.brokers._daemon: add notes around needed brokerd respawn tech

Author: goodboy

piker/brokers/_daemon.py

@@ -19,16 +19,22 @@
 ``brokerd``.
 
 '''
+from __future__ import annotations
 from contextlib import (
     asynccontextmanager as acm,
 )
+from typing import TYPE_CHECKING
+import exceptiongroup as eg
 
 import tractor
 import trio
 
 from . import _util
 from . import get_brokermod
 
+if TYPE_CHECKING:
+    from ..data import _FeedsBus
+
 # `brokerd` enabled modules
 # TODO: move this def to the `.data` subpkg..
 # NOTE: keeping this list as small as possible is part of our caps-sec
@@ -69,24 +75,40 @@ async def _setup_persistent_brokerd(
     # set global for this actor to this new process-wide instance B)
     _util.log = log
 
-    from piker.data.feed import (
-        _bus,
-        get_feed_bus,
-    )
-    global _bus
-    assert not _bus
-
-    async with trio.open_nursery() as service_nursery:
-        # assign a nursery to the feeds bus for spawning
-        # background tasks from clients
-        get_feed_bus(brokername, service_nursery)
-
-        # unblock caller
-        await ctx.started()
-
-        # we pin this task to keep the feeds manager active until the
-        # parent actor decides to tear it down
-        await trio.sleep_forever()
+    from piker.data import feed
+    assert not feed._bus
+
+    # allocate a nursery to the bus for spawning background
+    # tasks to service client IPC requests, normally
+    # `tractor.Context` connections to explicitly required
+    # `brokerd` endpoints such as:
+    # - `stream_quotes()`,
+    # - `manage_history()`,
+    # - `allocate_persistent_feed()`,
+    # - `open_symbol_search()`
+    # NOTE: see ep invocation details inside `.data.feed`.
+    try:
+        async with trio.open_nursery() as service_nursery:
+            bus: _FeedsBus = feed.get_feed_bus(
+                brokername,
+                service_nursery,
+            )
+            assert bus is feed._bus
+
+            # unblock caller
+            await ctx.started()
+
+            # we pin this task to keep the feeds manager active until the
+            # parent actor decides to tear it down
+            await trio.sleep_forever()
+
+    except eg.ExceptionGroup:
+        # TODO: likely some underlying `brokerd` IPC connection
+        # broke so here we handle a respawn and re-connect attempt!
+        # This likely should pair with development of the OCO task
+        # nusery in dev over @ `tractor` B)
+        # https://github.com/goodboy/tractor/pull/363
+        raise
 
 
 async def spawn_brokerd(

piker/data/feed.py

@@ -190,7 +190,7 @@ def remove_subs(
 
 def get_feed_bus(
     brokername: str,
-    nursery: Optional[trio.Nursery] = None,
+    nursery: trio.Nursery | None = None,
 
 ) -> _FeedsBus:
     '''