actor and we want to
+    # have it not be closed until all consumers have exited (which is
+    # currently difficult to implement any other way besides using our
+    # pre-allocated runtime instance..)
+    service_n: trio.Nursery = current_actor()._service_n
+
+    # TODO: is there any way to allocate
+    # a 'stays-open-till-last-task-finshed nursery?
+    # service_n: trio.Nursery
+    # async with maybe_open_nursery(_Cache.service_n) as service_n:
+    #     _Cache.service_n = service_n
+
     try:
         # **critical section** that should prevent other tasks from
         # checking the _Cache until complete otherwise the scheduler
@@ -214,9 +228,6 @@ async def maybe_open_context(
     except KeyError:
         log.info(f'Allocating new {acm_func} for {ctx_key}')
         mngr = acm_func(**kwargs)
-        service_n: Optional[trio.Nursery] = _Cache.nurseries.get(fid)
-        async with maybe_open_nursery(service_n) as service_n:
-            _Cache.nurseries[fid] = service_n
         resources = _Cache.resources
         assert not resources.get(ctx_key), f'Resource exists? {ctx_key}'
         resources[ctx_key] = (service_n, trio.Event())