Replace the hand-rolled CallbackExporter (TCP listener + custom
OBJREF) with a real `windows-rs` `#[implement]` COM class for
INmxSvcCallback, marshalled via CoMarshalInterface. NmxSvc validates
the callback OBJREF by calling IObjectExporter::ResolveOxid against
the local RPCSS at 127.0.0.1:135; hand-rolled OXIDs aren't registered
there, which is why RegisterEngine2 returned RPC_S_SERVER_UNAVAILABLE
(1722) on every live attempt. CoMarshalInterface registers the OXID
with RPCSS automatically, so the SCM-side resolution succeeds.
Mirrors MxNativeSession.CreateRegisteredService (cs:624), which is
the .NET reference's working path:
ComObjRefProvider.MarshalInterfaceObjRef(callback,
INmxSvcCallback, DifferentMachine)
Layout:
- mxaccess-callback::dcom_sink — INmxSvcCallback + DcomCallbackSink
+ create_dcom_callback_sink_objref. Forwards inbound calls into
the same CallbackEvent::CallbackInvoked { opnum, body } shape the
legacy exporter produces, so callback_router stays path-agnostic.
- Session::from_nmx_client — branched on `windows-com`. Real DCOM
sink when on; legacy CallbackExporter when off (kept for unit
tests that run against an in-process fake NMX peer).
- SessionInner.dcom_sink_holder: Option<IUnknownHolder> — keeps the
COM ref alive for the session's lifetime; shutdown_nmx drops it.
- mxaccess-rpc + mxaccess-callback: windows-rs 0.59 → 0.62. The 0.59
#[implement] macro generates code that doesn't compile under
edition 2024; 0.62 is fixed.
Live result: cargo test -p mxaccess-compat --features
live-windows-com --test lmx_write_complete_live -- --ignored
--nocapture passes end-to-end. RegisterEngine2 OK, write
round-trips, OnWriteComplete fires with the captured MxStatus shape.
Unblocks F49 step 5; F55 marked Resolved in design/followups.md.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Joseph Doherty
2fc327a8d5