feat(FN-4508): complete Step 1 — harden inspectBranchConflict ghost/tip guards

Fusion-Task-Id: FN-4508
Fusion-Task-Lineage: 948a9cae-3975-4f15-bd47-2f88b379171d

packages/engine/src/__tests__/branch-conflicts-ghost-references.test.ts

@@ -0,0 +1,147 @@
+import { afterEach, describe, expect, it } from "vitest";
+import { appendFile, mkdtemp, mkdir, rm, writeFile } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import path from "node:path";
+import { exec } from "node:child_process";
+import { promisify } from "node:util";
+import { inspectBranchConflict } from "../branch-conflicts.js";
+
+const execAsync = promisify(exec);
+
+async function run(command: string, cwd: string): Promise<string> {
+  const { stdout } = await execAsync(command, { cwd, encoding: "utf-8" });
+  return stdout.trim();
+}
+
+describe("inspectBranchConflict ghost references", () => {
+  const dirs: string[] = [];
+
+  afterEach(async () => {
+    await Promise.all(dirs.splice(0).map((dir) => rm(dir, { recursive: true, force: true })));
+  });
+
+  async function setupRepo() {
+    const repoDir = await mkdtemp(path.join(tmpdir(), "fn-4508-branch-conflict-"));
+    dirs.push(repoDir);
+    await run("git init -b main", repoDir);
+    await run("git config user.email test@example.com", repoDir);
+    await run("git config user.name 'Test User'", repoDir);
+    await writeFile(path.join(repoDir, "note.txt"), "base\n", "utf-8");
+    await run("git add note.txt && git commit -m 'chore: base'", repoDir);
+    return repoDir;
+  }
+
+  it("returns stale-resolved when live branch mapping points to missing ghost path", async () => {
+    const repoDir = await setupRepo();
+    await run("git checkout -b fusion/fn-9999", repoDir);
+    await run("git checkout main", repoDir);
+    const livePath = path.join(repoDir, ".worktrees/ghost-cat");
+    await run(`git worktree add ${JSON.stringify(livePath)} fusion/fn-9999`, repoDir);
+    await rm(livePath, { recursive: true, force: true });
+    const conflictingPath = path.join(repoDir, "conflict-path");
+    await mkdir(conflictingPath, { recursive: true });
+
+    const result = await inspectBranchConflict({
+      repoDir,
+      branchName: "fusion/fn-9999",
+      conflictingWorktreePath: conflictingPath,
+      requestingTaskId: "FN-9999",
+      ownerTaskId: "FN-9999",
+      startPoint: "main",
+    });
+
+    expect(result.kind).toBe("stale-resolved");
+  });
+
+  it("returns tip-already-merged when branch tip is reachable from main despite stale startPoint", async () => {
+    const repoDir = await setupRepo();
+    const staleStartPoint = await run("git rev-parse HEAD", repoDir);
+    for (let i = 0; i < 5; i += 1) {
+      await appendFile(path.join(repoDir, "note.txt"), `m${i}\n`, "utf-8");
+      await run(`git add note.txt && git commit -m 'chore: main-${i}'`, repoDir);
+    }
+    await run("git branch fusion/fn-9999", repoDir);
+    const livePath = path.join(repoDir, "wt-live");
+    await run(`git worktree add ${JSON.stringify(livePath)} fusion/fn-9999`, repoDir);
+    const conflictingPath = path.join(repoDir, "conflict-live");
+    await mkdir(conflictingPath, { recursive: true });
+
+    const result = await inspectBranchConflict({
+      repoDir,
+      branchName: "fusion/fn-9999",
+      conflictingWorktreePath: conflictingPath,
+      requestingTaskId: "FN-9999",
+      ownerTaskId: "FN-9999",
+      startPoint: staleStartPoint,
+    });
+
+    expect(result.kind).toBe("tip-already-merged");
+    if (result.kind === "tip-already-merged") {
+      expect(result.integrationRef).toBe("main");
+      expect(result.tipSha).toBe(await run("git rev-parse fusion/fn-9999", repoDir));
+    }
+  });
+
+  it("returns tip-already-merged when startPoint is HEAD and tip is ancestor", async () => {
+    const repoDir = await setupRepo();
+    await run("git branch fusion/fn-9999", repoDir);
+    const livePath = path.join(repoDir, "wt-head");
+    await run(`git worktree add ${JSON.stringify(livePath)} fusion/fn-9999`, repoDir);
+    const conflictingPath = path.join(repoDir, "conflict-head");
+    await mkdir(conflictingPath, { recursive: true });
+
+    const result = await inspectBranchConflict({
+      repoDir,
+      branchName: "fusion/fn-9999",
+      conflictingWorktreePath: conflictingPath,
+      requestingTaskId: "FN-9999",
+      ownerTaskId: "FN-9999",
+      startPoint: "HEAD",
+    });
+
+    expect(result.kind).toBe("tip-already-merged");
+  });
+
+  it("keeps genuine live-foreign conflicts unchanged", async () => {
+    const repoDir = await setupRepo();
+    await run("git checkout -b topic/other", repoDir);
+    await appendFile(path.join(repoDir, "note.txt"), "foreign\n", "utf-8");
+    await run("git add note.txt", repoDir);
+    await run("git commit -m 'chore: foreign work'", repoDir);
+    await run("git checkout main", repoDir);
+    const livePath = path.join(repoDir, "wt-foreign");
+    await run(`git worktree add ${JSON.stringify(livePath)} topic/other`, repoDir);
+    const conflictingPath = path.join(repoDir, "conflict-foreign");
+    await mkdir(conflictingPath, { recursive: true });
+
+    const result = await inspectBranchConflict({
+      repoDir,
+      branchName: "topic/other",
+      conflictingWorktreePath: conflictingPath,
+      requestingTaskId: "FN-9999",
+      ownerTaskId: "FN-9999",
+      startPoint: "main",
+    });
+
+    expect(result.kind).toBe("live-foreign");
+    if (result.kind === "live-foreign") {
+      expect(result.error.name).toBe("BranchConflictError");
+    }
+  });
+
+  it("keeps stale conflictingWorktreePath short-circuit behavior", async () => {
+    const repoDir = await setupRepo();
+    await run("git branch fusion/fn-9999", repoDir);
+
+    const result = await inspectBranchConflict({
+      repoDir,
+      branchName: "fusion/fn-9999",
+      conflictingWorktreePath: path.join(repoDir, "missing-conflict-path"),
+      requestingTaskId: "FN-9999",
+      ownerTaskId: "FN-9999",
+      startPoint: "main",
+    });
+
+    expect(result.kind).toBe("stale");
+  });
+});