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/_discovery.py

@@ -150,7 +150,7 @@ async def query_actor(
     regaddr: UnwrappedAddress|None = None,
 
 ) -> AsyncGenerator[
-    UnwrappedAddress|None,
+    tuple[UnwrappedAddress|None, Portal|None],
     None,
 ]:
     '''
@@ -164,7 +164,8 @@ async def query_actor(
     actor: Actor = current_actor()
     if (
         name == 'registrar'
-        and actor.is_registrar
+        and
+        actor.is_registrar
     ):
         raise RuntimeError(
             'The current actor IS the registry!?'
@@ -172,7 +173,7 @@ async def query_actor(
 
     maybe_peers: list[Channel]|None = get_peer_by_name(name)
     if maybe_peers:
-        yield maybe_peers[0].raddr
+        yield maybe_peers[0].raddr, None
         return
 
     reg_portal: Portal
@@ -185,8 +186,7 @@ async def query_actor(
             'find_actor',
             name=name,
         )
-        yield addr
+        yield addr, reg_portal
-
 
 @acm
 async def maybe_open_portal(
@@ -196,15 +196,36 @@ async def maybe_open_portal(
     async with query_actor(
         name=name,
         regaddr=addr,
-    ) as addr:
+    ) as (addr, reg_portal):
-        pass
+        if not addr:
+            yield None
+            return
 
-    if addr:
+        try:
             async with _connect_chan(addr) as chan:
                 async with open_portal(chan) as portal:
                     yield portal
-    else:
+
-        yield None
+        # 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 reg_portal.run_from_ns(
+                'self',
+                'delete_sockaddr',
+                sockaddr=addr.unwrap(),
+            )
+            log.error(
+                f'Deleted stale registry entry !\n'
+                f'addr: {addr!r}\n'
+                f'uid: {uid!r}\n'
+            )
 
 
 @acm
@@ -272,7 +293,7 @@ async def find_actor(
         if not any(portals):
             if raise_on_none:
                 raise RuntimeError(
-                    f'No actor "{name}" found registered @ {registry_addrs}'
+                    f'No actor {name!r} found registered @ {registry_addrs!r}'
                 )
             yield None
             return
@@ -288,6 +309,7 @@ async def find_actor(
             yield portals
 
 
+
 @acm
 async def wait_for_actor(
     name: str,