test(agent-presets): stabilize generation race coverage

packages/preset/agent-presets/tests/mount.spec.ts

@@ -1,4 +1,4 @@
-import { mkdir, mkdtemp, readFile, rm, writeFile } from 'node:fs/promises'
+import { mkdir, mkdtemp, readFile, rm, stat, writeFile } from 'node:fs/promises'
 import { tmpdir } from 'node:os'
 import { dirname, join } from 'node:path'
 import { fileURLToPath, pathToFileURL } from 'node:url'
@@ -542,6 +542,34 @@ describe('editing a composition file', () => {
     expect(livePresetMounts().filter(mount => mount.presetId === 'raced')).toHaveLength(2)
   })
 
+  it('keeps a newer generation pointer when a stale refresh loses the swap race', async () => {
+    const { scoped, path } = await editable('guarded-refresh')
+    const preset = await scoped.agentPresets.resolve('guarded-refresh')
+    await agentOn(scoped, 'sess-guarded-refresh-seed', 'guarded-refresh')
+    const service = scoped.agentPresets as unknown as {
+      standing: Map<string, Promise<{
+        key: unknown
+        scope: unknown
+        stamp: { mtimeMs: number; size: number }
+      }>>
+      ensureStanding(current: typeof preset): Promise<unknown>
+    }
+    const stalePromise = service.standing.get(preset.id)!
+    const stale = await stalePromise
+    await writeFile(path, rowFor('afterwards'))
+    const { mtimeMs, size } = await stat(path)
+    const newer = { ...stale, stamp: { mtimeMs, size } }
+    const newerPromise = Promise.resolve(newer)
+
+    // `await pending` yields before the guarded delete, letting the winning
+    // refresher replace the pointer deterministically instead of by timing.
+    const refresh = service.ensureStanding(preset)
+    service.standing.set(preset.id, newerPromise)
+
+    expect(await refresh).toBe(newer)
+    expect(service.standing.get(preset.id)).toBe(newerPromise)
+  })
+
   it('hands a host reader the standing key without starting an agent', async () => {
     const { scoped } = await editable('cold-read')