Handle stale registrar entries; detect and delete

In cases where an actor's transport server task (by default handling new
TCP connections) terminates early but does not de-register from the
pertaining registry (aka the registrar) actor's address table, the
trying-to-connect client actor will get a connection error on that
address. In the case where client handles a (local) `OSError` (meaning
the target actor address is likely being contacted over `localhost`)
exception, make a further call to the registrar to delete the stale
entry and `yield None` gracefully indicating to calling code that no
`Portal` can be delivered to the target address.

This issue was originally discovered in `piker` where the `emsd`
(clearing engine) actor would sometimes crash on rapid client
re-connects and then leave a `pikerd` stale entry. With this fix new
clients will attempt connect via an endpoint which will re-spawn the
`emsd` when a `None` portal is delivered (via `maybe_spawn_em()`).

tractor/__init__.py

@@ -31,6 +31,7 @@ from ._streaming import (
 )
 from ._discovery import (
     get_arbiter,
+    get_registrar,
     find_actor,
     wait_for_actor,
     query_actor,
@@ -77,6 +78,7 @@ __all__ = [
     'find_actor',
     'query_actor',
     'get_arbiter',
+    'get_registrar',
     'is_root_process',
     'msg',
     'open_actor_cluster',

tractor/_discovery.py

@@ -35,7 +35,7 @@ from ._state import current_actor, _runtime_vars
 
 
 @acm
-async def get_arbiter(
+async def get_registrar(
 
     host: str,
     port: int,
@@ -56,11 +56,14 @@ async def get_arbiter(
         # (likely a re-entrant call from the arbiter actor)
         yield LocalPortal(actor, Channel((host, port)))
     else:
-        async with _connect_chan(host, port) as chan:
+        async with (
+            _connect_chan(host, port) as chan,
+            open_portal(chan) as arb_portal,
+        ):
+            yield arb_portal
 
-            async with open_portal(chan) as arb_portal:
 
-                yield arb_portal
+get_arbiter = get_registrar
 
 
 @acm
@@ -101,7 +104,10 @@ async def query_actor(
 
         # TODO: return portals to all available actors - for now just
         # the last one that registered
-        if name == 'arbiter' and actor.is_arbiter:
+        if (
+            name == 'arbiter'
+            and actor.is_arbiter
+        ):
             raise RuntimeError("The current actor is the arbiter")
 
         yield sockaddr if sockaddr else None
@@ -112,7 +118,7 @@ async def find_actor(
     name: str,
     arbiter_sockaddr: tuple[str, int] | None = None
 
-) -> AsyncGenerator[Optional[Portal], None]:
+) -> AsyncGenerator[Portal | None, None]:
     '''
     Ask the arbiter to find actor(s) by name.
 
@@ -120,17 +126,49 @@ async def find_actor(
     known to the arbiter.
 
     '''
-    async with query_actor(
-        name=name,
-        arbiter_sockaddr=arbiter_sockaddr,
-    ) as sockaddr:
+    actor = current_actor()
+    async with get_arbiter(
+        *arbiter_sockaddr or actor._arb_addr
+    ) as arb_portal:
+
+        sockaddr = await arb_portal.run_from_ns(
+            'self',
+            'find_actor',
+            name=name,
+        )
+
+        # TODO: return portals to all available actors - for now just
+        # the last one that registered
+        if (
+            name == 'arbiter'
+            and actor.is_arbiter
+        ):
+            raise RuntimeError("The current actor is the arbiter")
 
         if sockaddr:
-            async with _connect_chan(*sockaddr) as chan:
-                async with open_portal(chan) as portal:
-                    yield portal
-        else:
-            yield None
+            try:
+                async with _connect_chan(*sockaddr) as chan:
+                    async with open_portal(chan) as portal:
+                        yield portal
+                        return
+
+            # most likely we were unable to connect the
+            # transport and there is likely a stale entry in
+            # the registry actor's table, thus we need to
+            # instruct it to clear that stale entry and then
+            # more silently (pretend there was no reason but
+            # to) indicate that the target actor can't be
+            # contacted at that addr.
+            except OSError:
+                # NOTE: ensure we delete the stale entry from the
+                # registar actor.
+                uid: tuple[str, str] = await arb_portal.run_from_ns(
+                    'self',
+                    'delete_sockaddr',
+                    sockaddr=sockaddr,
+                )
+
+        yield None
 
 
 @acm