Use the actor's service nursery instead

In order to ensure the lifetime of the feed can in fact be kept open
until the last consumer task has completed we need to maintain
a lifetime which is hierarchically greater then all consumer tasks.

This solution is somewhat hacky but seems to work well: we just use the
`tractor` actor's "service nursery" (the one normally used to invoke rpc
tasks) to launch the task which will start and keep open the target
cached async context manager. To make this more "proper" we may want to
offer a "root nursery" in all piker actors that is exposed through some
singleton api or even introduce a public api for it into `tractor`
directly.

piker/_cacheables.py

@@ -36,6 +36,7 @@ from contextlib import (
 
 import trio
 from trio_typing import TaskStatus
+import tractor
 from tractor._portal import maybe_open_nursery
 
 from .brokers import get_brokermod
@@ -180,43 +181,49 @@ async def maybe_open_ctx(
 
     ctx_key = id(mngr)
 
-    # TODO: does this need to be a tractor "root nursery"?
-    async with maybe_open_nursery(cache.nurseries.get(ctx_key)) as n:
-        cache.nurseries[ctx_key] = n
 
-        value = None
-        try:
-            # lock feed acquisition around task racing  / ``trio``'s
-            # scheduler protocol
-            value = cache.values[key]
-            log.info(f'Reusing cached feed for {key}')
-            cache.users += 1
-            cache.lock.release()
-            yield True, value
+    value = None
+    try:
+        # lock feed acquisition around task racing  / ``trio``'s
+        # scheduler protocol
+        value = cache.values[key]
+        log.info(f'Reusing cached feed for {key}')
+        cache.users += 1
+        cache.lock.release()
+        yield True, value
 
-        except KeyError:
-            log.info(f'Allocating new feed for {key}')
+    except KeyError:
+        log.info(f'Allocating new feed for {key}')
 
-            # **critical section** that should prevent other tasks from
-            # checking the cache until complete otherwise the scheduler
-            # may switch and by accident we create more then one feed.
+        # **critical section** that should prevent other tasks from
+        # checking the cache until complete otherwise the scheduler
+        # may switch and by accident we create more then one feed.
 
-            value = await n.start(cache.run_ctx, mngr, key)
-            cache.users += 1
+        # TODO: vaoid pulling from ``tractor`` internals and
+        # instead offer a "root nursery" in 
+        service_n = tractor.current_actor()._service_n
+
+        # TODO: does this need to be a tractor "root nursery"?
+        ln = cache.nurseries.get(ctx_key)
+        assert not ln
+        ln = cache.nurseries[ctx_key] = service_n
+
+        value = await ln.start(cache.run_ctx, mngr, key)
+        cache.users += 1
+        cache.lock.release()
+
+        yield False, value
+
+    finally:
+        cache.users -= 1
+
+        if cache.lock.locked():
             cache.lock.release()
 
-            yield False, value
+        if value is not None:
+            # if no more consumers, teardown the client
+            if cache.users <= 0:
+                log.warning(f'De-allocating feed for {key}')
 
-        finally:
-            cache.users -= 1
-
-            if cache.lock.locked():
-                cache.lock.release()
-
-            if value is not None:
-                # if no more consumers, teardown the client
-                if cache.users <= 0:
-                    log.warning(f'De-allocating feed for {key}')
-
-                    # terminate mngr nursery
-                    cache.no_more_users.set()
+                # terminate mngr nursery
+                cache.no_more_users.set()