Captured OBJREF byte structures from both paths via the .NET probe:
- `--probe-callback-marshal`: DCOM-marshalled, 338 bytes, succeeds
(when used inside `MxNativeSession.Open` → `CreateRegisteredService`).
- `--probe-register-managed-callback`: hand-rolled, 162 bytes, fails
with `RegisterEngine2 → 0x800706BA RPC_S_SERVER_UNAVAILABLE`.
The structural diff:
- `std_flags`: DCOM=`0x0A80` (SORF_OXRES4+6+8) vs hand-rolled=`0x280`
(SORF_OXRES4+6). Bit `0x0800` (SORF_OXRES8) only set in DCOM.
- ncacn_ip_tcp bindings: DCOM=4 with no ports; hand-rolled=1 with
explicit `[port]`.
- Total size: 338 vs 162 bytes.
Tested the simplest fix (hand-rolled `std_flags = 0x0A80` to match
DCOM): **still fails with the same 1722.** Reverted.
**Diagnosis updated in F55:** NmxSvc on receiving RegisterEngine2
appears to call `IObjectExporter::ResolveOxid` against the local
SCM (`127.0.0.1:135`) to resolve the callback OBJREF's OXID, then
dial the resulting bindings. Our hand-rolled OXID is never
registered with RPCSS, so the SCM-side resolution fails and NmxSvc
returns RPC_S_SERVER_UNAVAILABLE — matching:
- the symptom (1722),
- the sub-second timing (no TCP dial-back to our listener attempted),
- the fact that the .NET `ManagedCallbackExporter` (same hand-rolled
approach) ALSO fails identically.
DCOM marshalling fixes this because `CoMarshalInterface` internally
registers the OXID with RPCSS. The bindings have no port because
RPCSS returns the dynamic port from the DCOM stub layer.
**Conclusion: Path A is the architecturally correct fix** — the
callback exporter must be a DCOM-managed object (e.g. via
`windows-rs` `#[implement]`) for NmxSvc to accept the callback.
The hand-rolled-listener-with-explicit-port approach is
fundamentally incompatible with NmxSvc's callback validation, in
both Rust and the .NET reference.
Path C (cheap investigation) is exhausted; F55 verdict updated to
recommend Path A explicitly.
`cargo test --workspace` 824 passing; clippy `-D warnings` clean
across both feature configurations.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Joseph Doherty
0a274af76f