ottyl/tickets/108-rework-engine-test-harness-for-actor-split.md

108 — Rework the engine test harness for the primary/profile actor split

Summary

src/engine/tests.rs (2300+ lines) and the test_handle pattern are built entirely around one actor with one channel and one snapshot. After tickets 103-107 land, tests need to exercise three shapes that don't exist today: the primary actor alone, the profile actor alone, and primary+profile interaction (including lag/backfill scenarios). This is explicitly called out as a gap in 0001-actor-split's "Not yet considered" section — real rework, not just extension.

Where

• src/engine/tests.rs — the whole file; most existing tests construct an EngineSnapshot directly or drive it through a single test_handle-obtained channel and assert against one snapshot's query results. Every test that declares a metric, ingests a matching line, and asserts on bucket/query output now spans two actors.
• src/engine/handle.rs test_handle (see ticket 099's history for the existing single-actor test-visibility pattern) — needs an equivalent for the profile actor, and likely a combined test harness helper that spins up both and wires them together the way spawn does in production.
• src/api/mod.rs's test module (ingest_lines/similar helpers, e.g. around :1817 per prior research) and src/config/profile.rs's test module — both currently assume one engine handle; they need updating wherever they exercise metric/alert behavior that's now profile-actor-owned.

Scope

• Add primary-only test helpers (construct a primary actor, ingest lines, assert on raw storage/param extraction only — no metrics).
• Add profile-only test helpers (construct a profile actor against a fake/minimal primary read-side, declare metrics, assert on bucket computation and alert evaluation in isolation).
• Add combined-harness helpers for tests that need real end-to-end behavior (ingest via primary, wait for the profile actor to catch up via the watch signal — needs a deterministic "wait until profile actor has processed up to sequence N" helper rather than a sleep, to avoid flaky tests) — a barrier-equivalent for the profile actor, mirroring EngineHandle::barrier (src/engine/handle.rs:89-101 per prior research).
• Add explicit tests for the accepted lag/eviction tradeoff from 0001-actor-split's resolved decisions: primary evicts on schedule regardless of profile-actor progress, and a lagging profile actor silently misses data for the evicted range — this behavior should be asserted, not just assumed.
• Migrate existing tests in src/engine/tests.rs to the new harness incrementally; tests that only exercise primary-side behavior (param declaration, log storage, eviction) need minimal changes, while tests exercising metrics/alerts need the combined harness.
• Out of scope: adding new test coverage beyond what's needed to preserve today's behavior guarantees across the split — this is a harness migration, not a coverage expansion project.