test: Durable Object harness for SessionDO

[schmug]

Requested by Schmug · project thread

Closes #31.

Before: src/session-do.ts had no direct coverage. Deleting axisCreationsInFlight, the failed-embed pump kick, or the expanded write left the suite green.

After: test/session-do.test.ts drives a real SessionDO through its own SQL, alarm pump and restarts, inside the normal npm test, with no network or credentials. Each of those three deletions now fails a test (checked by mutating the code and rerunning).

How

test/helpers/do-harness.ts fakes only what the DO touches:

• ctx.storage.sql over a real in-memory SQLite (node:sqlite, built into Node 22, which CI already pins). Two runtime behaviours are reproduced on purpose: multi-statement exec (what migrate() relies on) and BLOBs reading back as ArrayBuffer. Node returns Uint8Array, which new Float32Array(...) would silently corrupt.
• Alarms are a recorded time that tests fire with fireAlarm() / drainAlarms().
• restart() builds a fresh instance over the same database, which is what eviction looks like to a DO.
• AI goes through the production selectBudgetedAiRunner with the binding set to fakeAiRunner from dev-fake-ai.ts, wrapped so tests can count calls and fail or hold specific ones. fakeAiRunner gained an optional latencyMs; the default stays 250.

The four tests cover the acceptance criteria: construct, persist and hydrate; an axis with expanded: false surviving restart through real SQL; a failed pole embed kicking the pump and a later pass filling it; and a second concurrent create at 2/3 axes spending zero AI calls. They add about 200 ms to the suite.

Why not @cloudflare/vitest-pool-workers: it needs a second vitest pool running inside workerd, and this project's remote: true AI binding pulls in the Access-gated proxy that kills wrangler dev without credentials. The approach is documented in CLAUDE.md.

Not done: BoardDO uses key-value get/put and the stubs in test/board-guards.test.ts. Adding those to FakeStorage is a natural follow-up, and so is updating the "no DO harness (issue #31)" comments in the board files.

Gates

• npm run typecheck: clean
• npm test: 44 files, 892 passing, 0 failing. Node prints an ExperimentalWarning for node:sqlite; it's harmless.

🤖 Generated with Claude Code

https://claude.ai/code/session_011jTqcyCetskFRRENsms9wZ

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