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q14-dispatcher-affinity.md

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Q14 — Dispatcher affinity measurement (deferred)

Spec 047 §14 Phase 1 (1.6) decision: keep the existing Debug-only DispatcherQueue.HasThreadAccess check; do not tighten to an unconditional throw in Release until Phase 1's measurement run confirms no in-tree callers trip it.

Status

Deferred to a perf run on the Phase 0 baseline machines (LAPTOP-4MEP83VI ARM64-native and CPC-ander-YTZ3O x64-native). The 1.6 implementation ships:

  • MountContext / UpdateContext / UnmountContext are readonly ref structs — they cannot escape the dispatcher-affine call stack.
  • XML doc comments on each context type call out the UI-thread guarantee (Q14).
  • The existing Debug-only HasThreadAccess check in Reconciler stays as-is.

What the Phase 1 measurement needs to capture

  1. Run a Release build of the in-tree test suite (dotnet test tests/Reactor.Tests) with the check toggled to unconditional and record any violations.
  2. Repeat for StressPerf.Reactor + StressPerf.ReactorV2 to catch non-test call sites.
  3. If zero violations: ship the tighten in 1.19 as part of the final perf gate sign-off.
  4. If non-zero: capture the call sites and decide between (a) fixing each to dispatch onto the UI thread, or (b) keeping the Debug-only check and documenting the gap in extensibility-preview.md.

Why this gates after the protocol surface lands

The decision needs the V1 dispatch path (1.6) live so test suites exercise the new context types — measuring before any handler ports exist would only re-validate the legacy path's existing behavior.