fix: attach the bridge session before it can serve messages

The worker created its SyncBridgeGateway inside host.call and only
attached it to the sync gateway afterwards on the main thread.  A
coordinator message arriving in that window (STATUS, CHANNEL_EXEC as
the first action on a fresh gateway) dispatched into a gateway whose
_trio_session was still None, so the reply fell through to the sync IO
stub and killed the session -- the whole test suite under pytest -n 12
showed this as freshly created gateways being dead on arrival.  The
race predates the B.5 facade (ProtocolSession had the same window).

SyncBridgeGateway now attaches itself in __init__, before its serve
task can dispatch anything, and the redundant attach calls at the two
construction sites are gone.

Also drop the apipkg-era unknown-attribute test from the namespace
tests -- unknown attributes are plain Python module behaviour now.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: RonnyPfannschmidt

src/execnet/_trio_host.py

@@ -220,6 +220,10 @@ def __init__(
         self._done_sync = threading.Event()
         self._send_closed = False
         self._send_lock = threading.Lock()
+        # Attach before any serving can happen: the first inbound message
+        # may need to reply through gateway._send, which must already
+        # route to this session (not the sync IO stub).
+        sync_gateway._attach_trio_session(self)
 
     # -- engine hooks (run on the host loop) --
 
@@ -455,11 +459,9 @@ def _make_gateway(self, stream: ByteStream, spec: Any) -> AsyncGateway:
         import execnet
 
         sync_gw = execnet.Gateway(_TempIO(self.group.execmodel), spec)
-        bridge = SyncBridgeGateway(
+        return SyncBridgeGateway(
             stream, id=spec.id, sync_gateway=sync_gw, host=self.host
         )
-        sync_gw._attach_trio_session(bridge)
-        return bridge
 
     async def _open_via_stream(self, spec: Any) -> ByteStream:
         from . import _provision

src/execnet/_trio_worker.py

@@ -225,10 +225,11 @@ def _run_worker(host: _trio_host.TrioHost, io: Any, id: str, model: ExecModel) -
     gateway, trio_exec, main_thread_only = _build_worker_gateway(host, id, model)
 
     async def _start() -> _trio_host.SyncBridgeGateway:
+        # The bridge attaches itself to the gateway before serving starts,
+        # so inbound messages can reply through gateway._send right away.
         return await host.start_session(gateway, io)
 
-    session = host.call(_start)
-    gateway._attach_trio_session(session)
+    host.call(_start)
 
     try:
         if main_thread_only:

testing/test_namespaces.py

@@ -74,12 +74,3 @@ def test_import_execnet_does_not_import_trio() -> None:
         check=True,
     )
     assert out.stdout.strip() == "False"
-
-
-def test_unknown_attribute_raises() -> None:
-    try:
-        execnet.does_not_exist
-    except AttributeError as exc:
-        assert "does_not_exist" in str(exc)
-    else:
-        raise AssertionError("expected AttributeError")