Joseph Doherty 4de94fab0d Phase 6.1 Stream A remaining — IPerCallHostResolver + DriverNodeManager per-call host dispatch (decision #144) ...

Closes the per-device isolation gap flagged at the Phase 6.1 Stream A wire-up
(PR #78 used driver.DriverInstanceId as the pipeline host for every call, so
multi-host drivers like Modbus with N PLCs shared one pipeline — one dead PLC
poisoned sibling breakers). Decision #144 requires per-device isolation; this
PR wires it without breaking single-host drivers.

Core.Abstractions:
- IPerCallHostResolver interface. Optional driver capability. Drivers with
  multi-host topology (Modbus across N PLCs, AB CIP across a rack, etc.)
  implement this; single-host drivers (Galaxy, S7 against one PLC, OpcUaClient
  against one remote server) leave it alone. Must be fast + allocation-free
  — called once per tag on the hot path. Unknown refs return empty so dispatch
  falls back to single-host without throwing.

Server/OpcUa/DriverNodeManager:
- Captures `driver as IPerCallHostResolver` at construction alongside the
  existing capability casts.
- New `ResolveHostFor(fullReference)` helper returns either the resolver's
  answer or the driver's DriverInstanceId (single-host fallback). Empty /
  whitespace resolver output also falls back to DriverInstanceId.
- Every dispatch site now passes `ResolveHostFor(fullRef)` to the invoker
  instead of `_driver.DriverInstanceId` — OnReadValue, OnWriteValue, all four
  HistoryRead paths. The HistoryRead Events path tolerates fullRef=null and
  falls back to DriverInstanceId for those cluster-wide event queries.
- Drivers without IPerCallHostResolver observe zero behavioural change:
  every call still keys on DriverInstanceId, same as before.

Tests (4 new PerCallHostResolverDispatchTests, all pass):
- DeadPlc_DoesNotOpenBreaker_For_HealthyPlc_With_Resolver — 2 PLCs behind
  one driver; hammer the dead PLC past its breaker threshold; assert the
  healthy PLC's first call succeeds on its first attempt (decision #144).
- EmptyString / unknown-ref fallback behaviour documented via test.
- WithoutResolver_SameHost_Shares_One_Pipeline — regression guard for the
  single-host pre-existing behaviour.
- WithResolver_TwoHosts_Get_Two_Pipelines — builds the CachedPipelineCount
  assertion to confirm the shared-builder cache keys correctly.

Full solution dotnet test: 1219 passing (was 1215, +4). Pre-existing
Client.CLI Subscribe flake unchanged.

Adoption: Modbus driver (#120 follow-up), AB CIP / AB Legacy / TwinCAT
drivers (also #120) implement the interface and return the per-tag PLC host
string. Single-host drivers stay silent and pay zero cost.

Remaining sub-items of #160 still deferred:
- IAlarmSource.SubscribeAlarmsAsync + AcknowledgeAsync invoker wrapping.
  Non-trivial because alarm subscription is push-based from driver through
  IAlarmConditionSink — the wrap has to happen at the driver-to-server glue
  rather than a synchronous dispatch site.
- Roslyn analyzer asserting every capability-interface call routes through
  CapabilityInvoker. Substantial (separate analyzer project + test harness);
  noise-value ratio favors shipping this post-v2-GA once the coverage is
  known-stable.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

2026-04-19 12:31:24 -04:00

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