test(FN-4475): add fallback empty cherry-pick regression coverage

Fusion-Task-Id: FN-4475
Fusion-Task-Lineage: cb285fff-18e6-4bad-8f22-9158c99337f5

packages/engine/src/__tests__/merger-empty-cherry-pick-fallback.test.ts

@@ -0,0 +1,182 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { execSync, spawnSync } from "node:child_process";
+import { existsSync, mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import type { Settings, Task, TaskStore } from "@fusion/core";
+import { DEFAULT_SETTINGS } from "@fusion/core";
+import { aiMergeTask } from "../merger.js";
+
+const hasGit = spawnSync("git", ["--version"], { stdio: "pipe" }).status === 0;
+const describeIfGit = hasGit ? describe : describe.skip;
+
+type TaskWithPromptOverride = Partial<Task> & Pick<Task, "id"> & { prompt?: string };
+
+function git(repo: string, command: string): string {
+  return execSync(command, { cwd: repo, encoding: "utf-8", stdio: ["pipe", "pipe", "pipe"] }).trim();
+}
+
+function makeTask(overrides: TaskWithPromptOverride): Task {
+  const { id, ...rest } = overrides;
+  return {
+    ...rest,
+    id,
+    title: overrides.title ?? id,
+    description: overrides.description ?? id,
+    column: overrides.column ?? "in-review",
+    dependencies: overrides.dependencies ?? [],
+    steps: overrides.steps ?? [],
+    currentStep: overrides.currentStep ?? 0,
+    log: overrides.log ?? [],
+    createdAt: overrides.createdAt ?? new Date().toISOString(),
+    updatedAt: overrides.updatedAt ?? new Date().toISOString(),
+  } as Task;
+}
+
+function createStore(task: Task, settings: Partial<Settings>): TaskStore {
+  let currentTask = { ...task };
+  const mergedSettings: Settings = {
+    ...DEFAULT_SETTINGS,
+    mergeStrategy: "direct",
+    directMergeCommitStrategy: "always-rebase",
+    mergeConflictStrategy: "fail-fast",
+    smartConflictResolution: true,
+    autoMerge: true,
+    includeTaskIdInCommit: false,
+    commitAuthorEnabled: false,
+    useAiMergeCommitSummary: false,
+    ...settings,
+  } as Settings;
+
+  return {
+    getTask: vi.fn(async () => currentTask),
+    getSettings: vi.fn(async () => mergedSettings),
+    listTasks: vi.fn(async () => [currentTask]),
+    updateTask: vi.fn(async (_id: string, updates: Partial<Task>) => {
+      currentTask = { ...currentTask, ...updates, updatedAt: new Date().toISOString() } as Task;
+      return currentTask;
+    }),
+    moveTask: vi.fn(async (_id: string, column: Task["column"]) => {
+      currentTask = {
+        ...currentTask,
+        column,
+        columnMovedAt: new Date().toISOString(),
+        updatedAt: new Date().toISOString(),
+      } as Task;
+      return currentTask;
+    }),
+    logEntry: vi.fn(async () => undefined),
+    appendAgentLog: vi.fn(async () => undefined),
+    updateSettings: vi.fn(async () => mergedSettings),
+    getActiveMergingTask: vi.fn(() => null),
+    emit: vi.fn(),
+    on: vi.fn(),
+    clearStaleExecutionStartBranchReferences: vi.fn(() => []),
+    getVerificationCacheHit: vi.fn(() => null),
+    recordVerificationCachePass: vi.fn(() => undefined),
+    upsertTaskCommitAssociation: vi.fn(async () => undefined),
+  } as unknown as TaskStore;
+}
+
+describeIfGit("FN-4475 empty cherry-pick fallback handling", () => {
+  const repos: string[] = [];
+
+  afterEach(() => {
+    for (const repo of repos.splice(0)) {
+      rmSync(repo, { recursive: true, force: true });
+    }
+  });
+
+  function setupRepo(prefix: string): string {
+    const repo = mkdtempSync(join(tmpdir(), prefix));
+    repos.push(repo);
+    git(repo, "git init -b main");
+    git(repo, 'git config user.email "test@example.com"');
+    git(repo, 'git config user.name "Test User"');
+    writeFileSync(join(repo, "README.md"), "init\n", "utf-8");
+    git(repo, "git add README.md && git commit -m 'chore: init'");
+    return repo;
+  }
+
+  it("FN-4475 treats -X ours fallback empty cherry-pick as already-on-main and completes done", async () => {
+    const repo = setupRepo("fusion-merger-empty-fallback-ours-");
+    writeFileSync(join(repo, "conflict.txt"), "base\n", "utf-8");
+    git(repo, "git add conflict.txt && git commit -m 'chore: base conflict file'");
+    const beforeConflictSha = git(repo, "git rev-parse HEAD");
+
+    const branch = "fusion/fn-4475-fallback-ours";
+    git(repo, `git checkout -b ${branch}`);
+    writeFileSync(join(repo, "conflict.txt"), "branch-change\n", "utf-8");
+    git(repo, "git add conflict.txt && git commit -m 'feat: branch conflict change'");
+
+    git(repo, "git checkout main");
+    writeFileSync(join(repo, "conflict.txt"), "main-change\n", "utf-8");
+    git(repo, "git add conflict.txt && git commit -m 'feat: main conflict change'");
+    const mainHeadBeforeMerge = git(repo, "git rev-parse HEAD");
+
+    const task = makeTask({ id: "FN-4475-A", branch, baseBranch: "main", column: "in-review", prompt: "# Task\n" });
+    const store = createStore(task, {
+      mergeConflictStrategy: "smart-prefer-main",
+      smartConflictResolution: true,
+    });
+
+    await aiMergeTask(store, repo, task.id);
+
+    expect((store.moveTask as ReturnType<typeof vi.fn>).mock.calls.some(([, column]) => column === "done")).toBe(true);
+    expect(git(repo, "git rev-parse HEAD")).toBe(mainHeadBeforeMerge);
+    expect(existsSync(join(repo, ".git", "CHERRY_PICK_HEAD"))).toBe(false);
+    expect((store.logEntry as ReturnType<typeof vi.fn>).mock.calls.some(([id, msg]) => id === task.id
+      && String(msg).includes("Auto-merge skipped: branch fully subsumed by main"))).toBe(true);
+    expect((store.logEntry as ReturnType<typeof vi.fn>).mock.calls.some(([, msg]) => String(msg).includes("MergeConflictGiveUp"))).toBe(false);
+    expect(beforeConflictSha).not.toBe(mainHeadBeforeMerge);
+  }, 20_000);
+
+  it("FN-4475 keeps smart-prefer-branch fallback non-empty picks as landed commits", async () => {
+    const repo = setupRepo("fusion-merger-empty-fallback-theirs-nonempty-");
+    writeFileSync(join(repo, "conflict.txt"), "base\n", "utf-8");
+    git(repo, "git add conflict.txt && git commit -m 'chore: base conflict file'");
+
+    const branch = "fusion/fn-4475-fallback-theirs";
+    git(repo, `git checkout -b ${branch}`);
+    writeFileSync(join(repo, "conflict.txt"), "branch-change\n", "utf-8");
+    git(repo, "git add conflict.txt && git commit -m 'feat: branch conflict change'");
+
+    git(repo, "git checkout main");
+    writeFileSync(join(repo, "conflict.txt"), "main-change\n", "utf-8");
+    git(repo, "git add conflict.txt && git commit -m 'feat: main conflict change'");
+
+    const task = makeTask({ id: "FN-4475-B", branch, baseBranch: "main", column: "in-review", prompt: "# Task\n" });
+    const store = createStore(task, {
+      mergeConflictStrategy: "smart-prefer-branch",
+      smartConflictResolution: true,
+    });
+
+    await aiMergeTask(store, repo, task.id);
+
+    expect((store.moveTask as ReturnType<typeof vi.fn>).mock.calls.some(([, column]) => column === "done")).toBe(true);
+    expect(git(repo, "git show HEAD:conflict.txt")).toBe("branch-change");
+    expect((store.logEntry as ReturnType<typeof vi.fn>).mock.calls.some(([id, msg]) => id === task.id
+      && String(msg).includes("Auto-merge skipped"))).toBe(false);
+  }, 20_000);
+
+  it("FN-4475 still propagates genuine conflicts when fail-fast strategy is used", async () => {
+    const repo = setupRepo("fusion-merger-empty-fallback-fail-fast-");
+    writeFileSync(join(repo, "conflict.txt"), "base\n", "utf-8");
+    git(repo, "git add conflict.txt && git commit -m 'chore: base conflict file'");
+
+    const branch = "fusion/fn-4475-fail-fast";
+    git(repo, `git checkout -b ${branch}`);
+    writeFileSync(join(repo, "conflict.txt"), "branch-change\n", "utf-8");
+    git(repo, "git add conflict.txt && git commit -m 'feat: branch conflict change'");
+
+    git(repo, "git checkout main");
+    writeFileSync(join(repo, "conflict.txt"), "main-change\n", "utf-8");
+    git(repo, "git add conflict.txt && git commit -m 'feat: main conflict change'");
+
+    const task = makeTask({ id: "FN-4475-C", branch, baseBranch: "main", column: "in-review", prompt: "# Task\n" });
+    const store = createStore(task, { mergeConflictStrategy: "fail-fast" });
+
+    await expect(aiMergeTask(store, repo, task.id)).rejects.toThrow();
+    expect((store.moveTask as ReturnType<typeof vi.fn>).mock.calls.some(([, column]) => column === "done")).toBe(false);
+  }, 20_000);
+});