test(FN-4954): complete Step 2 — reproduce pool rehydrate double-lease race

Fusion-Task-Id: FN-4954
Fusion-Task-Lineage: b3dd2d2f-3aa3-48f1-8a56-56e66518d139

packages/engine/src/__tests__/worktree-pool-double-lease.test.ts

@@ -0,0 +1,34 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+vi.mock("node:fs", () => ({
+  existsSync: vi.fn().mockReturnValue(true),
+  lstatSync: vi.fn().mockReturnValue({ isDirectory: () => true, isSymbolicLink: () => false }),
+  readdirSync: vi.fn().mockReturnValue([]),
+  rmSync: vi.fn(),
+  realpathSync: vi.fn((path: string) => path),
+}));
+
+import { WorktreePool } from "../worktree-pool.js";
+
+// FN-4954: deterministic repro for the rehydrate collision race.
+describe("WorktreePool double-lease reproduction", () => {
+  let pool: WorktreePool;
+
+  beforeEach(() => {
+    pool = new WorktreePool();
+  });
+
+  it("reproduces rehydrate re-adding a path that is already leased", () => {
+    pool.release("/tmp/wt-race");
+
+    const firstLease = pool.acquire();
+    expect(firstLease).toBe("/tmp/wt-race");
+
+    // Simulates scan/rehydrate colliding with an in-flight lease.
+    pool.rehydrate(["/tmp/wt-race"]);
+
+    // Expected invariant: leased paths must never be re-added to idle.
+    // Current behavior (pre-fix) returns the same path again here.
+    expect(pool.acquire()).toBeNull();
+  });
+});