test(acp): stabilize continuable subagent snapshot

examples/acp-agent/tests/fixtures/subagent-durability-failure.ts

@@ -1,9 +1,47 @@
 import type { Context } from 'cordis'
 
 export const name = 'subagent-durability-failure'
+export const inject = ['sessionPersistence', 'tasks']
 
-/** Fail a continuable child's provider-owned final durability confirmation. */
+const UNKNOWN_CHILD_ID = '22222222-2222-4222-8222-222222222222'
+const FOLLOW_UP_TASK_ID = 'subagent-2'
+
+/** Fail the child checkpoint and stabilize the authored follow-up failure ordering. */
 export function apply(ctx: Context): void {
+  const thirdStepEnded = Promise.withResolvers<undefined>()
+  const followUpSettled = Promise.withResolvers<undefined>()
+  const persistence = ctx.sessionPersistence
+  const load = persistence.load.bind(persistence)
+
+  // The unavailable-child lookup is real asynchronous I/O. Fence it between
+  // the authored step boundaries so runner speed cannot reorder the exact log.
+  persistence.load = async (id) => {
+    if (id === UNKNOWN_CHILD_ID) await thirdStepEnded.promise
+    return load.call(persistence, id)
+  }
+  ctx.effect(() => () => {
+    persistence.load = load
+    thirdStepEnded.resolve(undefined)
+    followUpSettled.resolve(undefined)
+  }, 'subagent snapshot ordering')
+
+  ctx.on('session/event', (session, event) => {
+    if (session.header.parentSession === undefined
+      && event.type === 'step/end'
+      && event.data.turn === 1
+      && event.data.step === 3) {
+      thirdStepEnded.resolve(undefined)
+    }
+  })
+  ctx.tasks.onTaskDone((snapshot) => {
+    if (snapshot.id === FOLLOW_UP_TASK_ID) followUpSettled.resolve(undefined)
+  })
+  ctx.on('agent/step', async (agent, turn, step) => {
+    if (agent.session.header.parentSession === undefined && turn === 1 && step === 4) {
+      await followUpSettled.promise
+    }
+  })
+
   const flushedTurnEnds = new WeakSet<object>()
   ctx.on('session/flush', (session) => {
     if (session.header.parentSession === undefined) return