test(FN-4954): complete Step 6 — add merger/pool reliability backstop

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

packages/engine/src/__tests__/reliability-interactions/worktree-pool-merger-release.test.ts

@@ -0,0 +1,71 @@
+import { mkdir } from "node:fs/promises";
+import { execSync } from "node:child_process";
+import { join } from "node:path";
+import { describe, expect, it, vi } from "vitest";
+
+vi.mock("../../pi.js", () => ({
+  createFnAgent: vi.fn(async () => ({
+    prompt: vi.fn(async () => undefined),
+    dispose: vi.fn(async () => undefined),
+  })),
+  describeModel: vi.fn(() => "mock-provider/mock-model"),
+  promptWithFallback: vi.fn(async (session: { prompt: (prompt: string) => Promise<unknown> }, prompt: string) => {
+    await session.prompt(prompt);
+  }),
+  compactSessionContext: vi.fn(),
+}));
+
+import { aiMergeTask } from "../../merger.js";
+import { WorktreePool } from "../../worktree-pool.js";
+import { git, hasGit, makeReliabilityFixture } from "./_helpers.js";
+
+describe("FN-4954 reliability interactions: merger pooled release ordering", () => {
+  it.skipIf(!hasGit)("detaches and clears task pointers before pooled release exposes the path", async () => {
+    const fixture = await makeReliabilityFixture({
+      taskId: "FN-4954-RI-A",
+      task: { steps: [] as any[] },
+      settings: { recycleWorktrees: true },
+    });
+
+    try {
+      const { rootDir, store, task } = fixture;
+      const seededTask = await store.getTask(task.id);
+      const completedSteps = (seededTask?.steps ?? []).map((step) => ({ ...step, status: "done" as const }));
+      await store.updateTask(task.id, {
+        steps: completedSteps,
+        currentStep: completedSteps.length,
+      } as any);
+      const branch = task.branch ?? `fusion/${task.id.toLowerCase()}`;
+      const worktreePath = join(rootDir, ".worktrees", "fn-4954-ri-a");
+
+      await fixture.createBranch(branch);
+      await fixture.writeAndCommit("packages/engine/src/fn-4954-ri-a.txt", "race guard\n", "feat: add merge content");
+      await fixture.checkout("main");
+
+      await mkdir(join(rootDir, ".worktrees"), { recursive: true });
+      git(rootDir, `git worktree add ${JSON.stringify(worktreePath)} ${JSON.stringify(branch)}`);
+      await store.updateTask(task.id, { branch, worktree: worktreePath, column: "in-review" });
+
+      const pool = new WorktreePool(rootDir);
+      const result = await aiMergeTask(store, rootDir, task.id, { pool });
+      expect(result.merged).toBe(true);
+      expect(result.worktreeRemoved).toBe(false);
+      expect(pool.size).toBe(1);
+
+      const acquired = pool.acquire("FN-4954-RI-B");
+      expect(acquired).toBe(worktreePath);
+
+      expect(() => execSync("git symbolic-ref --quiet HEAD", {
+        cwd: worktreePath,
+        stdio: "pipe",
+        encoding: "utf-8",
+      })).toThrow();
+
+      const mergedTask = await store.getTask(task.id);
+      expect(mergedTask?.worktree ?? null).toBeNull();
+      expect(mergedTask?.branch ?? null).toBeNull();
+    } finally {
+      await fixture.cleanup();
+    }
+  }, 30_000);
+});